sailboat folding propeller

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Choosing the Proper Sailboat Prop

• By David Schmidt
• Updated: July 21, 2021

As sailors, it’s romantic to think that our locomotion comes from the wind, but most skippers, when pressed, admit to motoring a fair portion of the time. Like it or not, your boat’s propeller is a key element of the propulsion system, and just as there are myriad considerations involved when ordering sails, decisions abound when selecting the prop that best suits your wallet and sailing plans.

Fixed-Blade Options

As the moniker implies, fixed-blade propellers are typically cast out of an alloy such as manganese bronze or nickel, bronze and aluminum. Individual propeller blades stem from a central hub, which in turn attaches to the sailboat’s prop shaft or saildrive. Depending on the manufacturer, these blades typically are pitched in a way that propels the boat forward or backward when the transmission is engaged. The term “pitch” refers to how far forward or reverse a propeller will theoretically travel through a solid material (picture a screw twisting into a wood block). For example, a prop with a 12-inch pitch will theoretically move through 1 foot of solid material with each full rotation.

Sailors have choices available when buying a fixed-blade propeller, with the biggest variables being the ­diameter, the number of propeller blades, and the pitch. Just as sailboats create “dirty air” that spills off their sails, propeller blades push water against the hull, which then reverberates back (call this “dirty water”). When the transmission is in forward, this dirty water is sent back toward the rudder, which can influence steering. An important rule of thumb for all propeller designs and types is to ensure that the propeller has a tip clearance (the amount of space between a blade’s tip and the hull when the blade is in the 12 o’clock position) that’s at least 10 percent of the length of the prop blade (hub to tip). If you don’t have enough tip clearance, the next step is to spec a smaller-diameter propeller with an extra blade. Fixed props are available with two, three or four blades.

While this can be an easy fix, additional blades create additional drag when the boat is sailing. Because of this, sailors who opt for a fixed-blade prop are typically best served choosing a screw with the fewest number of blades that can properly fit their boat and their engine (see “Choosing Screws,” right). That said, additional blades equate to more power when motoring into a stiff headwind, current or big seas. Additional propeller blades can also accommodate a more powerful engine at a given diameter. These latter points are important if you’re planning on cruising unfamiliar waters and might find yourself negotiating a lee shore under power.

While fixed-blade ­propellers are strong, simple, relatively inexpensive, and ­easier to have repaired in remote locales, they do affect sailing performance. Moreover, they’re more likely to foul a lobster pot or other fishing gear than a folding prop when the boat is sailing. If you care about sailing performance, especially in the light stuff, or if you’re planning a long-distance cruise, purchasing a folding or feathering prop is a wise investment. But if you’re seeking maximum power or punch for your buck, and you sail on breezy waters such as San Francisco Bay, where extra drag isn’t a huge concern, a fixed-blade prop could be the way to go.

Folding Propellers

Folding propellers employ a series of gears (situated around the prop’s central hub and on the inboard end of each prop blade) and centrifugal force to open when the engine’s transmission is engaged. The slipstream action of the passing water acts to fold them closed when sailing, and their gear mechanisms hold the blades in their closed position even in light airs. As with fixed-blade props, sailors can choose between models that feature two, three or four blades. Unlike fixed-blade props, however, folding propellers create far less drag when the boat is sailing.

“For the same diameter and number of blades, folding propellers have 85 to 100 percent less drag than a fixed prop, depending on the fixed propeller’s blade area and the folding propeller’s make and model,” says Geoff Prior, owner of AB Marine, the Newport, Rhode Island-based importer and distributor for Gori folding props and AutoProp, Variprofile, and Variprop feathering propellers. “Geared folding propellers stay closed or folded when the boat is sailing, so a blade cannot drop down to be exposed to the water flow, and does not catch lines and weeds like fixed and feathering propellers can.”

Others agree. “Most sailboats will gain 0.5 to even 1 knot of speed by using a low-drag folding propeller,” says Keld Willberg, Flexofold’s general manager. “Many boat owners don’t look just at the speed gains, but also the ability to sail in very light wind.” Also, he says, when tacking, a boat that’s equipped with a low-drag prop won’t lose as much momentum.

This latter point is critical to anyone who sails in places with typically light breeze. “The folding propeller’s drag reduction and increased sailing speeds produce less turbulence over the rudder, enabling you to point higher, steer easier and more precisely, and tack and jibe the boat easier,” Prior says. “The biggest speed difference is in light or medium air and with clean water flow over the rudder—the boat is a lot more responsive, especially when fine sailing on the wind.”

One important consideration when selecting a folding prop is ensuring that your boat can physically accommodate the length of the prop when it’s folded. This is especially important on full-keel boats that have an aperture between the keel and rudder. For anyone who cruises with a saildrive(s), Prior advises that saildrive propellers must be approved for use by the saildrive’s manufacturer to ensure that the prop is electrically isolated from the saildrive’s leg and shaft; also the prop’s hub must incorporate a rubberized bushing to absorb shock and inertia, as well as an easily changed and ­streamlined anode.

As with all props, ­selecting the diameter, number of blades, and pitch of the propeller are all key decisions. “Diameter is the single most critical factor in determining the amount of power that a propeller absorbs and transmits into the water,” Prior says. “It’s the most important single factor in determining the amount of thrust delivered. The larger the diameter, the greater the efficiency. A small increase in diameter dramatically increases thrust and torque load on the engine and shaft.”

Blade count is also critical, but Willberg cautions that it’s overly simplistic to think that additional blades equate to additional power (see “Calling the Pitch,” right). “What ­matters is to get the right combination of blade area and pitch for the engine and gear box,” he says. “On flat water, a two-blade propeller can be just as efficient or even slightly better than a three-blade. However, when motoring against wind and waves, and when maneuvering, you get more power from a ­three-blade prop.”

Blade shape also matters. “Propellers with flat, planar blades are like a paddle,” Prior says. “The flat shape is not ­efficient at producing thrust because a flat blade is overloaded at the tip and underloaded at the blade base, whereas a fully shaped airfoil blade shape has a constant loading over its whole surface.” Much like fixed-blade props, most folding propellers employ an efficient blade shape, while feathering props use flatter and more ­planar-shaped blades.

One drawback of most (but not all) folding props emerges when the transmission is put into reverse. “In reverse, the suction side is on the back or aft side of a blade, and with the typical folding propeller design, the blades are trying to close in reverse, giving low thrust and lots of prop walk,” says Prior, who added that Gori’s three-blade prop presents identical leading-blade edges in both forward and reverse, negating this issue. Willberg advises that folding props require slightly different operation than fixed-blade or feathering props to ensure that they stay open. “For more stopping and reverse power, it’s just a matter of giving more revs on the throttle,” he says.

RELATED: Understanding Your Sailboat Propellers

As with all props, folding propellers must be kept clean of marine growth, and anodes must be regularly changed to ensure cathodic protection. “One millimeter of growth on a blade and your propeller loses approximately 12 percent efficiency,” Prior says, adding that this applies to any make, model or design. And, he notes, “folding propellers are water-lubricated, so they do not need to be greased.”

The final consideration is cost. Folding props are roughly four to six times more expensive than a fixed-blade propeller for a given diameter and blade count. While this is a big difference, the gains are huge, and—for bluewater cruisers—can translate to ­significantly faster passage times (potentially measured in days) and happier crews.

Feathering Propellers

As do folding propellers, feathering props greatly reduce drag when the boat is sailing; however, the way that they achieve this gain is significantly different. Moreover, the physics behind how the prop transitions from its sailing mode to motoring mode is also different.

