catamaran hulls

Catamarans: A Complete Guide to Multihull Boats

Catamarans have been a part of sailing history for centuries and continue to be popular for their stability, spaciousness, and performance. Developed by various cultures around the world, the principles of catamaran design have evolved over time to become optimized for both pleasure cruising and racing. This complete guide will help you understand the essentials of catamarans, their unique characteristics, and how to choose the right one for your needs.

From the basic concepts of multihull design, performance, and handling, we will explore the advantages and benefits of a catamaran in terms of safety and comfort on board.

Along the way, we will discuss maintenance considerations, distinctive catamaran brands and models, and how a catamaran lifestyle can compare to more traditional sailing options .

Finally, we will provide learning resources and frequently asked questions tailored to both seasoned sailors and newcomers to the world of catamarans.

Key Takeaways

• Catamarans are known for their stability, spaciousness, and performance
• This guide covers aspects like design, handling, safety, and choosing the right catamaran
• Resources and frequently asked questions provide additional insights for potential catamaran owners

Understanding Catamarans

Design Characteristics

Catamarans are known for their unique design, which features two parallel hulls connected by a deck. This design provides several advantages over traditional monohull boats, such as stability and speed.

With their wide beam, catamarans have a reduced risk of capsizing and can access shallow waters due to their shallow drafts 1 .

One of the notable aspects of a catamaran is its twin hulls, which offer increased living space and comfort compared to a monohull. Additionally, catamarans are often favored by recreational and competitive sailors for their excellent maneuverability 2 .

The materials used for constructing catamarans range from wood to fiberglass, and even aluminum for high-performance vessels. Aluminum catamarans are known for their strength, lightweight structure, and resistance to corrosion 3 .

Hulls and Construction

The hulls in a catamaran are crucial to its stability and performance. These hulls help distribute the weight evenly across the water surface, minimizing drag and allowing for smoother sailing.

In general, the hulls can be categorized into two types:

• Symmetrical Hulls : The hull shape is similar on both sides, which enhances balance and stability in various sailing conditions.
• Asymmetrical Hulls : One side of the hull is designed differently than the other, which can be advantageous when sailing upwind.

The construction materials used in building catamaran hulls also play a vital role in the boat's performance and durability. Common materials include:

• Fiberglass : A popular choice due to its lightweight, strength, and ease of maintenance.
• Wood : Traditional material that offers a classic look, but requires more maintenance than fiberglass or aluminum.
• Aluminum : Lightweight and strong, aluminum is an excellent choice for high-performance catamarans 4 .

Multihulls vs Monohulls

There's often a debate between the benefits of multihull boats, such as catamarans or trimarans, and monohull boats. Here are some key differences between the two:

• Stability : Due to their wide beam and reduced heeling, catamarans offer improved stability compared to monohulls. This makes them an attractive option for those who want to avoid seasickness or feel more comfortable on the water 5 .
• Speed : Multihull boats are known for their speed, which results from their ability to minimize drag and maintain a level sail.
• Living Space : Catamarans and other multihulls generally have more living space, as both the hulls and the connecting deck can be utilized for accommodation and storage.
• Maneuverability : While monohulls are known for their agility and ability to point close to the wind, catamarans can still offer exceptional maneuverability when properly sailed 6 .

Performance and Handling

Speed and Efficiency

Power catamarans have gained popularity for offering a unique combination of speed, efficiency, and stability. Their dual-hull design allows for less water resistance, which directly translates to higher speeds and better fuel efficiency compared to traditional monohull boats.

In addition, the wide beam provided by the two hulls ensures a stable ride even at higher speeds. This makes power catamarans ideal for cruising, fishing, and watersports ( Boating Beast ).

Sailing Dynamics

When it comes to sailing catamarans , the performance is affected by factors such as keel, rudders, mast, and sails.

Their wide beam and dual-hull design provide inherent stability and reduced heeling effect, making them less likely to capsize compared to monohulls.

I should also note that catamarans have a shallow draft, which gives them the ability to access shallow waters that may be off-limits to other boats ( Navigating the Waters ).

In my experience, the lighter weight of a catamaran and its aerodynamic design can contribute to remarkable sailing performance under different wind conditions.

The larger sail area relative to hull weight allows them to harness more wind power, further enhancing their speed and agility on the water.

