modern trimaran design

Comparing the Cross 18 with the newer W17

QUES:   As your W17 appears to be ‘not unlike the older Cross 18’ trimaran design, would you care to comment on any differences, and why you felt it necessary to design the W17, with the Cross 18 design still being available?   .......   Harvey, CA 2019

ANSWER:   Be pleased to - being a timely excuse to share some subtleties of design.   But first, let me say that back in the 1980’s when I first got hooked on Trimarans, I was quite a fan of Norman Cross’s designs.   I thought they were rightly ‘conservative in design’ but also that they looked better than most of the competition of that era. Around that time, I also made a point of meeting Mr Cross at one of the early World Multihull Symposiums organized by the late Editor/Founder of Multihulls Magazine Charles Chiodi, along with designers Brown, Crowther & Wharram.  It was to be all part of my early Multihull learning experience and I am still very thankful for all that we can learn from these early pioneers.

But there’s been ‘a lot of water under the bridge’ since then.   His Cross 18 was probably his smallest boat and designed back in the late 1960’s .. now some 50 years back!     Despite that, I would not dismiss a design based on age alone - (still too many Herreshoff beauties around ;) – but for me, I later become aware of a sufficient number of potential design-upgrades, that I felt a new boat could take things to another level.

I will go through these briefly and explain what they offer … as always, from my personal study, tests, knowledge & experience, but acknowledging that others may come to somewhat different conclusions.

Most hull designs to be built of plywood, from the 50’s right up to the end of the century, attempted to replicate the round bilge hulls of carvel, moulded ply or fiberglass construction, by using multiple chines.   The Brown, Crowther and Cross boats were early examples of that while the Scarab’s are more recent ones.   (We also have Constant Camber (CC) that has no chines, though this method is more limited in what shape can be produced.  see Construction Methods ).   A system using multiple chines works very well for a hull with a fair amount of rocker or what I call ‘a banana hull’, as the lay of each plywood strake tends to be fairer if the ends rise up.  This also gives a fairly Vee'd section up forward that blends most naturally into a rounded, cutaway forefoot.   The majority of boats designed before 2000 show these features, so many have become conditioned into thinking this is generally the best shape for a boat.   But I will ‘stick my neck out’ and say, my studies have shown this not to be entirely true.

I worked for a while on submarines and was reminded that a volume of displacement well underwater creates far less residuary (wave) resistance than one that disturbs the surface water, so when I am looking for a specific volume of displacement to support a given weight, I am looking to see how I can place this low under the surface, with the least disturbance of the water surface.    What NA’s call SWATH vessels are an ultimate example of this with a submarine form under the water to support the weight but with a very fine part passing up through the water surface, offering minimal wave making.   While this is not practical for a heeling sailboat, it shows a direction we may want to consider. 

Also, another thing to consider is that the vee’d hulls of the multi-chine (or CC) construction, also give a rapid increase in buoyancy at and above the waterline.   ‘ This is great’ the old guard may say, but it’s been my observation that this rapid increase in buoyancy has a couple of concerning negative effects.

It not only disturbs the surface water, throwing it outward as white spume that flies in the air and back into the boat making it pretty wet, but it also overacts on the bow, throwing it way up in the air, starting a pendulum pitching that can rapidly increase, especially if the wave spacing & speed are ‘just right’ (or perhaps I should say, 'just wrong' ;).   This is further enhanced when the hull profile is of a banana shape, making a see-saw out of the boat.    It’s now possible to look at the banana shape of an older trimaran AMA and say with some certainty, ‘she will pitch a lot’ .   This is aggravated when the center of flotation of the ama is longitudinally close to that of the main hull, making a too-perfect pivot point right across the boat.

So, when designing the W17, I have done all I can to get away from both a flared vee’d in section and a banana hull in profile, as well as designing the two hulls with a center of flotation as far apart as practical to dampen out any synchronous pitching.  This has required nearly vertical hull sides, plus hulls with minimal rocker, and with any chine, as straight (‘non-banana’) as physically possible.   Center of flotation was pushed forward on the amas but kept back on the main hull.