Feathering props are available with two-, three-, four- and five-blade configurations. A central hub attaches to the prop shaft or a saildrive, and it features a series of internal beveled gears. Geared propeller blades attach to the hub and rotate through 180 degrees, depending on if the transmission is in forward or reverse (more on this later); this articulation is governed by machined stops inside the hub that prevent the blades from overrotating. When the transmission is engaged, the blades present their broad sides to the slipstream, but when the boat is under sail, the blades rotate 90 degrees to align with the flow of the water.

“Feathering props work off torque from the shaft or saildrive,” says Fred Hutchison, at PYI, which distributes Max-Prop feathering propellers. Because feathering props use torque—not centrifugal force—to open, it’s “instantaneous,” says Hutchison, who points to this as the design’s biggest advantage. “It takes a feathering prop only three-quarters of a turn to go from full forward to full reverse. In that time, the blades rotate 180 degrees, and it doesn’t generate much shock load.”

Prior adds, “In the sailing position, the water flow turns the blades to the feathered position when the engine is off for vastly reduced drag.” He estimates that this drag reduction is approximately 85 percent compared with fixed-blade props.

As previously mentioned, feathering-prop blades rotate through 180 degrees, which is a big advantage when operating in reverse compared with most folding props. “A feathering propeller gives reverse thrust with the same efficiency as in forward, as the leading blade edge in forward is turned 180 degrees, and without prop walk,” Prior says.

Another advantage of feathering props for boats with full keels or with tightly spaced skegs or rudders is that they “can fit in a small aperture as they have shorter overall lengths compared to folding propellers, which have a greater length when folded,” Prior says. “If there is a diameter constraint, the feathering prop can easily be designed and built with a higher pitch to make up for the loss of diameter and blade area.”

Most feathering props are machined out of a nickel, bronze and aluminum alloy, which, Hutchison says, makes them easy to repair. Unlike fixed or folding ­propellers, feathering propellers employ flat-shaped blades to reduce drag when the boat is sailing. “Feathering propeller blades are foiled and aren’t as efficient as the helical twist found on most fixed-blade or folding props,” Hutchison says, adding that manufacturers compensate for this less-efficient shape by increasing each blade’s surface area. “Blade surface area and shape make a big difference in how efficient it is,” he says. “No low-drag prop will be as efficient under power as a fixed-blade propeller.”

Lobster pots and fishing nets are common concerns when operating under auxiliary power, and feathering props are not snag-proof. “When motoring, feathering props are the same as fixed-blade and folding propellers,” Hutchison advises. “But when sailing, a folding propeller has less of a chance of catching a pot or fishing net than a fixed-blade or feathering propeller.” This is a consideration for anyone who cruises extensively in, say, Maine or in heavily fished waters off the West Coast.

As with all propellers, feathering props require basic care and maintenance to ensure their efficiency and the engine’s longevity. This starts with a properly spec’d and regularly refreshed anode, and the prop blades must be kept clean.

Unlike folding props, which typically use exposed self-­lubricating gears, the internal gears of feathering propellers need to be greased annually. While this adds an item to an owner’s yearly to-do list, Hutchison says that this can be tackled by a diver.

Finally, while feathering props are a great upgrade, they don’t come cheap. “Feathering props are more expensive than folding props,” says Hutchison, who ­advises that a feathering prop is roughly 10 to 15 percent more ­expensive than a folding prop with the same blade count and a ­comparable diameter. “This is primarily because of ­machining costs,” he says.

David Schmidt is CW ’s electronics editor and also writes about topics ranging from sailing gear to environmental issues from his home in Bellingham, Washington.

Choosing Screws

Choosing the right propeller is complex, but the most important variables for selecting the correct prop are your boat’s make, model, LOA and displacement, the engine’s horsepower rating and its maximum rpm, as well as the transmission’s reduction ratio. While most of this information is easily ascertained, determining the reduction ratio can be challenging. But, Prior says, “the best method to get the actual ratio is to use your cellphone camera and take a photo of the plate on the transmission,” adding that this beats hanging upside down and trying to read this information using a mirror and flashlight. For the DIYer, online calculators exist that can help crunch the numbers. “After that, it gets into the art,” Hutchison says. His advice: Ask an expert.

Calling the Pitch

Unlike fixed-blade propellers, feathering and folding propellers are often designed so that the pitch can be adjusted, perchance it doesn’t perfectly match your vessel, engine and transmission right out of the box. Depending on the prop, this adjustment is typically made by changing a screw(s) or swapping out the prop blades. These adjustments can usually be done by a diver while the boat is in the water. Some general rules of thumb as to when to change the pitch involve engine rpm: If the prop doesn’t meet the engine’s ideal rpm, the blade’s angle needs to be reduced; conversely, if the prop is exceeding the engine’s rpm metrics, the blade’s angle of attack needs to be increased. As a cautionary note, overpitching a propeller can cause the engine to overheat, and it can also lead to injector-choking problems and create excessive internal engine pressure, so be sure to stay within the engine ­manufacturer’s lines.

Vendor Information

EWOL: ewoltech.com , 410-317-8104; from $2,120

Flexofold: flexofold.com , 781-797-0809; from $980

GORI: gori-propeller.com , 401-847-7960; from $650

J Prop: betamarinenc.com , 252-249-2473; from $2,500

Kiwiprop: kiwiprops.co.nz , 877-549-4872; from $1,350

Max-Prop: pyiinc.com , 425-355-3669; from $1,900

Michigan Wheel: ­ miwheel.com , 800-369-4335; from $550

Variprofile: spw-gmbh.de , 401-847-7960; from $800

Variprop: spw-gmbh.de , 401-847-7960; from $1,200

Volvo: volvopenta.us , 800-522-1959; call for pricing

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Folding and feathering propeller test

• Emrhys Barrell
• March 24, 2015

Which is the best folding or feathering propeller for your boat? Emrhys Barrell compares 15 props in a test of speed, thrust, drag, stopping speed and propwalk

in 2009, Yachting Monthly tester Emrhys Barrell put 15 folding and feathering props through the most comprehensive test ever Credit: Graham Snook/YM

What’s the difference between a bucket and a fixed, three-bladed propeller? Not much, if you want to slow your boat down by at least half a knot when you are sailing. That’s the conclusion of our fixed, folding and feathering propeller test – the most detailed ever conducted as far as we are aware. A fixed three-bladed prop, locked in gear, exerts as much drag as hanging a bucket over the stern.

So what is the solution if you don’t want to sacrifice precious speed? The answer is, fit one of the many folding or feathering props on the market. But which one, and what are the drawbacks?

We measured the force generated by each propeller across the engine’s rev range

We tested all the props we could get our hands on (except the Volvo, Radice and J-Prop, which turned up too late for the test), measuring top speed under power, thrust (bollard pull) ahead and astern, stopping distance, and – for the first time anywhere – we measured the side-force (propwalk) generated when you put the engine into reverse.

We also measured propwalk while running astern. It’s a lot stronger than you think

It is this vital, unwanted component that sends you swinging off to one side when you try to stop suddenly, or back-up in the marina. We then towed three typical props behind a test boat to measure their drag, and how it compares to the overall drag of a yacht under sail.

Prop drag has been an issue for sailing vessels since the propeller was invented. Early trials were carried out on English warships, using jointed shafts that could be lifted into a trunk in the hull when sailing. In the 1890s, Danish pilot boats used similar lifting shafts.

For many years, long keels reduced the drag of two-bladed props on yachts, but the advent of the fin keel and exposed shafts brought the problem back again, with the added factor of more powerful engines making three-bladed props a necessity. The need to manoeuvre precisely in tightly packed marinas and overcrowded harbours has convinced even the owners of many long-keeled yachts to fit three-bladed props.