Maneuvering and Docking

Maneuvering and docking a power catamaran involves understanding its unique handling characteristics.

The presence of two engines in separate hulls allows for more precise control in confined spaces such as marinas.

The maneuverability of these boats is typically improved by the use of dual rudders that are located close to each powered hull for efficient steering ( BoatUS ).

When docking under power, I find it helpful to carefully assess the wind and current conditions beforehand.

This is because catamarans can be more sensitive to windage due to their larger surface area above the waterline.

By understanding how these forces may affect the boat, I can make adjustments to my approach and successfully dock the catamaran without any incidents.

Safety and Comfort on Board

Safety Features

Safety is a top priority when sailing any type of vessel, including catamarans. A well-built catamaran offers several features aimed at ensuring the safety of those onboard.

First, catamarans have inherent stability due to their wide beam and twin hull design . This makes them less prone to capsizing than monohull boats. This stability allows me to confidently navigate various water conditions .

In addition to stability, catamarans are designed with positive buoyancy, making them almost unsinkable . Of course, safety equipment such as lifejackets, flares, and first aid kits should always be onboard and well-maintained.

Furthermore, you should also stay updated on weather conditions, avoid sailing in high-risk areas, and learn your boat's safe sail limits.

Living Spaces and Comfort

When it comes to living spaces, I value comfort and practicality as essential features for my time on the water. Catamarans offer a unique advantage in this regard, as their dual hulls create spacious living areas.

Most catamarans are designed with separate cabins in each hull, allowing for privacy and comfort when sleeping. Additionally, these boats typically feature shallow drafts , which means I can access shallow waters and anchor close to shore.

The main living area, or salon, is situated on the bridge deck between the hulls. It usually includes a seating area, a dining table, and a galley (kitchen). Large windows provide ample natural light and panoramic views, making the space feel open and bright. Some catamarans even have the option for an additional living area on the upper deck where you can enjoy the sun and breeze.

One aspect of catamaran living I truly appreciate is the ample storage available. Each cabin typically has built-in storage spaces for clothes, gear, and personal items. There are also designated areas for equipment such as spare sails, tools, and water toys. This makes it easy for me to keep my belongings organized and make the most of my time on the water.

Maintaining a Catamaran

Routine Maintenance

In order to keep my catamaran in the best possible shape, I make sure to perform routine maintenance tasks. These tasks are essential to extend the life of the components and ensure smooth sailing:

• Cleaning : Regularly cleaning the deck, hulls, and sails prevents buildup of dirt, algae, and other debris that could affect performance.
• Inspection : Periodically inspecting my catamaran allows me to detect any potential issues before they become significant problems. I pay close attention to the rigging, sails, and lines on my boat.
• Lubrication : Keeping all moving parts lubricated is vital to prevent friction and wear on components such as winches and pulleys.
• Antifouling : Applying antifouling paint to the hulls of my catamaran helps prevent the growth of marine organisms that can damage the boat and reduce its speed. Make sure to do this at least once a year.

Dealing with Wear and Tear

Despite my best efforts to keep my catamaran well-maintained, wear and tear is inevitable. Here's how I deal with common issues that could arise from regular use:

• Repairs : When I notice signs of wear on sails, lines, or rigging components, I make it a priority to repair or replace them promptly. Neglecting these issues can lead to more significant problems and affect the boat's performance.
• Hull maintenance : If I find dents, scratches, or stiff rudders on my catamaran's hulls, I address them immediately. Repairing any damage not only ensures smooth sailing but also prevents further issues from developing.
• Sail care : Over time, my sails can become stretched, torn, or damaged due to exposure to sun, wind, and saltwater. Regularly inspecting them for signs of wear and making any necessary repairs or replacements helps maintain optimal performance.
• Rust and corrosion prevention : Since my catamaran is made of various metal components, I need to protect them from rust and corrosion. I routinely check for signs of corrosion and apply anti-corrosive treatments when needed.

Catamaran Brands and Models

High-Performance Models

In recent years, there has been a growing interest in high-performance catamarans. I have seen a variety of brands and models that have impressed me with their performance capabilities. One notable brand is Fountaine Pajot , which has a long history of producing a range of sailing catamarans and power catamarans. Some of their popular models include the Tanna 47 and the Bali 4.4 .