But this is not the end of the hull form difference.   We must also consider the cross section.    As most boats, including the multi-chine of the Cross, have a rounded cross section, we must acknowledge that this will permit some transverse flow under the hull.  This will occur when lateral wind force presses on the boat, causing what we define as ‘leeway’.    On a keel boat, the deep continuous side skin greatly limits that side flow, but on a hull without a keel, it takes a very efficient foil to limit that side slip … typically a deep aero-foil dagger (or center) board.    But just pull up that board when sailing to windward and see what happens!  The boat crabs sideways, slows down and steering control is much reduced.  So despite adding its own wetted surface, it has to stay down just to get to windward..

But what if we change the sections of the main hull so that it’s NOT round and make it into a box section with very small-radius corners that will discourage any transverse flow?  Now most of the flow will be along its length and the bonus is that we’ve now also moved some displacement lower down, well under the water surface, enabling the section to have a narrower waterline for the same displacement.

The above thinking is not totally new, even if this application may be.  Sharpie designs by the likes of Ralph Monroe, Howard Chapelle, Van de Stadt*, Phil Bolger and others, share many of these attributes but there’s one very important difference.   Monohull sharpie’s are long and narrow and therefore limited in basic transverse stability, while with a trimaran, stability comes from the outer amas.   * [I grew up sailing the Solent when Van de Stadt's Zeevalk and later Black Soo , were flying around the British south coast ... advertisements for Kees Bruynzeel’s new marine plywood at the time, and I loved these slim, near vertical-sided sharpies ... but as monos, they needed very deep fin keels with ~50% ballast down there to carry their sail ... a major compromise that even slimmer tri main hulls do not have to deal with.] 

Another advantage of the near vertical-sided boxy main hull, is the way these hulls do not significantly change the shape of the water while passing through waves.  This seems to be overlooked by many designers today, as vee'd hulls are still very prevalent.  Rather than explain this again here, I'd like to reference this article that I prepared for an Australian club magazine, as it includes some diagrams that clearly show the potential negative effect of highly vee'd hulls in waves.

For the amas (outriggers), there are also several significant differences between those of the Cross 18 and the W17.   First, the W17 pushes its volume and center of flotation farther forward to reduce the pitching as noted above and also reduces nose diving too.  (In his WB review of the W17, Geoff Kerr noted " ... I saw no sign of that dreaded beach-cat tendency to bury the lee bow." .) 

The W17 also totally avoids any banana profile by having a deeper bow and straighter lines.   Most trimarans use symmetrical hulls port & starboard as does the Cross, but contrary to catamarans where symmetry makes good sense, trimaran amas are not both in the water at the same time and the one that is significantly immersed is typically always to leeward, so it presents an interesting opportunity to be asymmetrical, IF indeed there is seen to be any advantage.    

(for more on this, see:    https://smalltridesign.com/pdfs/W17ProBoatOctNov2017.pdf )

                                                                                                   .

Other design differences are noted here:

Cross beams (akas) on the Cross 18 are straight and slightly below deck level, whereas on the W17, they are raised higher above waves to rarely contact them, making the boat either drier or adding capability in the rough stuff.

Although both boats have split-aka pivot-hinges, the W17 design is flat and wider, so can be walked and sat on.    Today, both hinges & latches are home molded in fiberglass for the W17, making them lighter, stronger and corrosion free.  See Folding Systems Part 3 for more on this. The W17 beams are far more robust than those on the Cross, adding to its rough water capability as well as to its appearance.   As many have commented, the uniquely faired forward beam of the W17 gives it the look of a miniature ocean racer as well as less wind and water resistance. 

The conservative Norman Cross was known to not like rotating masts, but he was missing out, and today all top performing multihulls have them.   Rotating wing-masts are standard on the W17 and their Build-it-Yourself designs are offered to suit each rig, in wood or carbon fiber. These masts add significantly to upwind performance.

One weakness on many small boats is the commonly suspended rudder attachment.  On boats that cruise long distances, rudder damage is by far the most common.  (In the late 60's, a teenaged Peter Clutterbuck cruised his British 16ft open Wayfarer dinghy to wildly-unrealistic places such as Denmark, Sweden, France and even across the Bay of Biscay!  While he survived 2 capsizes he also reportedly suffered 9  rudder failures !).   On the W17 a totally different mount is employed, that spreads the load from the deck to the hull bottom, creating a very strong mount.  In addition, the W17 sports a balanced spade rudder totally under the hull, which still pops up with an auto-release in case of touching the bottom.  Its improved location permits it to have less depth than a transom-hung rudder and still be highly effective. With a raised daggerboard and the low hull-leeway, this permits the W17 to be sailed to windward in as little as 2ft of water if so needed.