1970s: the first folding props

Most cruising skippers simply ignored the unwanted drag and loss of speed, but in racing circles it became increasingly significant. This led to the development in the 1960s and 70s of propellers whose blades folded backwards when sailing, reducing the drag markedly. The blades flew out under centrifugal force when the engine was put into ahead or astern. The earliest folding props had blades that moved independently, but this could lead to the lower blade dropping down when sailing, so the roots of the blades were linked with geared teeth, ensuring that they opened and closed together.

At the same time, an alternative approach was developed: the feathering propeller. Here, the blades were set at right-angles to the boss, as in a conventional prop, but on swivelling hubs. When motoring, the blades swing to the ahead or astern angled position, but under sail they ‘feather’, turning parallel to the boss.

One great advantage of feathering props is that they’re suitable for the many fin-and-skeg and long-keel yachts that have a small prop aperture in the rudder, where a folding prop won’t fit.

In the earliest feathering props, the blades remain in the same orientation to the boss in ahead or astern, as with a conventional fixed blade prop. In astern the aerofoil is working backwards, giving less thrust than in ahead. Some new feathering props have blades that swing right round, so the same leading edge is presented to the water in ahead or astern.

Cut prop drag by 90-95%

Under sail, the drag of folding and feathering props is tiny compared to a standard fixed-blade unit. Feathering props create about 5-10% of a fixed prop’s drag, sometimes less, while folding props have almost zero drag. This gives a significant gain in sailing speed, between half a knot and one knot, with the greatest percentage saving at low speeds.

Inevitably, there are drawbacks. The first is expense: a folding or feathering prop costs between two and six times more than a fixed equivalent. The second issue is complication – the gearing and folding mechanisms are prone to wear and corrosion in the salty and sandy environment, leading to reduced performance, and even loss of blades in extreme circumstances.

The third problem was performance under power. The early folding and feathering props produced less thrust than an equivalent fixed prop, particularly in astern, with dire consequences if the blades fail to deploy when you need to make a crash stop.

Now, the manufacturers claim to have addressed all the issues except cost. They claim the latest generation of their products give as good a performance or better under power, and major gains in speed under sail. We put their claims to the test on two chilly days in February. To our folding and feathering props we added a fixed three-blade prop as a benchmark, and the Axiom prop, a radical new development in fixed propellers, just to see how it compared.

Pitch, diameter, number of blades and handedness all determine a propeller’s performance

To help you understand our test, we will take a quick look at propeller theory and numbers. The four main figures you will see used to describe any prop are diameter, pitch, number of blades, and rotation.

Diameter is twice the distance from the centre of the boss to the tip of the blade. As a rule of thumb, the more powerful your engine, the bigger the diameter you will need.

Pitch is the measure of how far the propeller will advance in one revolution and thus how fast it will push your boat through the water for a given engine rpm (revolutions per minute). To understand pitch, imagine screwing a screw into a block of wood. The angle of the spiral thread governs how far it goes in for each turn. Similarly, the blades of a prop are set at an angle to the boss. The greater the angle, the greater the pitch. However, this is only the theoretical pitch. In practice, as water is not solid, the prop will slip to a degree and not advance so far. The amount of slip is around 30% for the props and speeds we are looking at.

Diameter and pitch are still measured in inches throughout the world – a quirk of history that would delight Henry VIII, and have Napoleon turning in his grave. But pitch can also be measured in degrees, especially relevant with feathering props whose blade angle can be varied.

The number of blades will vary between two, three or four, or even five in some high-speed craft. In practice, more blades will take greater power for a given diameter. For many years sailing boats used two-bladed props as they gave least drag in a boat with a full-length keel and an aperture for the prop, provided it could be locked in the vertical position. Today, most fixed props have three blades. Folding or feathering props have either two for cheapness, or three for higher power.

The hand of the propeller is the direction of rotation in ahead, when you are looking from astern. A right hand prop is one that turns clockwise in ahead.

Blade Area Ratio (BAR), sometimes called Disc Area Ratio, (DAR) is the area of the blades as a percentage of the area of a circle the same diameter as the prop. A prop with a greater BAR will take more power, but have more drag. Figures for sailing boat props are around 60%.

Incidentally, the explanation that a propeller advances just because its blades are angled forward is a convenient way to imagine what is happening, but not strictly correct. The blades of a standard prop are in fact aerofoil in section, like the wing of a plane, and move the boat forwards because as they spin round they develop lift. This lift is caused by a reduction in pressure on the back of the blade (‘back’ being confusingly the blade’s forward face). The faster they go, the greater the reduction in pressure. Once a certain point is reached, the reduction in pressure causes the water next to the blade to vapourise, and form bubbles. This is called cavitation, and limits the amount of power that a given area of blade can cope with. Also, as the bubbles collapse, they erode the metal of the prop, resulting in surface pitting on the back of the blade.

Our test boat was a Beneteau Oceanis 323 with a shaft-mounted fixed three-blade prop

We used a Bénéteau Oceanis 323, kindly leant to us by Sailtime in Lymington. She has a typical fin keel form, but atypically she has a built-in skeg carrying the shaft, rather than the P-bracket or saildrive of most other modern yachts. The skeg protects the shaft and prop from underwater damage, but a downside is increased vibration as the blades of the prop pass through the disturbed waterflow behind the skeg. This is resolved in normal use by fitting a three-bladed prop as standard rather than two, but otherwise did not alter the propulsive element of our test for either two- or three-bladed units.

The engine was a Yanmar YM20, giving 21hp at a maximum 3600rpm. The gearbox ratio is 2.6:1 in ahead, but somewhat confusingly a higher ratio of 3:1 in reverse. This is a very common engine/gearbox combination, so not an unreasonable test. Yanmar says it gives better thrust in astern, but in practice it meant the folding props were forced to use a compromise pitch. Some of the feathering props were able to set a different astern pitch.

We measured the force generated by the engine when running astern

We measured thrust, or ‘bollard pull’, in ahead and astern, throughout the rev range, using a load-cell borrowed from Diverse Yachts, with a remote read-out.

We then measured the side-thrust at full power in astern. This enabled us to predict the prop-walk created when you go into astern. To put this figure into context, the thrust produced by the worst prop in our test is the same as a 3hp outboard mounted on the transom, driving at right-angles at full throttle. No wonder so many yachts veer off to one side!

On the water, we measured speed through the rev range to maximum. We then carried out a crash stop from 6 knots. We recorded the time it took to bring the boat to rest at full throttle from the moment we engaged reverse gear.

To put these times into context, the distance the boat would travel before stopping would be 12m (39ft) with the best prop tested, but 17.4m (57ft) with the worst.

To measure the drag of all 18 propellers precisely enough to compare them with one another, making allowances for different yacht hull forms, we would have had to build a sophisticated testing rig, hire a team of scientists and spend several days in a research laboratory with a very large towing tank. Our objective was just to demonstrate the difference in drag caused by different types of propeller.

We tested the drag of three types of propeller using this test rig

We fitted a fixed prop, then a folding one, then a feathering one, to an outboard motor leg mounted on the transom of a lightweight 14ft skiff. We then towed the skiff at speeds up to 7 knots, and measured the difference in drag. We don’t claim this gave us the last degree of accuracy, but it was sufficient to compare with published drag figures. We then compared this drag to the hull-only drag of the Océanis 323 – a typical 10m cruising yacht.

A question of some gravity, Mr Newton

To keep things simple, we have presented our drag and thrust figures in kilograms (kg). Strictly speaking, thrust is a force and should be measured in Newtons – 1kg is multiplied by the acceleration due to gravity to give a figure of 9.81N. On another planet, with different gravity, our units would be wrong, but until we hear they are holding regattas on the canals of Mars, we will rest our case.