Another high-performance catamaran I've come across is the Leopard 40 . Known for their speed and exceptional handling in various conditions, the Leopard brand started with sailing catamarans and has since expanded to include power catamarans. Their models range from 40 to 53 feet long, offering both power and luxury for those looking for a thrilling experience on the water.

Cruising Catamarans

When it comes to cruising catamarans, the Lagoon brand is synonymous with luxury and comfort. With a range of sailing catamarans from 40 to 70 feet long, Lagoon offers spacious catamarans for extended bluewater cruising. Their 60- and 70-foot power catamarans are equally impressive, providing ample living space and smooth sailing experiences.

I've also found the Aquila 42 PC to be a remarkable cruising catamaran. With a focus on design and innovation, Aquila has produced catamarans perfect for exploring the open sea with friends and family. Their spacious, stable designs allow for a more enjoyable and serene journey, ensuring you arrive at your destination comfortably.

The Catamaran Lifestyle

Anchoring and Cruising

I find catamarans to be a fantastic choice for cruising and anchoring , which is a critical part of living the catamaran lifestyle . Catamarans have several advantages when it comes to anchoring and cruising, such as:

• Stability : Due to their wide beam and twin hulls, catamarans remain stable during anchoring, which reduces the risk of seasickness.
• Shallow draft : Thanks to their shallow draft , catamarans can anchor close to shore, enabling better access to protected coves and more beautiful beaches.
• Speed : Despite their large size for cruising vessels , catamarans are generally faster than monohulls. This is a result of their slim hulls and reduced water resistance.

When it comes to anchoring, catamarans can make use of their shallow draft to anchor in locations that other boats cannot. This allows for a greater range of cruising spots, which makes the overall experience much more enjoyable and unique.

Living on a Catamaran Full-time

For many catamaran enthusiasts, the dream of living full-time on a catamaran is entirely possible. While not without challenges, there are several factors that make living aboard a catamaran an enjoyable experience:

• Spacious living areas : Catamarans generally have more living area compared to monohulls, providing ample space for the whole crew.
• Privacy : The separate hulls allow for private cabins, ensuring that everyone on board has their space.
• Stability : As mentioned earlier, catamarans are stable vessels, making living on them more comfortable than monohulls.

Choosing Your Catamaran

Comparing Models and Features

When I start to look for the perfect catamaran, the first thing I focus on is comparing various models and features .

I determine the key factors that are essential for my needs, such as size, passenger comfort, and performance. By doing so, I can identify which catamaran models are most suitable for me.

For example, if I plan to sail with a large group, I would look for a catamaran that offers ample space both inside and out.

To help me with my comparisons, I usually create a table or list of the different models and their features:

This visual aid makes it easier for me to sort the options and prioritize my considerations, such as price, yacht type, and brand.

New vs. Second-Hand

Another critical aspect of choosing a catamaran is deciding between a new or second-hand boat.

Both options have their pros and cons, and ultimately it depends on my preferences and budget.

If I can afford a new catamaran, I get the advantage of the latest design , features, and technology. Plus, I typically receive better warranty coverage and support from the manufacturer.

However, new catamarans are more expensive and can have long wait times due to high demand.

On the other hand, purchasing a second-hand catamaran can save me a significant amount of money, and I might find a high-quality boat with low mileage or well-maintained by the previous owner.

However, this option carries more risks, as I need to be knowledgeable about potential maintenance issues and conduct a thorough inspection before purchase.

Learning Resources

Books and Manuals

When it comes to learning about catamarans, there are plenty of books and manuals available.

One of the highly recommended books is Multihull Voyaging by Thomas Firth Jones. This book provides a comprehensive understanding of multihulls, including catamarans, and is an essential guide for any beginner sailor.

Another great book to check out is Catamarans: The Complete Guide for Cruising Sailors by Gregor Tarjan.

With a foreword by Charles K. Chiodi, publisher of Multihulls Magazine, this book covers all aspects of cruising catamarans. It includes detailed information on design, construction, and maintenance, as well as tips and tricks for sailing a catamaran.

Here are a few more books that I find valuable:

• The Catamaran Book by Tim Bartlett, an excellent resource for both beginners and experienced sailors
• Catamaran Sailing: From Start to Finish by Phil Berman and Lenny Rudow, a comprehensive guide to both catamaran racing and cruising

Online Content and Photography

In addition to books, you can find plenty of online content and photography about catamarans.

