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Lock Crowther Biography

Discussion in ' Multihulls ' started by oldsailor7 , Nov 20, 2009 .

oldsailor7 Senior Member

I was browsing the web and came across this Bio for Lock Crowther. It may be of interest to Locks fans. Lock Crowther Famed muli-hull designer of Australia. Among his most notable designs were the Buccaneer & Kraken series trimarans, Spindrift 45 & Catana 40 cats, as well as a great number of other cruising, racing, charter/commercial and power cats and tris. Lock Crowther built his first boat, a trimaran called BUNYIP, in 1959 while still a teenager. The following year he raced in the Easter regatta at Paynesville, Victoria, Australia, beating 300 boats to the finish. The success of his boat inspired several of his friends to build similar boats and thus Lock Crowther’s design career started, though he did not expect to make a living out of it, and studied electrical engineering. In the early 1960s, when there were only a handful of people designing multihulls, Lock Crowther was involved in this international scene and also a member of the Amateur Yacht Research Society. BUNYIP was followed by the Kraken 25 design, which somewhat widened Crowther’s recognition. His reputation was established internationally in 1996, when his first offshore racing trimaran BRANDERSNATCH won the Sydney to Hobart multihull race. Even more notice was taken in 1966, when a Kraken 40 won the New York to Bermuda race with him aboard. During Lock Crowther’s career, more than 2500 of his designs were built. He started out with racing boats, where most of his new ideas were developed, though he was successful drawing cruising and commercial craft as well. Among his outstanding designs was the trimaran SPIRIT OF AMERICA, which was an early user of GRP-foam sandwich construction and had innovative composite beams with uni-directional fibres and turned-down ends. Lock Crowther also developed the use of ‘bulbous bows’ as a measure to reduce pitching, and hence increase speed when sailing upwind in a swell. Since his death in 1993, Crowther Designs was run by Lock Crowther’s son, Brett. On the 27th of July 2004 Crowther Pty Ltd was shut down and the company name de-registered. Brett moved on to join INCAT P/L to form INCAT- CROWTHER, and concentrate on development of commercial vessels. Lock Crowthers DIY multihull sailboat plans are now permanently archived and no longer available.  

bad dog bad dog

I had the great pleasure of racing on Lock's own last boat, a 48' cat named Deguello (built 1989 in Gladstone QLD), which moored then in Pittwater (just north of Sydney), now sold and gone to Hobart. Lock actually died on board of a heart attack I think, and I'm sure his ghost assisted in getting us fired up when needed. He was a very clever man, and all multihull designers now stand on his shoulders (amongst several others!).  

bad dog said: ↑ Lock actually died on board of a heart attack I think, Click to expand...

sabahcat Senior Member

I put up some of his design notes and info on wave induced capsize on this thread here http://www.cruisersforum.com/forums/f48/lock-crowther-design-notes-as-promised-5910.html  

Lock had many, many friends and aquaintances. If any of you have anecdotes, or tales to tell, of Lock and his lifetime, it would be nice if you could see fit to tell them here in this thread. A lot of us would be really interested. For instance here's one. Lock had a very dry sense of humour. When He and Logan Apperley came over to Canada for the World Multihull Symposium in June 1976, They sailed with my wife and I on our Buccaneer 28 No:1 in an overnight race on Lake Ontario. The next morning we were sailing along in light winds when Logan went out onto the bow of the Stbd float to hank on the spinnaker pole downhaul----and promptly fell overboard. There was a moment or two of ghastly silence and then two sets of white knuckles showed up on the rail at the aft end of the float. Without missing a beat Lock shouted, " Logan--stop mucking about and get back on board" I'm sure Logan will never forget that.  

Ad Hoc Naval Architect

He worked as a customer rep, for a client, in the company i worked at in the early 90s. Was bloody tall, as i recall, nice bloke easy to chat with. He sadly died shortly after the project was finished.  