The results

Featherstream three-blade

Made in England at the time of writing (2009), this is a three-blade feathering unit with a bronze boss, and stainless steel blades. The pitch of the blades can be adjusted externally, and can be different for ahead and astern. The blades swivel 180º to present the same leading edge in ahead and astern. As a result, it performed well in astern: second best for bollard pull and third for stopping distance. However, occasionally it would not open in astern, and you had to learn to snap the throttle hard to get it to deploy.

Max Prop three-blade

Designed by Massimiliano Bianchi in 1976, the Max Prop was one of the first of the new generation of props. Boss and blades are bronze, and the pitch can be set as you assemble it, or by the factory if you want a different astern pitch. The blades swivel 180º to present the same leading edge in ahead and astern. It gave the best astern thrust, and was in the middle of the pack for stopping distance. It never missed a beat going into astern.

Max Prop two-blade

A two-blade version of the Max prop. As expected, it gave some vibration on our test boat, but this would not occur on a yacht with a P-bracket or saildrive. Mid-range performance for astern thrust and stopping distance.

Kiwi Prop three-blade

Designed in New Zealand in 2000, this is only available as a three-blade unit. The boss is stainless steel, and the blades are glass-reinforced Zytel plastic. Each blade has two different aerofoil sections, as you move out from the boss. It has no internal gears, so each blade feathers independently according to the waterflow over it. The blades do not swing completely round in reverse, so the trailing edge becomes the leading edge. It was one of the simplest to fit – just slide it on and tighten the nut. Mid-range for astern thrust and stopping distance, but the lowest top speed. It also went easily into astern.

Brunton’s Autoprop

Brunton’s Autoprop marked a completely different approach to feathering props when it was brought out in 1987. The three blades are linked together, going from fully feathered while sailing, or in neutral, to fully twisted under power. The difference is that when you go into gear, the amount they rotate depends on engine speed and loading. Thus the ahead or astern pitch varies according to engine rpm, which, Brunton claims, improves performance and fuel economy, with reduced rpm for a given cruising speed. We can confirm the latter: our tests show the Autoprop achieved 6 knots at 2,100rpm, compared to 2,500rpm for our standard prop and most of the others on test. However, previous drag tests have shown that this is achieved at the expense of slightly more drag than other feathering props, though still 80% less than a fixed prop. Bollard pulls were on the low side, but it still achieved near the maximum top speed, with mid-range stopping distance.

Autostream three-blade

From Australia comes the Autostream, a three-bladed feathering unit that has been in production for 20 years. Construction is all-stainless steel, with the blades swivelling 180º to give the same leading edge in astern as ahead. Separate ahead and astern pitch can be adjusted by the owner without dismantling the prop. It has been designed to stay feathered at speeds up to 25 knots, helped by extra blade area aft, making it suitable for fast multihulls. On test, it gave the fastest stopping time of all the folding or feathering units, plus significantly lower side-thrust than the rest, while still maintaining good ahead speed.

Variprofile three-blade

This is a three-blade feathering prop, made in Germany. The blades swivel 180º to give the same leading edge in astern, and the pitch can be set differently for ahead and astern. Usually, this is set in advance for the customer, but can be altered on site. Blades and boss are bronze, with stainless pins, and an anode aft of the nut. Ahead thrust was low, but stopping distance was good.

Flexofold three-blade

Made in Denmark at the time of writing (2009), the Flexofold is an all-bronze folding unit, with stainless steel pins, and the anode cleverly enclosed by the blades. On test, it gave the equal highest forward thrust of all props, and highest top speed. It also had one of the lowest side-thrusts. Astern performance was the best of the folding props. At a cruising speed of 6 knots, the engine was turning at 2,300rpm, compared to the 2,500rpm of our standard fixed prop and most of the others on test.

Flexofold two-blade

A two-blade version of the Flexofold, its performance was only slightly down on the three-blade, and in fact better for astern thrust. Again, it had some vibration due to the skeg. A racing two-blade version is also available.

Gori three-blade

Another of the earliest folding designs, the Gori has been made in Denmark since 1975, in two and three-blade versions, but the company only recommended the three-blade for this boat. They also make a racing two-blade version, with reduced drag. The blades have a three-way gearing system for opening and closing. Boss and blades are bronze, with stainless steel pins. A speciality is its overdrive feature, which sets the blades in a coarser pitch if you open the throttle gradually while the boat has forward way on, which allows you to cruise or motorsail at lower rpm for more comfort. Performance ahead and astern were towards the bottom of our list, with the longest stopping distance, but the prop-walk was third best.

Slipstream three-blade

From the same Australian company as the Autostream comes the Slipstream folding prop. Again the boss and blades are stainless steel, which gives greater strength, allows thinner section blades, and removes the need for an anode. The bearings are bronze bushes, while the blade gears are bevelled, with two rows each, claimed to better chew up any barnacles that might chose to grow on them. Polyethylene side thrust washers further improve the opening action. On test it performed in the middle of the folding props, though with comfortable low cruising revs of 2,250rpm at 6 knots.

Slipstream two-blade

Similar in design to the three-blade version, though with slightly worse astern figures, but it still made 6 knots at a low cruising rpm of 2,350.

Varifold two-blade

Made in the UK, this is mid-way between a racing and cruising prop, with the blades closing tightly for low drag, but still having enough shape to give good motoring performance. As a result of its close fit, you have to give the engine a burst of throttle to get the blades to open initially. It has a bronze boss and blades, and stainless steel pins. It gave equal highest top speed, but was near the bottom of astern performance.

Axiom three-blade

Axiom fixed three-blade

The Axiom is the joker in our pack. It is not a folding propeller, but it does have a revolutionary blade profile and section, if you will pardon the pun, and has never been tested on a yacht before, so we just had to put it into our trials to see how it compared. As the photograph shows, the blade profile is rectangular, while the blade section is almost S-shaped, and symmetrical in ahead and astern, with no twist. Its designers claim it gives greater thrust and stopping power, together with lower wash. So how did it stand up? Well the charts show the story, with its stopping time nearly a second better than any other model, and its side thrust again the lowest by far. However this was at the expense of lower top speed, which suggests some more tweaking is needed, but it is still one to watch.

Drag curves

Drag v speed

In the graph above, you can see that at 5 knots, a fixed three-blade prop with its shaft locked creates almost half as much drag as the entire hull. The drag can be halved by allowing the prop to spin, but the gearbox may suffer. By contrast, the drag of a feathering prop is negligible, and the drag of a folding prop is too small to plot on a graph of this scale.

The hull resistance curve for the Océanis 323 was calculated for YM by the Wolfson Unit, at the University of Southampton, using data from the Delft University Systematic Series. The propeller drag curves are based on data from SSPA Maritime Consulting, using Volvo S-drives. This data were verified by YM’s on-the-water drag test.

All but three of the propellers on test produced less prop walk than the standard fixed prop. The fixed Axiom and feathering Autostream were the best performers, but nearly all the folding props fared better than the rest of the feathering propellers.

Maximum speed

There was more than half a knot of difference between the best-performing props and the worst. Four folding propellers and one feathering prop managed to prove the claim of better performance than a standard fixed prop, but nine of them fell slightly short.

Interestingly, some of the best performers were two-bladed props, which are widely assumed to perform worse than three-blade versions.

Bollard pull ahead

The fastest props are generally also the most powerful in ahead, and most of the slower ones are among the least powerful. However, only one unit – the three-bladed Flexofold – generated a greater bollard pull than the standard fixed prop. The most powerful props produce almost a third more thrust than some of their rivals.