Websites like Sailaway Blog and Boating Guide offer tips, techniques, and how-to articles for sailing catamarans.

Many of these sites also include stunning photography, showcasing these beautiful vessels in action.

For those who prefer Kindle or e-books, many of these resources are available in digital format.

This makes it easier for you to access them anytime, anywhere, allowing you to keep learning and improving your catamaran sailing skills.

To further enhance your knowledge, you can also join online forums and communities dedicated to catamarans.

These platforms provide invaluable advice and first-hand experiences shared by fellow sailors, as well as recommendations for additional learning resources.

Frequently Asked Questions

What factors should be considered when choosing a catamaran for full-time living?

When choosing a catamaran for full-time living, consider its space and layout , as it will become your home.

Look for a design with a comfortable living area , ample storage, and sufficient berths for the number of people living aboard.

Also, consider fuel efficiency , ease of maintenance, and the catamaran's cruising range .

Lastly, the overall cost of ownership , including insurance and mooring fees, should be considered.

How do catamarans perform in rough sea conditions?

In general, catamarans are known for their stability, which is primarily due to their wide beams. This makes them less prone to capsizing when compared to monohulls.

However, their performance in rough sea conditions will depend on the specific model and design of the catamaran. Some may perform better in certain conditions than others, so researching and selecting the right design is essential.

What are the key differences between sailing a catamaran and a monohull?

One of the main differences between catamarans and monohulls is stability.

Catamarans have a wider beam , which makes them more stable and minimizes the risk of capsizing.

They also have shallower drafts, which allow them to access more shallow waters compared to monohulls.

Additionally, catamarans often have larger living spaces, making them more comfortable and suitable for cruising and full-time living.

What are the advantages of catamarans for long-distance cruising?

Catamarans offer several advantages for long-distance cruising.

Their wide, stable design provides a comfortable ride and reduces the risk of seasickness.

They can also attain higher speeds due to their reduced drag and generally sail faster than monohulls on certain points of sail.

The shallow draft allows them to explore more coastal areas and anchor closer to shore. Lastly, their spacious interiors make them ideal for extended cruises and living aboard.

How does one assess the value of a used catamaran on the market?

Assessing the value of a used catamaran requires thorough research and inspection.

Start by comparing the age, model, and condition of the catamaran to similar listings on the market.

Take note of any upgrades or additions made to the boat, as these can affect the price.

It's essential to inspect the boat in person or hire a professional surveyor to ensure there are no hidden issues that could affect its value.

What essential features should be looked for in a catamaran intended for ocean voyages?

For ocean voyages, look for a catamaran with a strong, well-built hull designed to handle rough conditions.

Safety features such as liferafts, adequate flotation, and sturdy deck hardware are crucial.

A reliable engine and well-maintained rigging and sails are also essential.

In terms of living space, opt for a catamaran with a comfortable, spacious interior and ample storage.

Last but not least, good navigation and communication systems are necessary for long-distance ocean voyages.

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Catamaran vs. Monohull: Which Hull Design is Safer? Advantages, Pros & Cons of Power and Sailing Catamarans

When it comes to choosing the right boat, especially for open waters, the debate between catamarans and monohulls is always top of mind. Each hull design has its unique characteristics, offering different advantages, safety features, and experiences on the water. In this guide, we’ll explore the safety aspects of catamarans compared to monohulls, the advantages of catamaran hull designs, and the pros and cons of both power and sailing catamarans.

1. Catamaran vs. Monohull: Understanding the Basics

A catamaran has two parallel hulls joined by a deck, while a monohull has a single hull. The fundamental difference in hull structure greatly affects the stability, speed, and handling of each type of vessel.

Monohulls are designed with a V-shaped or rounded hull and are typically more narrow. Their stability comes from a deep keel and the boat's weight, allowing it to stay upright even in rougher waters. However, this design requires the boat to heel, or lean, especially when sailing.

Catamarans , with two hulls, have a wider base and sit more level on the water. They don’t have a deep keel, relying on the beam (width) and buoyancy in both hulls for stability, which results in less heeling and a more comfortable ride.

2. Is a Catamaran Safer than a Monohull?

The question of safety is critical, especially for boaters venturing into open waters or rough conditions. Both catamarans and monohulls have safety benefits, but catamarans hold some advantages in certain situations.