Corley epoxy coated

I found the following on the web sadly it seems Howard Stephenson has passed away as I've tried to contact him without success to let him know I was doing up Lock's Kraken 25 Trimaran. Out of interest does anyone know what became of Trio or Bunyip? I'm also interested as to the fate of one of Lockie's later daysailing tri's Jabberwock. TRIMARANS ON THE GIPPSLAND LAKES I have learnt a lot from my friends. Stuart Harris and others taught me about amateur radio. David Theobald and I together learnt saltwater light game-fishing. David Williamson showed me how popular songs were constructed. Lock Crowther taught me a lot about how boats were designed and stimulated my interest in learning more. His real name was Lachlan, but he couldn't stand it, and insisted on being called Lock, or Lockie. I knew him slightly from our home town, Bairnsdale. In 1960 or 1961 we ended up by coincidence in the same Melbourne boarding-house. We were both part-time students. As Lockie had a car, he would sometimes give me a ride home to Bairnsdale for the weekend. On one of these weekends home I went to Paynesville, about 15 Km from Bairnsdale, on the Gippsland Lakes. This was one of my favourite places, because of the boats there. There was the Government Slipyard, with its two RAAF crashboats. I loved to peer through the portholes at their triple straight-eight Packard engines. Tied up near the slipyard were rows of fishing boats and cabin cruisers. Between Christmas and New Year there would be speedboat races on the lake. Home-built Bondwood three-pointers powered by modified V-8 car engines would tear around the course at speeds reaching thirty knots and more. There were sailing boats at Paynesville too, but they were too slow to attract my interest, until one Saturday afternoon I saw something that moved like a speedboat with sails. From a distance of half a kilometre or so I could see a huge plume of spray arcing out from either side of the boat as it rocketed towards Montague Point in a gust of wind. I wanted to be on that boat. I knew that Lockie and his family were keen on sailing and that they had built a trimaran. As the sailing speedboat I saw that afternoon was a trimaran, I guessed, correctly, that it belonged to Lockie's family. It was not long before I'd wangled an invitation to go sailing. Their trimaran, 6 metres long, had been designed by Lockie and built out of plywood on wooden frames and stringers by him, his two brothers and their father. Maybe his sister and mother had a hand in it too; they were a close family. The tri was sloop-rigged with a wooden mast. The balanced jib was attached to a boom. The decks between the main hull and the floats were plywood. In an effort to provide lift, the floats were flat-bottomed. Although this feature was not an absolute failure, more often than not it just created extra drag. The trimaran was named Bunyip, after an Australian mythical creature. Another feature, intended to prevent broaching, was the very full bow on the main hull. This was the reason for the twin jets of water that came off the forefoot, ensuring that at any speed at all the crew became absolutely saturated. On my first outing, we were three-up; I was just a passsenger. As the wind was fairly light, the Crowthers had rigged a single-luffed spinnaker as a genoa. One of the brothers was out on a trapeze and Lockie was steering. I just sat there, thrilled by the water hosing off the bow into my face, by the rush of wind and by the streaming wake. I was hooked, but I was still a student, with no spare money at all. Building a trimaran myself was out of the question, at least for the time being. Lockie, feeling that he could do better with a new design, started work, in a Melbourne suburban backyard, on a cat-rigged single-hander about 600 mm. shorter than his first effort. The sail was fully battened, with a huge roach. The side-decks were plywood. Instead of a trapeze, there was a sliding seat which could also be pivoted fore and aft. A new idea was the use of a daggerboard in each vee-bottomed float, canted inwards at forty-five degrees so as to provide both lift and lateral resistance. There was also a conventional daggerboard in the main hull. This design was not a success. It was too heavy, because of the sliding seat and because it was built out of heavy hardwood plywood instead of the fir plywood which had been to build the earlier design. Being deep in the water, having no jib and with two off-centre daggerboards, the tri would not tack properly. It was almost always necessary to back the main and reverse the rudder to get onto the new tack. Lockie's next attempt was an even bolder step into the unknown. Fortunately, it was much more successful. At that time racing in C-class catamarans for the "Little America's Cup" was all the rage. Lockie decided to design and build a C-class trimaran. The C-class rules required a maximum length of 25 feet (7.6 metres) with a sail area of 300 square feet (28 square metres). At that time his ideas were somewhat influenced by Scandinavian square-metre keelboats, whose main design restriction was also a fixed sail area. They seemed to go well with their large overlapping jib, a genoa, just as Lockie's first design went well in light airs with a genoa. So the new design featured 28 square metres of sail area, about 11 square metres of it in the genoa. This allowed a relatively short aluminium mast, just over nine metres tall, as I recall. The main hull was quite different from any of the C-class catamarans. The curved forefoot carried forward into a very fine overhanging curved bow. The mid-section was a conventional (for a multi-hull) semi-circle, but the bottom was flat by the time it reached the transom. The floats had a similar bow, but with a vee-shaped midsection; the