Bollard pull astern

Three propellers produced a more powerful bollard pull than the standard fixed prop in astern: two folding units and the newly designed Axiom. Nearly all the feathering props performed better in astern than the folding ones – some by a very wide margin. There’s a huge difference between the best and worst-performing props – the three-blade Max Prop has almost twice the bollard pull of the two-blade Varifold.

Stopping time

The new-concept Axiom prop excelled in this test, but nearly all the feathering props were better at bringing the boat to a standstill than the standard prop. The folding props were less effective and some of them were less than 100% reliable when called upon to perform an emergency stop. The difference between the best and worst stoppers was about 3½ seconds. It may not sound like much, but in a crunch, it could make all the difference.

If you want to add up to a knot to your boat speed, then fitting a folding or feathering propeller is a must, not just for racing boats. And as our test demonstrates, you can usually still retain the handling and performance under power that you had with a standard fixed-blade prop. In fact, often you’ll get better performance.

Five of the tested props gave more speed than the standard prop, with four of them being folding models, and both Flexofolds coming out top. And even though it was only 0.15 knots better, when you look at the hull resistance curve this is a considerable improvement. On the other hand, with astern performance, in general it was the feathering props that came out best, with better bollard pull than the standard, and better stopping times, and the Autostream coming out top. The folding props were generally not as good as the standard, though in the main only by no more than 10%.

But it was the prop-walk figures that proved the most interesting. Ten of our test units gave less prop-walk, with generally the folders coming out best, but the top units being the Axiom and Autostream, with 30% less side-thrust than the standard fixed three-blade, a considerable advantage when you have to stop suddenly or back-up in a marina.

In terms of resistance, our test shows that letting your fixed-blade prop spin, if the gearbox manufacturer will allow it, halves the drag. But to get the real improvement, fitting a folding prop will give at least 95% less drag than a locked, fixed propeller, while a feathering unit will give at least 92% of the drag – still an enormous saving.

But these benefits do have a price, literally on your wallet. A 16in fixed three-blade propeller will cost around £300 (2009 prices in UK). But the cheapest two-blade folding prop will be at least double this price, with most of them between £600-900. For a three-bladed folding unit expect to pay between £1,200 and £1,600. Feathering props are even more expensive, starting at around £1,200 for a two-blade model, rising to £2,100 for the most expensive three-blade version.

Because of the wide variation in cost, performance and specifications of the all the units tested, we have not felt it appropriate to recommend a best buy. The tables and curves give you the information you need at a glance, allowing you to make your own decisions as to which is best for you and your boat.

Some are easy to fit, others difficult. Professional installation is advised for such an important – and expensive – bit of kit

Some of the props on test are very simple to install, others are very complicated. However, while they all have instructions for DIY fitting, unless you’re very confident in your own skills, a piece of equipment as vital as a propeller ought to be professionally installed, both for safety and peace of mind. For our test, we had every prop fitted by the manufacturer’s representative, so there was no question about the installation, and they also observed all our measurements.

Bronze has been the material of choice for propellers almost since they were invented. Strong, resistant to salt-water corrosion, it is also easy to cast, with a low melting point, as our ancestors found out 4,000 years ago. Stainless steel has been making an appearance recently. Even stronger, it allows thinner and so more efficient blades. It is even more corrosion resistant, and also harder, so less vulnerable to impact damage. However, it has a much higher melting point, so it is more difficult and expensive to cast and machine.

Maintenance

A folding or feathering prop will need more maintenance than a fixed prop

Whatever prop you have, it should be checked every time the boat is lifted, for wear, corrosion and movement. Folding and feathering props do require more maintenance than fixed ones. Some bosses are packed with grease, which should be repacked annually. Some have nylon shims or bearings, which should be checked, especially in silted waters. Most have an anode, which should be checked and replaced if necessary.

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Flexofold Classic Sail Drive Propeller 17" X 2 Blade

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17" Flexofold 2 Blade folding Propeller  fits Volvo Penta, Yanmar,  Lombardini and Technodrive   Saildrives.

Propeller Sizing Form:  Click here to submit information

FLEXOFOLD PROPELLERS HOME

PRICES FOB DENMARK - AIR FREIGHT SURCHARGE WILL APPLY Please note: Returns / Exchanges on Flexofold Propellers are only valid when vessel data is reviewed by Getaprop and Flexofold prior to purchase. Expect delivery within 7-10 Day's

The Flexofold Folding Sailboat Propeller is the most efficient, lowest drag propeller you can buy. This claim is based on independent tests conducted over the past several years and reinforced today by sailboat builders and owners around the world.

Flexofold folding propellers deliver: • Maximum performance under power* • Superior performance in reverse* • Lowest drag under sail* • 2 and 3-blade design • Unmatched construction • Virtually no maintenance • No fouling of lines under sail • Approved use with sail drives as well as shafts • Easy pitch changes • Patented, twin helical gear design for reliable operation • Patented, locking screws • Fair pricing

The Flexofold Propeller does work with a hydraulic transmission. However, to get the prop to fold, you have to follow a simple procedure. The objective is to stop the rotation of the prop which is easily done with a mechanical transmission by putting the gear in reverse. With the mechanical transmission, the clutch stays engaged, and the prop can’t turn. With the hydraulic transmission, the clutches stay engaged only as long as there is hydraulic pressure being produced by the turning engine. When the engine is turned off, the clutches disengage and the prop is free to spin no matter where the transmission is. With the blades open and the boat moving forward, centrifugal force will keep the blades open unless you can stop the rotation for a moment while moving forward, forcing the blades to fold. Once folded, the blades will not reopen without rotating the engine again. To cause the Flexofold to fold with a hydraulic transmission, follow the steps below:

1. With the boat moving forward and the motor running in forward (the sails can be up, too) idle down and shift to reverse.

2. Immediately after putting the engine in reverse, shut the engine down. By putting the engine in reverse, you stop the rotation in the forward direction and the blades will fold because you are still going forward. After the engine is shut down, the blades remain folded until you restart the engine and put it in gear.

Sail drive Hub length: 2 and 3 blades

Hub for 2-blade propeller = 135 mm/5.314”

Hub for 3-blade propeller = 135 mm/5.314”

Propeller Attaching Hardware for Classic 2 Blade Propellers

1)  Tapered pivot pin locking screw (M8x25mm)

2)   Tab washer and locking screw (M8x20mm)

3)   Shaft nut 16mm

3)   Shaft nut 20mm

4)   Pivot pin 14mm

5)   Zinc Anode

6)   M6x10 Anode Screw

7)   Shock Absorber

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Gori Propeller

A Gori folding propeller is a specialized marine propulsion device used in sailboats and yachts. It's designed to reduce drag while sailing, optimizing performance. Gori propellers can be installed on various boat types and require regular maintenance to ensure their efficiency and longevity. The Gori propeller's patented single-drive and geared blade design not only provides excellent propulsion but also minimizes vibration both ahead and astern, offering a smoother sailing experience. These propellers are thoroughly tested and have a stellar reputation for quality and innovation. Renowned for their low water resistance and ability to provide one full knot of improvement in speed under sail, Gori propellers feature unique overdrive technologies that optimize thrust in reverse, greatly appreciated by sailors worldwide. This range includes the popular 4-blade models and custom-made options for specific commercial projects, making Gori a go-to choice for enhanced sailing performance.

How does the Gori Sailboat Propeller work?