Stability : Catamarans have a wider beam, making them more stable than monohulls. This stability makes them less likely to capsize or roll over, which can be especially reassuring in turbulent waters.

Heeling : Monohulls tend to heel significantly under sail, which can be a bit unnerving for some passengers. Catamarans remain more level, even in strong winds. The reduced heeling in catamarans generally makes for a safer and more comfortable ride.

Capsizing Risks : Catamarans are less prone to rolling due to the stability of their dual hulls, but they are harder to recover if they do capsize, as they don’t self-right like a monohull. However, they are less likely to flip in the first place, so the trade-off might be worth it for many boaters.

Shallow Draft : Catamarans have a shallow draft due to their lack of a deep keel, allowing them to navigate in shallower waters. This can be a safety advantage in areas with reefs or shallow coastlines.

3. Advantages of Catamaran Hull Design

The hull design of a catamaran brings multiple advantages beyond stability:

Fuel Efficiency : Power catamarans benefit from reduced water resistance, which means they’re often more fuel-efficient than comparable monohulls, offering longer range and lower fuel costs.

Speed : Catamarans, especially power catamarans, are generally faster due to reduced drag. With two hulls slicing through the water, they can reach higher speeds with less effort.

Deck Space and Comfort : Catamarans boast significantly more deck space and interior volume than monohulls of the same length. This added room offers more living and lounging space, making them ideal for families or larger groups.

Reduced Wake : The hull design of a catamaran allows it to produce less wake, which not only contributes to fuel efficiency but also minimizes environmental impact.

4. Pros and Cons of Power and Sailing Catamarans

• Speed and Efficiency : Power catamarans are efficient and tend to be faster than sailing catamarans, making them great for long trips.
• Easy Maneuverability : Equipped with dual engines, power catamarans have excellent maneuverability and can turn and dock with ease.
• Reduced Noise : Without sails, there’s less flapping and wind noise, resulting in a quieter experience.
• Fuel Costs : While generally more fuel-efficient than monohulls, power catamarans do consume fuel, which can be a factor in long-distance cruising.
• Limited Range : Power catamarans still have a more limited range than sailing catamarans, as they rely on fuel reserves for long trips.

Sailing Catamarans

• No Fuel Costs for Sailing : Sailing catamarans use wind as their main power source, making them a great choice for extended voyages.
• Stability and Comfort : Sailing catamarans are well-suited for a steady, comfortable ride with less heeling.
• Eco-Friendly : Without the need for fuel, sailing catamarans offer a more environmentally friendly option.
• Slower Speeds : Compared to power catamarans, they are generally slower, especially in low-wind conditions.
• Skill Requirements : Operating a sailing catamaran requires some skill in handling sails and understanding wind patterns, which may not appeal to all boaters.

5. Choosing Between a Power and Sailing Catamaran

Your choice between a power and a sailing catamaran largely depends on your boating needs and experience. Power catamarans are favored by those who prefer speed, convenience, and don’t mind refueling. Sailing catamarans, on the other hand, are ideal for adventurers who enjoy a quieter, fuel-free experience and don’t mind the slower pace of wind-powered travel.

Final Thoughts

Choosing between a catamaran and a monohull boils down to your priorities on the water. Catamarans offer excellent stability, space, and fuel efficiency, making them a great choice for those who prioritize comfort and a smoother ride. Monohulls, with their self-righting capabilities and traditional design, remain a popular choice for many sailors, especially those who prefer a more hands-on, classic sailing experience. Whether you go for a power or sailing catamaran, understanding the pros and cons of each type will help you make the best choice for your maritime adventures.

Ben Oaksmith

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Catamaran Design Formulas

• Post author By Rick
• Post date June 29, 2010
• 10 Comments on Catamaran Design Formulas

Part 2: W ith permission from Terho Halme – Naval Architect

While Part 1 showcased design comments from Richard Woods , this second webpage on catamaran design is from a paper on “How to dimension a sailing catamaran”, written by the Finnish boat designer, Terho Halme. I found his paper easy to follow and all the Catamaran hull design equations were in one place.  Terho was kind enough to grant permission to reproduce his work here.