afterbody tapered towards a triangular reverse transom. I am not sure where these ideas came from, but they certainly produced an elegant shape. I guess he was trying to minimize wetted surface area and reduce hobbyhorsing. Lockie drew up a nice set of plans for this boat, including an isometric projection which I greatly admired The three hulls were cold-moulded, with a lot of effort put in by his family, and some by me too, stapling the veneers, then pulling the staples out after the glue dried. (This reminds me of the way ships were built in the days before the steel plates were welded together: half the building crew would be making holes in the plates and the other half filling them up with rivets. Some aircraft are still built this way, although not with steel plate, of course.) By now I was earning a proper wage and I wanted to build my own trimaran. For someone who had never made anything more difficult than a doorstop, this was a major decision. I reckoned that I could rely on Lockie and other yachtie friends to advise me on boatbuilding, and save a lot of time by purchasing a ready-made main hull. But who made stock trimaran hulls? No-one, of course; but Charles Cunningham was turning out fibreglass shells for his 6-metre Austral 20 catamaran. I bought one of these, decked it with plywood, built timber-and-ply crossbeams and floats to Lockie's design and fabricated much of the hardware from stainless-steel plate. E.F. Prior of St Kilda made up a genoa and fully battened, heavily roached main out of sky-blue Terylene. The spaces between the hulls were filled with two canvas trampolines, also made by Prior at what my mother considered an exorbitant price. The result was a cut-down version of Lockie's latest creation, with seven percent less sail area, less beam and length and a lot less weight. Instead of a daggerboard, I followed Cunningham's idea of using a leeboard. Once we solved the engineering problems -- I broke several boards -- this set-up worked quite well, but obviously was less efficient than a daggerboard. Eventually I discarded the leeboard, glass-and-epoxied a daggerboard case into the hull and made a nice hardwood daggerboard which never looked like breaking. This arrangement proved to be much more efficient. This was the first time I had used epoxy resin. In those days it was very expensive and hard to come by. One of the Bulmer brothers was an industrial chemist and he was able to purchase some for me at a reasonable price. It was as thick as treacle on a winter's day, so I would scoop out each small batch into a pot, heat it on the kitchen stove to bring it to a manageable viscosity, mix it with the right amount of hardener, then apply it very quickly before it started to set. Meanwhile, Lockie's larger tri, which he named Kraken, was performing well on the Gippsland Lakes. Admittedly the only competition came from Lightweight Sharpies and Flying Dutchmen, so it was hard to gain a comparison with other similar boats -- C-Class catamarans, for instance. In due course I finished my boat. It should have been up with his, at least in light airs but, partly because of my inexperience, my "Trio" was hardly ever a match for the bigger tri. There was one occasion when Trio beat them all, Kraken included: one of the Gippsland Lakes Yacht Club's annual long-distance races. The course was set from the yacht club at Paynesville, south-west along Lake Victoria to Waddy Point and return, a round-trip of about 15 nautical miles. As there was a gentle south-easterly sea-breeze that day, the course was virtually two long reaches, with no tacking and just a gybe at Waddy Point. We won easily, in record time. We also used to cruise the lakes although, without auxilliary power, we ran the risk of being becalmed. Once Hugh Crowther and I took Trio for a day-sail to Lakes Entrance, about 15 nautical miles from Paynesville. We arrived there in good time, had our picnic lunch and headed for home. By the time we got to Metung, only half-way back, it was dark and we had run out of wind. I had to call my father to come and drive us home. We sailed the boat back to Paynesville the following weekend. Lockie once took Kraken to compete on Port Phillip Bay. It turned out not to be competitive against large catamarans. They were designed for the bigger seas and stronger winds of the Bay. Our tris excelled in the light breezes and small chop which we mainly encountered on the Gippsland Lakes. This was fun, but even more fun was when there was a beam reach in a stronger breeze. The sheets would be just started, the apparent wind being well forward of abeam. My crew would be out on a trapeze and I would be out there on a trapeze too, barely maintaining control via the tiller extension, nearly three metres long. I also enjoyed light-weather sailing. Neither tri had a spinnaker, but we hardly needed them: our technique was to tack downwind, heading up in the lulls and bearing off in the puffs. Even to windward both tris were faster than all the monohulls, although we would lose out if there was any short tacking to be done. These good times eventually came to an end. I left for Queensland, returned briefly, sold my boat and went overseas. Lockie decided to give up his day job and move to Sydney where he became a famous and successful designer of trimarans and catamarans. Nearly forty years on and living over a thousand kilometres from the sea, I still daydream about boats. These days my interest has returned to powerboats, because I enjoy fishing and need plenty of shade; my skin no longer lets me sit out under the sun for hours. Maybe I can find a design somewhere for an easy-to-build trimaran which could be fitted out with minimal camping-style accommodation and powered by a smallish outboard engine. Howard Stephenson November 1999  