A Gori Propeller is a specialized propeller designed for sailboats and yachts. It reduces drag during sailing by folding its blades inwards, improving sailing performance. When the engine is engaged, the blades can open up to provide effective propulsion and maneuverability. Gori Propellers are made of durable materials, often with adjustable pitch settings for optimal performance. The Gori propeller's ingenious design allows it to function effectively on sailing yachts fitted with engines up to approximately 60 hp. Its feathering and folding capabilities reduce total drag while enhancing speed and maneuverability under sail. The propeller's geared blade design ensures that it opens and closes seamlessly through centrifugal force, similar to many other folding propeller types on the market. This system provides less vibration ahead and astern, optimizing comfort and performance. Gori propellers are also known for their ability to give optimum thrust in reverse, a feature particularly beneficial in tight docking situations, contributing to the brand's reputation for high quality and reliability in the boating industry.

Are there specific boat types or sizes that are better suited for a Gori Propeller?

Gori Propellers, including the folding propeller models, are versatile and suitable for a wide range of boat types and sizes. Their design and performance make them an excellent choice for both sailboats and motorboats. Gori propellers are known for their efficiency, making them a preferred option for boat owners looking to enhance their vessel's propulsion and fuel efficiency, regardless of the boat's size or type. The Gori propeller, known for its unique blade shape and profile, provides exceptional performance for both cruising and high-speed operation. Its patented single-drive technology enables the blades to open and close using centrifugal force, a mechanism that ensures smooth operation and efficient power delivery. This design reduces water resistance significantly, allowing the propeller to give the optimum thrust even in reverse, which enhances maneuverability and control. Furthermore, Gori's commitment to innovation is evident in their full range of folding propellers and custom-made options for specialized projects. These propellers have been thoroughly tested and have gone on to build a reputation without parallel in the industry, favored by sailors and commercial operators worldwide for their ability to deliver power and efficiency with less vibration.

Why is regular maintenance essential for the optimal functioning of a Gori Folding Propeller?

What types of gori propeller items do you have in your catalogue.

• Gori 2 Blade Folding Propeller
• Gori 3 Blade Folding Propeller
• Gori Propeller Parts
• Gori Racing Propeller
• Gori Propeller Zinc Anodes

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Racing sailboat propeller MARK III ELIPTEC folding propeller shaft 2-blade

Characteristics

Description.

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Other Martec products

• Sacrificial anode
• Boat sacrificial anode
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• Propeller shaft sacrificial anode

GORI 3-Blade Propeller

The gori 3-blade folding propeller is available for installation on vessels with engines from around 10bhp to 300+bhp..

This unique design offers an “overdrive feature”, with 2 controlled pitch settings in forward and full reverse thrust. The “overdrive feature” gives lower engine RPM for the same cruising speed when in calm waters or motor-sailing.

The 3-blade propeller is available in diameters from 15” ~ 30+” , and pitch settings between 60% and 80% of the diameter. They are available in both LH and RH configurations for both saildrive and shaft installations. The GORI 3-blade propeller won the “DAME” design award at the Meets on November 1994 and the “HISWA” award in December 1994. In November 1994 the German Magazine “Die Yacht” published results of independent tests carried out by the Technical University of Berlin. These results showed conclusively the advantages of the GORI 3-blade propeller.

Ahead: GORI was 11+% more efficient than 3-blade feathering props tested.

Astern: GORI was 6.5+% more efficient than 3-blade feathering props tested.

Sailing: Drag was reduced by nearly 50% compared to the 3-blade feathering propeller under test.

The German sailing magazine “Segeln” tested propeller drag at the German Naval Architecture Test Center in Potsdam and results show the GORI 3 Blade propeller had less drag than all the other makes of 2-blades & 3-blades (except the GORI Race) with 1.4 Newton.

3-Blade Standard Shaft

3-Blade Saildrive

GORI 3-Blade Measurement Table

Propeller and Shaft Clearance Dimensions

A. Shaft Diameter B. Aft end of cutlass to start of taper/forward edge of hub E. Shaft centerline to hull at forward edge of hub F. Shaft centerline to hull at end of shaft thread H. End of shaft to leading edge of rudder I. Start of taper/forward edge of hub to rudder

Download Propeller and Shaft Clearance Form

Saildrive Installation

Please read these instructions carefully before installing your new GORI propeller

Warning ~ do not remove the blades from the blade housing when installing or removing the propeller from the shaft!!!! To remove blades – the center Fixing Bolt “MUST” be removed first!!!!

Installing the propeller.

The propeller is delivered assembled. This ensures that at the factory the propeller has been checked, and balanced prior to shipping.

Take apart the retaining cap (1). To do this … First remove the 3 x allen head bolts (item #5) holding the retaining cap (1) in place.

Now undo the retaining cap using the C-Spanner (16). The thread on the blade housing assembly (6), is a standard right to tight and left to loose. You may use a hammer with care, to tap the C-Spanner to get the retaining cap started as it is a firm, tight fit.

You will now have the retaining cap and blade housing assembly (6), (with blades installed) separate.

Check the shaft nut (3) & the nut-locking bolt (4.1) with the output shaft threads.

Remove the inner hub (2), flexible bushing (2.2), and PEEK bushings (2.1-2.3) from the blade housing (6).

Slide the retaining cap (1), PEEK bushing (2.1), inner hub (2), flexible bushing (2.2), and PEEK bushing (2.3) on to the saildrive shaft, matching the splines on the output shaft with those in the inner hub (2). Smear splines & shaft with waterproof lubricant.

Fit the nut (3), and tighten. Typically 72ft/lb. of torque for 1.00” dia. shaft. It is very important to always use the Gori supplied propeller nut for the installation. An incorrect nut can lead to loss of the propeller, part of the propeller or cause an electrical connection between the propeller and the saildrive.

Smear locking glue (Loctite 243) on the thread of the nut-locking bolt (4.1). Place the washer (4) into the shaft hut and then thread the nut-locking bolt (4.1) using a 5mm allen key (19). Typically tighten to 10 ft/lbs of torque.

Slide the complete blade housing assembly (6 thru 17.1), onto the flexible bushing (2.2), so that the 2 x security cams slide into the grooves of the flexible bushing (2.2). Wet the bushing to allow it to slide in easily.

Continue to slide the blade housing onto the assembly until the thread of the retaining cap (1), and the blade housing (6) are touching. Smear a threads with waterproof lubricant.

Screw the retaining cap (1) RH or clockwise onto the blade housing assembly (6). Using the C-spanner (16) tighten until the 3 x holes in the retaining cap are aligned with the 3 x half circle cutouts in the blade housing assembly.

Now using the Loctite-243 (21), re-install the 3 allen head bolts (5) using the 5mm allen key (19), into the holes and tighten firmly. Check that the blades will move freely from fwd to reverse and that the tip clearance with the hull is 10% of the blade diameter.

Removing the propeller

Warning ~ DO NOT remove the blades from the blade housing when removing the propeller from the saildrive leg.

First remove the 3 x allen head bolts (5), from the retaining cap

Now using the C-Spanner (16), undo the retaining cap so that it separates from the blade housing assembly (6). This is a right hand thread… simple right is tight…left is loose.

Carefully remove the blade housing assembly, by pulling off from the flexible bushing (2.2) and inner hub (2). These two items along with the bushings (2.1-2.3) will remain on the output shaft

Unscrew the nut-locking bolt (4.1) and also remove the washer (4) using a 5mm allen key (19).

Unscrew the shaft nut (3). It will be necessary to lock the output shaft when undoing the nut

Pull the inner hub (2), flexible bushing (2.20, end bushings (2.1 & 2.3), and retaining cap (1) off the output shaft.

Replacing the Aft Zinc

This should be done if more than 50% of the zinc (7), has been eroded away.