Below are basic equations and parameters of catamaran design, courtesy of Terho Halme. There are also a few references from ISO boat standards. The first step of catamaran design is to decide the length of the boat and her purpose. Then we’ll try to optimize other dimensions, to give her decent performance. All dimensions on this page are metric, linear dimensions are in meters (m), areas are in square meters (m2), displacement volumes in cubic meters (m3), masses (displacement, weight) are in kilograms (kg), forces in Newton’s (N), powers in kilowatts (kW) and speeds in knots. 

Please see our catamarans for sale by owner page if you are looking for great deals on affordable catamarans sold directly by their owners.

Length, Draft and Beam

There are two major dimensions of a boat hull: The length of the hull L H  and length of waterline L WL  . The following consist of arbitrary values to illustrate a calculated example. 

L H  = 12.20      L WL  = 12.00

After deciding how big a boat we want we next enter the length/beam ratio of each hull, L BR . Heavy boats have low value and light racers high value. L BR  below “8” leads to increased wave making and this should be avoided. Lower values increase loading capacity. Normal L BR  for a cruiser is somewhere between 9 and 12. L BR  has a definitive effect on boat displacement estimate.  

• Tags Buying Advice , Catamaran Designers

Owner of a Catalac 8M and Catamaransite webmaster.

10 replies on “Catamaran Design Formulas”

Im working though these formuals to help in the conversion of a cat from diesel to electric. Range, Speed, effect of extra weight on the boat….. Im having a bit of trouble with the B_TR. First off what is it? You don’t call it out as to what it is anywhere that i could find. Second its listed as B TR = B WL / T c but then directly after that you have T c = B WL / B TR. these two equasion are circular….

Yes, I noted the same thing. I guess that TR means resistance.

I am new here and very intetested to continue the discussion! I believe that TR had to be looked at as in Btr (small letter = underscore). B = beam, t= draft and r (I believe) = ratio! As in Lbr, here it is Btr = Beam to draft ratio! This goes along with the further elaboration on the subject! Let me know if I am wrong! Regards PETER

I posted the author’s contact info. You have to contact him as he’s not going to answer here. – Rick

Thank you these formulas as I am planning a catamaran hull/ house boat. The planned length will be about thirty six ft. In length. This will help me in this new venture.

You have to ask the author. His link was above. https://www.facebook.com/terho.halme

I understood everything, accept nothing makes sense from Cm=Am/Tc*Bwl. Almost all equations from here on after is basically the answer to the dividend being divided into itself, which gives a constant answer of “1”. What am I missing? I contacted the original author on Facebook, but due to Facebook regulations, he’s bound never to receive it.

Hi Brian, B WL is the maximum hull breadth at the waterline and Tc is the maximum draft.

The equation B TW = B WL/Tc can be rearranged by multiplying both sides of the equation by Tc:

B TW * Tc = Tc * B WL / Tc

On the right hand side the Tc on the top is divided by the Tc on the bottom so the equal 1 and can both be crossed out.

Then divide both sides by B TW:

Cross out that B TW when it is on the top and the bottom and you get the new equation:

Tc = B WL/ B TW

Thank you all for this very useful article

Parfait j aimerais participer à une formation en ligne (perfect I would like to participate in an online training)

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• PERFORMANCE TOPICS

Optimising Hull Lines for Performance

This article was inspired by a question about the rocker line in the new 8.5m cat Design 256 and I want to stick to the point, so we won’t turn it into a book, but I’ll discuss two issues, hull fineness ratio and some aspects of the rocker profile.

When you manipulate the hull form you’re adjusting the lines in three planes, waterplanes (plan view), buttocks (side view including the keel rocker) and the section shapes. So you need to be aware of how the shapes are changing in the other two planes as you manipulate any one of these three, or all three globally as is now possible with computer modelling.

There are two fundamental constants that you start with and don’t change throughout the process. The big one is the displacement or the amount of buoyancy you need.

If you make the hull finer by narrowing the waterlines you have to increase the draft or make the ends fuller to get back to the required displacement number.

If you flatten the rocker line you have to increase the hull width, fill out the ends, or square up the section shapes rather than having a V or rounded V. 

The other constant is the longitudinal centre of buoyancy. You really can’t do any meaningful shaping of the hull form until you have settled on the these two constants.

A third number that we can plug in as a constant if we want to is the prismatic coefficient which describes bow much volume there is end the ends relative to the cross section shape in the middle of the boat, but in sailing boats this is of less importance compared to other factors. 