The W17 mainsheet arrangement is upgraded with a full width radiused traveler, permitting the mainsail to be sheeted down flat without use of a kicking strap (boom vang), adding to boat control and safety in windy conditions.  

The more efficient flat-top full-battened mainsail is also stowed on a boom that is rotated via a removable handle and this not only adds life to the sail, but also keeps the cockpit tidy ... (see photo below).

The cockpit seat backs contain 6 small deck lockers.

To reduce fatigue and give variety, the W17 can be sailed from at least 6 different areas on the boat as the helm is very light.  Two lightweight, 6ft extensions that reach well forward and to either ama, enable this.

It’s also acknowledged that, while the hulls of the W17 are basically easier to build (using the ABC System) and retain a cleaner frame-free interior, the shaped beams of the W17 take a little more time, so the total construction period might be a little more.   W17 builders think it’s well worth it though, noting the advantages and improved esthetics that this newer design offers.

Yet having clarified all that, a few builders will still appreciate to build and own the old classic C-18 design so we hope its plans will still be available for years to come.   As someone who always tries to works with ‘facts’, it’s fair to add that the extra length of the Cross 18 should theoretically help speed and seaworthiness; the quasi-rounded bottom of the hulls should give a gentle ride and lower skin drag at slow speeds; and the more vee’d hull will sink slightly less with excess load.  But for other aspects, it will miss the many W17’s perks.

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The complete list of trimarans.

There is no single trimaran that is best for everyone. Where some prefer luxury cruisers for long trips with family and friends, others might opt for a high performance racing tri for thrilling rides at breakneck speeds. With the recent spike in trimaran popularity, these days there is a perfect tri for every sailor. So to help prospective trimaran owners decide which boat is just right for them, we here at WindRider have put together a comprehensive list of the best trimarans on the market today! Read through for simple at-a-glance trimaran comparisons of boats both big and small, exhilarating and relaxing, and for all price points.

Jump to a specific sailing trimaran: Neel Weta Corsair WindRider Dragonfly Catri Astus Hobie Sea Pearl Farrier Sea Cart Multi 23 Triak SeaRail Warren Lightcraft Diam Radikal Challenger

Known for their award-winning luxury trimarans,   NEEL   is based in La Rochelle, the capital city of sailing in France. NEEL trimarans are built for fast cruising with an average cruising speed of about 10 knots, and are even configured to facilitate that sustained speed under motor propulsion. The NEEL 45 was notably named Cruising World’s Most Innovative Vessel in 2013, and by all accounts is an easy-to-sail, high performance boat that is just plain fun.

At a glance:

Models: NEEL 45, 65

Length: 45’ – 65’

Cost:   $$$$$

Use: Luxury cruiser

A fan favorite,   Weta trimarans   are fast, stable, and remarkably easy to rig. This single-sailor tri has a capacity of up to three, and the ease with which it can be transported and stored makes this a great, versatile boat for beginners. The Weta was named Sailing World’s 2010 Boat of the Year, and one ride is enough to know why: simply put, the Weta is an absolute ton of fun to sail regardless of skill level.

Models: Weta

Length: 14’5”

Cost:   $$ $$$

The high-end   Corsair trimaran   definitely holds its own in the categories of versatility, performance, and convenience. Boasting a rigging time of 30 minutes from trailer to sailor ,   the Corsair 42 – whose convenient folding amas makes trailering possible – is a simple option even for single sailors, though cabin space is suitable for two adults. These boats are wicked fast, capable of reaching speeds of 20+ knots, and were made for skilled sailors seeking solid construction and high performance vessels, not for beginners.

Models: Pulse 600, Sprint 750 MKII, Dash 750 MKII, Corsair 28, Cruze 970, Corsair 37, Corsair 42

Length: 19’8” – 37’

Cost:   $$$$ $

Use: Sports cruisers

Built for the sailor who wants to maximize the joys of sailing while minimizing any hassle, WindRider trimarans are notoriously fast, very safe, and a blast to sail from start to finish. With several models that can hold between 1 and 6 riders, including adaptive designs to allow participation from sailors of all levels of mobility, there’s something to suit every sailor’s needs. The WindRider 17, an exhilarating ride perfect for families or camper sailors, has been known to reach speeds of up to 20mph. This easy day sailor goes from trailer to sailing in under 30 minutes and is sure to fit in perfectly with whatever adventures you have planned.