Undo the allen head bolt (8) and remove the old zinc (7) … if not gone completely.

Clean the propeller so as to ensure a clean strong bond with the new zinc.

Replace the zinc with new zinc, align the zinc with the end of the hub & the alignment hole with the pin in the end of the hub.

Use the new allen head bolt supplied … smear with loctite 243 also supplied, before re-installing the new bolt. Check that the blades will swing freely from fwd thru to reverse. If not and there are tolerance issues it may be necessary to realign or lightly file the sides of the zinc.

Replacing the Fwd Collar Zinc

This should be done if more than 50% of the zinc (17), has been eroded away.

First remove the 4 x allen-head bolts (17.1) and remove what is left of the zinc

Clean the surface of the propeller hub to ensure a good clean contact between the hub and the new zinc

Install the new zinc (17) using the Loctite 243 (supplied) on each of the 4 x allen-head bolts.

Replacing Flexible Stops

These can be replaced without removing the blades. Using a flat screw driver … pry out the old or worn Flexi-stops (15). Push or tap the new flexi-stops in place. It may be necessary to swivel the blades open and closed to obtain the best angle for re-installing the new ones. Remember they are flexible.

Removal of blades from the Blade Housing

To remove blades – the center Fixing Bolt “MUST” be removed first!!!! This must only be done when cleaning and a full service of the propeller is required. NOTE: Blades are not removed for the installation and removal of the propeller

Remove the zinc centre bolt (8) and the zinc anode (7), using a 5mm allen key (19). On propellers 18.0” and larger it will also be necessary to remove the threaded pin (9) using a 6mm allen key (20) … then remove the fixing bolt (10) using a 5mm allen key (19).

Failure to remove all these pins(8-9-10) first, will result in damage to the internal threads of the blade pins (11).

Disassemble the blade pins (11 & 11.1) from the blade housing using a 6mm allen key (20). Note that blades, pins and the housing are matched and numbered. They should only be reassembled in the correct location … that is #1 – # 2 – # 3.

Remove the blades. Remove the gear-wheel (13) and the spacer (14). Remove the flexible stops (15) using either a flat blade screw driver or pliers.

When re-mounting the blades and gears to the housing it is important to apply loctite-243 to … blade pins (11 +11.1), lock pin (10), threaded pin (9), fixing bolt (8).

The lock bolt (10) and the fixing bolt (8) are the very last items to be reassembled.

Be sure that all components fit back together and that the blade pins (11) are located as before removal, as they are indexed inside the hub to lock bolts (10 +8).

Note: If more than 50% of either of the zincs (7-17) has been eroded away they should be replaced. If the gear wheel (13) has been damaged or worn…it should be replaced If the flexible stops (15) have been damaged or worn…they must be replaced.

– Do not start the engine while the boat is out of the water – The prop may have sharp edges… be careful not to cut yourself – Make sure the blades do not open or close suddenly and trap your fingers – Stop the engine before diving or swimming in the vicinity of the boat – Propeller blades can cause considerable damage when rotating … be careful. – Do not remove fish nets, rope or similar from the prop with the engine running. – Check that the prop works in both fwd and reverse before each trip. If any strange sounds or vibrations are noticed coming from the propeller stop the engine and investigate the reasons/solve the problem.

Download GORI 3 Blade Saildrive Installation Maintenance Instructions

The propeller is delivered assembled. This ensures that at the factory the propeller has been checked, and balanced before shipping.

If your propeller has a fwd collar zinc installed, remove this first (22 + 23) Take apart the jacket (1) from the blade housing (6).

First remove the 3 x allen head bolts (5) holding the jacket (1) in place. If supplied with a fwd collar zinc remove this zinc first.

Now undo the jacket using the C-Spanner (16). The thread on the blade housing assembly is a standard right to tight and left to loose. You may use a hammer with care, to get the jacket started as it is a firm fit.

You will now have the jacket and blade housing (with blades installed) and the hub cone (2) separate.

Check the fit of the hub cone (2) onto the shaft. Adjust the key if necessary.

Check the shaft nut (3) with the shaft threads.

Slide the jacket (1) onto the shaft first. Now slide the hub cone (2) onto the shaft and tighten the shaft nut (3) very tightly. Typically up to 72ft/lbs of torque

Cover the allen screws (4) with Loctite-242 and tighten these dog point allen screws into the hub cone (2), locking the shaft nut(3), firmly in place. To do this you will need to rotate the jacket to line up an access hole with the allen screws (4).

Slide the blade housing assembly onto the hub cone (4), making sure that the tabs engage the hub cone. Push it fwd until you can start the thread of the jacket (1).

Tighten the jacket (1) to the blade housing assembly. Use the C-Spanner (16) to tighten and align the 3 threaded holes (5) with the holes in the jacket.

Now using Loctite-242, re-install the 3 allen head bolts (5) into the holes and tighten firmly.

Check that the blades will move freely from fwd to reverse.

Warning ~ do not remove the blades from the blade housing when removing the propeller from the shaft

First remove the Fwd split collar zinc, then the 3 x allen cap head bolts (5), from the jacket.

Now using the C-Spanner, place it on the stb side, & hammer down, unscrewing the jacket so that it separates from the blade housing and push it fwd on the shaft.

Carefully remove the blade housing assembly by pulling straight aft.

Undo the allen screws (4) to allow removal of the shaft nut (3). It is not necessary to remove them completely from the Hub cone (2).

Unscrew the shaft nut (3)

Install the bronze puller (17) into the hub cone (2). Thread it in all the way.

Tighten the large bolt in the centre of the puller (17) and this will draw the hub cone off of the shaft.

This should be done if more then 50% of the zinc (7), has been eroded away.

Undo the allen head bolt (8) and remove the old zinc (7) … if not gone completely. Clean the contact area.

Replace the zinc with a new one … index the fwd end of zinc with the end of the hub pin to the zinc hole.

Use the new allen head bolt supplied … smear with loctite before re-installing the bolt.

First remove the 4 x allen-head bolts and remove what is left of the zinc

Install the new zinc using the Loctite (supplied) on each of the 4 x allen-head bolts

These can be replaced without removing the blades.

Using a flat screw driver … pry out the old or worn Flexi-stops (15)

Push the new flexi-stops in place. It may be necessary to swivel the blades open and closed to obtain the best angle for re-installing the new ones. Use the blades to “push” home the stop into its location hole. Remember they are flexible.

Removal of blades from the Blade Housing !!!

This must only be done when cleaning and full service of the propeller is required.

Blades are not removed for the installation and removal of the propeller

Remove the centre bolt (8) and the zinc anode (7), using a 5mm allen key (19).

On propellers 18.0” and larger it will also be necessary to: remove the threaded pin (9) using a 6mm allen key … then remove the lock bolt (10) using a 5mm allen key.

Failure to remove these pins first will result in damage to the internal threads of the blade pins (11).

Disassemble the blade pins (11 & 11.1) from the blade housing using a 6mm allen key. Note that blades, pins and the housing are matched and numbered. They should only be reassembled in the correct location … that is #1 – # 2 – # 3.

Remove the blades.

Remove the gear-wheel (13) and the spacer (14).

Remove the flexible stops (15) using either a flat blade screw driver or pliers.

When re-mounting the blades and gears to the housing it is important to apply loctite-242 to … blade pins (11 +11.1), lock pin (10), threaded pin (9), fixing bolt (8).

Be sure that all components fit back together and that the blade pins (11) are located as before removal as they are indexed inside the hub to lock bolts (10 +8).