The hull lines for Design 256, 8.5m Cat. It's that hump in the rocker line - right under the back of the cabin that brought up the question and is one of the key points discussed here.

Hull fineness.

Fine hulls are fast, but only in the higher speed range. There’s a misconception I come across quite a bit that you can add weight and windage and you’ll still be fast as long as your hulls are fine.

Well you won’t be. Your boat will simply sink to find the new state of equilibrium. If your transoms are submerged you’ll have more drag. If your bridge deck is too close to the water you’ll have slamming. Much better to be conservative with your displacement figure in the design stage than overly optimistic.

And fine hulls have more wetted area so you have more drag in light air where friction resistance is the primary drag factor. 

I’ve seen promotional material for catamarans stating that the boat has less wetted area because it has fine hulls. For a given displacement the minimum wetted area is described by a sphere (or a semi sphere in the case of a floating object). The more you stretch it out in length, keeping the displacement constant, the more wetted area you have.

The more you make the section shape into a deep V or a broad U with tight corners, as opposed to a semicircle, the more wetted area you have. Add into the equation finer hulls are slower to tack.

So fine hulls are only an advantage if your boat is light and has enough sail area to ensure you’re travelling at speeds where form resistance is greater than skin resistance.

In my view the advantage of fine hulls is often overrated as it applies to cruising cats.

At the other end of the scale the resistance curve is fairly flat up to about 1:9 which is still quite fast in most conditions. From there the resistance rises steeply as the hull gets fatter and at 1:8 and fatter you’re suffering from some serious form drag.

This is the rocker line isolated from the lines plan above (in blue) and and the red line shows a more moderate rocker line that achieves the same buoyancy and maintains the centre of buoyancy in the same position.  The bow is to the right.

In the image lower right I've squashed it up and increased the height to make the difference in the lines more obvious.

The difference in the two lines is quite subtle, but races are often won or lost by seconds.

Rocker Profile

So if we’re looking for low wetted area we would want a rocker profile that was even and rounded, relatively deep in the middle and rising smoothly to the surface at each end. But this would give us a low prismatic which is not ideal in the higher speed range, and it’s not ideal for pitch damping which in my view is the critical design factor that is often underrated. 

Pitching is slow. It destroys the airflow in your sails and the flow around the hulls, and your performance is suffering from slamming loads.

The single most effective way to counter pitching is with asymmetry in the water planes. You can achieve that in the with a fine bow and broad transom. Or you can achieve it with V sections forward and a flattened U shape aft. Or you can achieve it in the profile view with a very straight run forward and a bump in the aft sections. A flatter rocker line is better for resisting pitching than an evenly curved one with deeper draft in the middle.

The final result is a combination of all three of these factors.

On a cat like Design 256 the weight is concentrated well aft so we need to get buoyancy well aft.

The kink you see in the rocker profile helps to do this. It also helps to keep the rocker straight for most of its length and smooth the water flow exiting the hull aft at higher speeds, possibly promoting some planing effect.

If we had a more even rocker line we would slightly reduce the wetted area, but we would increase the pitching and the water would exit the hull aft at a steeper angle, increasing form drag in the higher speed range.

How much of a bump can you put in there without creating a flow separation, and how damaging would that flow separation be? I really don’t know. The way all of these factors interplay in the various conditions we sail in is very complex.

Ultimately a lot of this work is gut feel nurtured by experience, observing things in nature and most importantly experimenting and trying new ideas.

Is the new Groupama AC45 a breakthrough that will influence the form of racing catamarans into the future? I don’t think anyone has a computer that can answer that. We have to wait and see.

Symmetric and non symmetric water-planes. The blue line with grey fill is the DWL from the design above. As is typical with modern cat hulls the bow is long and fine, the stern is full and rounded. This is the asymmetry that has a damping effect on pitching. The red line on the other hand is more like you would see on a double ended monohull and quite a few multihulls have also used this shape in the past. It's quite symmetric about the pitch axis and does not have good pitch resistance.

The hull lines of the new 8.5m Sports Cat Design 256

Mad Max , Previously Carbon Copy . She was designed in 1997 but she's the current (2016) title holder of the Australian Multihull Chamionships (2 successive years) and the fastest inshore racing boat in Australian waters.

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