Models: WR 16, 17, Tango, Rave V

Length: 10’11” – 18’3”

Cost:   $ $$$$

Use: Day sailor

The Danish-built   Dragonfly   trimarans come in a variety of models ranging from 25’ – 35’, all known for their spry performance, comfortable ride, and ease of use. Every model comes equipped with the unique “SwingWing” feature, a motorized system that can unfold the amas even while the boat is already underway – making it accessible to marinas and slips, and even makes trailering possible. Perfect for those who don’t want to sacrifice their comfort for high performance, the Dragonfly can breeze along at 13 knots while remaining one of the quietest compact cruisers out there.

Models: Dragonfly 25, 28, 32, 35, 1200

Length: 25’ – 39’

Designed for both safe cruising as well as for high speed racing,   Catri trimarans   will make your day. Especially noteworthy is the Catri 25, a stable yet wildly fast foiling trimaran with accommodations for up to 6 people. With profiles optimized for speeds of 25+ knots when foiling, this is no beginner’s sailboat. The special attention paid to stability in the foil design allows the Catri to be a single sailor vessel, even at foiling speed, with no special physical abilities. Whether you’re taking a small crew for longer rides at shuddering speeds or bringing the whole family along for a shorter, but still thrilling sail, the Catri is truly one of a kind.

Models: Catri 25

Length: 25’

Use: Cruiser/racer

A popular brand of trimaran in Europe,   Astus   has recently made its way to the US market to the delight of sailors on this side of the pond. Designed to offer maximum pleasure with minimum hassle, all models of Astus trimarans are fast to set up, quick on the water, inherently stable, and always a joy to sail. Their outriggers are mounted on telescopic tubes for easy stowage and towing, and can even be extended and retracted on the water for access to narrow passageways and monohull slips in marinas. With models in all sizes and price points, Astus trimarans are a great option for any sailor.

Models: Astus 16.5, 18.2, 20.2, 22, 24

Cabin: Some models

Length: 16’ – 24’

Use: Sport cruisers

HOBIE ADVENTURE ISLAND

Great for beginners and adventurers alike, the   Hobie Mirage Adventure Island   series is nothing if not just plain fun. With the option to use as a kayak or as a very basic trimaran, the Hobie is transportable, versatile, unintimidating, lightweight, and wonderfully affordable. The pedal system known as “Mirage Drive” allows a person to pedal the kayak using their legs for an extra kick of movement in slow winds. Amas tuck close to the main hull for docking or car-topping, adding serious ease and convenience to the exhilarating experience of the Hobie.

Models: Hobie Mirage Adventure Island, Mirage Tandem Island

Length: 16’7” – 18’6”

Use: Convertible kayak/trimarans

Best known for its use in camp cruising excursions, the   Sea Pearl   offers a roomy main hull and particular ability to sail in very shallow waters, making beaching and launching a breeze. The lightweight Sea Pearl trimaran is easy to tow, and the larger-than-expected cabin opens this vessel up for overnight adventures with plenty of storage space. The simple design makes the Sea Pearl notoriously low maintenance, and the ease it takes to rig and sail it add to the overall delight of owning this boat.

Models: Sea Pearl

Length: 21’

Use: Camper cruiser

Quick, lightweight, roomy, and trailerable,   Farrier trimarans   are made for versatility to fit every sailor’s needs. Different Farrier models are available in plan or kit boat form for those who appreciate building their boat themselves, but of course, also as the full production sail-away boat for the rest of us. Single-handed rigging and launching takes under 10 minutes from start to finish, minimizing hassle and getting you on the water fast. All non-racing Farrier designs use a minimum wind capsize speed of 30 knots or more to ensure safety for all those aboard. Add the roomy cabin and high speed capabilities to the equation and you’ve got a boat that is great fun for everyone.