– Do not start the engine while the boat is out of the water – The prop may have sharp edges… be careful not to cut yourself – Make sure the blades do not open or close suddenly and trap your fingers – Stop the engine before diving or swimming in the vicinity of the boat – Propeller blades can cause considerable damage when rotating … be careful. – Do not remove fish nets, rope or similar from the prop with the engine running. – Check that the prop works in both fwd and reverse before each trip. – If any strange sounds or vibrations are noticed coming from the propeller stop the engine and investigate the reasons/solve the problem.

Download GORI 3 Blade Standard Shaft Installation Maintenance Instructions

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Model of Interest:  2-Blade 3-Blade 3-Blade Single Pitch 4-Blade Race Series All Models

BOAT INFORMATION

Boat Make and Model (required)

Waterline Length (required)

ENGINE INFORMATION

Number of Engines (required)

Engine Manufacturer (required)

Model Number (required)

Engine hp/kW (required)

Max Rated RPM (required)

Type of Gearbox (recommended)

hydraulic mechanical sail drive

Forward Gear Ratio (required)

Prop Installation Type:

1. P Bracket Strut 2. Shaft Log 3. Aperture 4. Saildrive

Existing Prop Size:

Shaft Diameter:     Prop Diameter:     Pitch:    

# of Blades:   Right Hand Left Hand

Additional Notes:

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Sailboat Propellers

Experience the difference....

Uncompromising, Impartial Solutions

King propulsion offers uncompromising and impartial solutions to all your marine propulsion needs.  our unique portfolio of sailboat propellers ensure that we only match the customer with the propeller they need.  folding and feathering propellers have unique characteristics that do not apply to every boat or sailor in them.  we strive to make this connection for you and give you the best technical advice possible. our team are all highly qualified and respected naval architects with countless years of experience in all types and sizes of propulsion systems from sailboats to warships., in addition king propulsion also supports and distributes sigmadrive, and exceptional drive train solution for coupling propeller shafts to engines quietly and efficiently., us distributor for autoprop feathering propellers, varifold folding propellers & sigmadrive, based in usa.

King Propulsion is a US owned and operated company based in Virginia Beach, VA supporting the USA, Canada and the Caribbean.

Tech Support

King Propulsion is staffed by Professional Naval Architects who can meet all of your propulsion needs. From accurately sizing a propeller to shafting and alignment issues, we have you covered.

King Propulsion is a technical consultancy with substantial experience in propeller analysis and propeller design.  King Propulsion represents Bruntons Propellers, a 150 year old propeller company based in the UK.

King Propulsion is an exclusive Bruntons dealer selling the full line of Autoprop propellers, Varifold propellers and SigmaDrives.  King Propulsion also provides full after sales support and propeller refurbishment.

Choosing a Sailboat Propeller

In the market for a new sailboat propeller ow about a folding sailboat propeller a feathering sailboat propeller what about autoprop, they are. supposed to be good or was it varifold, maxprop or gori choosing between them can be an impossible task and understanding the basis of each manufacturers claims and counter claims even harder. at king propulsion we are professional engineers first, technical sales second and we pride ourselves on the fact that we offer all flavors of propeller to suit every boat and every sailor. we don’t need to fiddle facts on propellers, we use science to help you choose what is right for you., if you look at the market you can distill the sailboat propeller types into 3 broad categories, the fixed pitch propeller, the folding propeller and the feathering propeller, each suits a type of sailing, a type of boat and a type of sailor. there is no one propeller to fit all. as any engineer will tell you, design is a compromise to achieve your objectives. so let us look at some of the compromises all the companies don’t want you to know about. the descriptions below are features that transcend the designs to give you some idea of the choices and options available., remember at king propulsion we sell all 3 types of sailboat propeller, so it is all about the right choice for you, propeller drag, the reason sailboat propellers exist in the first place is to reduce the appendage drag of the sailboat and sail faster. each family of designs achieves this in a different way  and compromises on very different parts of the puzzle to get there. for serious racers, this category is the only consideration.  for the regular sailor, it is a choice, low drag, backing performance, motor sailing as you can’t have everything in this category the folders and the feathering propellers fight for the title, autoprop is a bronze medal winner and fixed pitch propellers just add plain old drag., cruising speed at 2500 rpm, not only is the hull shape and the size of the main engine a factor in speed through the water, but propeller type play a big part of this as well.  fixed pitch propellers are the benchmark but autoprop with its self pitching mechanism gets more speed.  the folder and the feathering options are sub optimal here., backing and maneuvering ability, backing down can be one of the most fraught times for a sailboat owner. for some race boats it is not a consideration, for day sailing and backing into tight marina spots it is a requirement., fuel consumption, the wind is of course free, but catching the wind is expensive, especially if you have to motor long island sound or the icw to get to wind. all of the sailboat propellers impact fuel burn in different ways with autoprop the clear winner and the feathering propeller requiring more fuel for the same voyage. the values below are in us gallons per hour., initial cost of the propeller, sailboat propellers are an aspirational product as soon as you move away from the stock fixed pitch propeller. the cost increases as the complexity of the propeller goes up. the folding propellers are the least expensive of the sailboat propellers; the feathering propellers typically the most expensive and autoprop is slightly behind the feathering propellers. these prices are based on 2024 18″ diameter shaft drive propellers., so what type of sailer are you, what type of boat do you have, find out how a sailboat propeller can improve your sailing performance and heighten the sailing experience. simply fill out the form below with basic details about your boat and we will work with you to size and select a propeller that will be a perfect fit for you and the boat, please note, the above data is drawn from actual sea trials, magazine publications and academic research performed by king propulsion and bruntons propellers on their line of fixed, folding and feathering propellers..

Customer Feedback

Our products are in use around the world on a vast array of vessels. here are some of the comments we have had back for our propellers, couplings and service..

Jeanneau Sun Odyssey 43

Halberg Rassy 31

4-Blade Shaft Folding Propeller

A powerful hydrodynamic design that ensures efficiency

The 4-blade Flexofold folding propeller provides trouble-free handling and superior power in both forward and reverse. The powerful hydrodynamic design provides efficient use of the engine. Despite the relatively large propeller and 4 blades, the low drag makes your boat sail faster and allows the sails to stay up even in very light wind.

Original and correct propeller for Beneteau, Hanse Yachts, Sunreef Catamarans and more!​ 

Danish quality

All Flexofold propellers are designed and manufactured in Denmark in our single-purpose factory, equipped with modern CNC equipment and robot technology.

In 2017 Flexofold was acquired by Yanmar Marine International. However, Flexofold is still operating from the factory in Denmark, maintaining own distinct brand identity and remaining a Danish registered company.

The propellers are sold directly to boat builders as well as private boat owners worldwide. Due to our flexible production setup, we can manufacture and ship worldwide within a few days.

Special features

• Helical gearing between the blades ensures a syncronized folding mechanism.
• Protective cover over the gearing
• Easily replaceable anode
• Large shock absorbers for quiet opening of the propeller
• Mounting screws are pre-applied with Loctite® for mounting above as well as under water.
• Water lubricated - no need for maintenance

Propeller sizes

The 4 blade prop for shaft installation are available in the diameters of 15" to 27" - all in a wide range of pitches.

Supplied for both LEFT and RIGHT turning shafts.

Compatible with...

The Flexofold propellers fit perfectly on all common shaft diameters with ISO, SAE or IMP cones

ISO (European standard) = 1:10 cone

SAE (US standard) = 1:16 cone

IMP (British standard) = 1:12 cone

Installation guide, 4-Blade Folding Propeller, Shaft (pdf)

Propeller test - Yachting Monthly (pdf)

It's not too late!

Get a new folding propeller before the boat goes into the water. If you are fast, we will send your new propeller within 3-5 days.