Models:   F-22, 24, 25, 82, 27, 28, 31, 9A, 9AX, 9R, 32, 33, 33R, 33ST, 36, 39, 41, 44R

Length: 23’ – 39’4”

Cost:   $$$ $$

Use: Sport cruisers/racers

One of the biggest names in the game,   SeaCart   is internationally noted for its high performance trimarans that far exceed expectations for a production boat of its size. The SeaCart trimaran performs as brilliantly off the water as it does on with its super-light and efficient harbor folding system, making light work of trailering. Notoriously easy to manage and maintain, the SeaCart 26 One Design is the ultimate day racing trimaran, designed for both course and inshore/coastal distance racing. Absolutely worth the international buzz it has garnered, the SeaCart is a thrill from beginning to end.

Models:   SeaCart 26

Length: 26’

A high performance racer class, the   Multi 23   is a lightweight, powerful trimaran known for its wicked speed of up to 25 knots. Multi trimarans of both available configurations were designed to give beach cat thrills and speed without any of the stability or seaworthy concerns. Open ocean sailing is no issue for the Multi’s big bows, which do their job to keep her stable. Built for sailors with a need for speed, the Multi makes a perfect weekend boat for racers, especially those with a taste for boat camping.

Models:   Multi 23

Length: 23’

Another dual outrigger sailing kayak/canoe design,   the Triak trimaran   was designed to be effortless and fun, especially for beginners. Paddle the kayak with sails furled, use the foot pedals for an extra kick of momentum, or sail with just the mainsail – the only boat in its class to feature an asymmetrical spinnaker – for exhilarating speeds and a blast on the water. Car-top the Triak anywhere for a quick sail or plan for a week long expedition, but always count on having a great time on this easy little boat.

Models:   Triak

Length: 18’

Use: Convertible kayak/trimaran

SeaRail trimarans   are known for being affordable, light weight, trailerable trimarans that offer the perfect combination of exciting and relaxing experiences to a wide range of sailors. Whether it’s day sailing with your family, resort or camper sailing, SeaRail trimarans are ideal leisure vessels. Leave the hassle to the other boats – the SeaRail takes you from trailer to sailor in 15 minutes. But don’t let its reputation as a leisure tri fool you: if speed is what you want, rest assured that the SeaRail can deliver that as well.

Models:   SeaRail 19

WARREN LIGHTCRAFT

Warren Lightcraft trimarans , another example of a convertible kayak-to-sailboat option, are known for their aesthetically pleasing designs that are also, as the name implies, very light for simple transportation and ease of use. Convert the kayak into a fast, high performance sailboat in just minutes, fly around on the waves all day long, then simply car-top the 68lb Warren for a maximum enjoyment, low-hassle day on the water. Perfect for sailors and paddlers of all skill levels, the Warren Lightcraft is the best of both worlds and an absolute joy to sail.

Models:   Warren Lightcraft

Length: 15’6”

Built strictly with racing in mind,   the Diam 24   is a light, powerful one-design class trimaran and a notoriously exceptional performer. Boasting blistering speeds of up to 30 knots, Diam trimarans are not intended for beginners. For racers who crave the very best in terms of intense speeds, smooth handling and impeccable performance, the Diam is the red-hot one-design racing tri for you.

Models:   Diam 24

Length: 24’

For the sailor who prefers the finer things in life, the   Radikal 26   delivers. Perfect for bringing the whole family out for a day on the water, this high performance, trailerable sailing trimaran strikes the most luxurious balance between quicksilver speeds and a smooth, comfortable ride. The Radikal 26 trimaran is as convenient to transport and set up as it is pleasant to sail, with a folding system that minimizes rigging hassle and also makes this a trailerable tri. Built for a fast and comfortable sail rather than a hold-onto-your-seats thrill, one-the-water safety and overall pleasure makes the Radikal 26 what it is.

Models:   Radikal 26

Use: Sport cruiser

A solidly-built, single-handed trimaran, the Challenger also doubles as an adaptive design – meaning it is made to accommodate sailors of all levels of physical mobility. Best suited to lakes, the Challenger is a very safe, seaworthy boat for sailors of all ages and experience levels. Add to this the ease of owning, transporting and maintaining the Challenger trimaran and what you get is a simple, fun sailboat perfect both for beginners and those seeking a cheap thrill alike.

Models:   Challenger

At a glance comparison:

Did we miss one? Let us know. Tell us what you sail and what you like about each boat in the comments below.

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