rebel1 New Member

I too had been "touched" by Lock in late 1969 and 1970 but not by involvement with his early racing designs. In January 1970 I was transferred by my employer to Townsville NQ., where I found two of Locks early cruiser/racing designs had recently been launched. Both boats were 33ft long and of a design by Lock called I think "tempest" versions. One boat called Jindivik was owned by a Don McGrath, the other by a local engineer, Ian Hamilton, was called Nomad. These boats were full bridge deck versions of what was known as the Kracken 33. Slightly wider in the main hull than a Kracken, a habitable cabin, and in the case of Jindivik a small cabin aft of the rear beam, cold molded round bilge plywood construction with what today could almost be called a center cockpit. Nomad's cockpit was in the stern and one would sit on the rear beam and tiller steer. Both of these boats "owned" the Townsville Cruising Yacht Clubs easter race to Dunk and the Townsville to Whitsunday series in the early to mid 70's. Jindivik in fact held the Townsville Dunk Island race record for many years prior to it being beaten by a later Lock Crowther design. The race was 96Nm as the crow flies and Jindivik finished the course in 8 hours 16 minutes. Very quick in those days. I had the pleasure to be crew on Jindivik during that race ( and many other races) and can remember the boat surfing down huge waves, completely controllable, and us, the crew, celebrating whenever the log showed over 20knots. Mid 70's saw Don Hinch sell Manta 11 to a local doctor and I had the pleasure to sail on this boat during a Townsville to Brampton Island race. In short, I have a huge respect for Lock I must, now my memories have been revived, contact Lock Crowther’s son, Brett to see if he has copies of the "Tempest" design.  

brian eiland Senior Member

Lock Crowther's Work sabahcat said: ↑ I put up some of his design notes and info on wave induced capsize on this thread here http://www.cruisersforum.com/forums/f48/lock-crowther-design-notes-as-promised-5910.html Click to expand...

Barba 22 New Member

Crowther Tempest 37 question Hello everyone, I have a question regarding a Crowther Tempest 37 trimaran. Can anyone give me some information on that boat? I found virtually nothing on the internet, so every bit of information will be very appreciated. The intended use would be cruising in the Indian Ocean. The actual boat can be seen here: http://tri4sale.net/ Thanks.  

Barba 22 said: ↑ Hello everyone, I have a question regarding a Crowther Tempest 37 trimaran. Can anyone give me some information on that boat? I found virtually nothing on the internet, so every bit of information will be very appreciated. The intended use would be cruising in the Indian Ocean. The actual boat can be seen here: http://tri4sale.net/ Thanks. Click to expand...

Gary Baigent Senior Member

Barba22, here is a crappy reproduction from an old AYRS.  

Attached Files:

Maelstrom.jpg.

Gary, Thanks for that copy. Sure brings back some memories of not only very pretty boats but very quick ones at that. I consider myself very fortunate to have sailed on designs by two pioneers of Tri's in Aussie land. Lock and Hedley Nicoll were the driving force of the industry at that time.  

catsketcher Senior Member

I never sailed the Tempest but had a Twiggy. The Tempest was in Lock's first attempt at a cruising tri range - Zephyr, Tempest, 38 foot Impala and a larger one as well. I thought the Tempest looked a little fine. It was drawn before Lock and John Hitch drew the Kraken 33 (Lock told me the design was a collaboration) After Bandersnatch he beefed up the designs. The tri will be an old design with little load carrying or volume inside for its length. cheers Phil  

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Hello and thanks for the input. It's very much appreciated. I understand that the boat doesn't have very much space inside but it's not so much of an issue in the tropics. It's the space outside that you want there. A small cabin to hide from the rain when sailing, is more or less sufficient. At anchor, you rig a peace of cloth over the boom and you're fine. However, I was not able to figure from those pictures how much space really there is on the deck. Also, with no daggerboards, I wonder about the capacty of that boat to sail to weather, especially in short, steep seas? The carrying/loading capacity is more of an issue than the size of the cabin. Even though I intend to sail "light" (no 40KW genset to run the 15' plasma TV...) , I have to carry some rice and beens, and some chain and anchors... Cheers  

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Crowther Buccaneer 37

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Crowther 45

So, take off your beret and replace it with a white hanky tied with a granny knot in each corner so it stays on your head. Now get out a leg of lamb. Coat it liberally with balsamic vinegar and then refrigerate it for 24 hours. Remove the leg and warm to room temperature. Mix Worcestershire sauce with plum jam until it is not too sweet. Stick some garlic cloves into the lamb, pour the sauce over it and sprinkle with pepper, fresh basil, rosemary and thyme. Add a little water to the pan to prevent sticking and the sauce from burning. Put into 400-degree oven, reduce to 350 degrees and cook to an internal temperature of 155 degrees. Remove from the oven and let sit for 15 minutes before carving. Do not undercook!

The biggest difference between the two cruising cats is that the Crowther weighs 13,650 pounds loaded. This gives the Crowther a D/L of 68.7 based upon a DWL of 41.5 feet. In short, the Crowther weighs about half of what the Bahia 46 weighs. Keep in mind that all published displacements are suspect. Designers tend to be optimistic when it comes to "as-built" weights.

This difference in displacements is reflected in the accommodations of the Crowther. Note that in this cat there are no accommodations in the hulls aft of the main saloon. The accommodations are designed for two couples, both sharing the head in the starboard hull. Galley and nav station are in the saloon. Unfortunately, the printed comments that came with this design do not match the drawings I have. The comments indicate that there are staterooms aft. I don't see them on the drawings.

Probably the most interesting aspect of this design is the fact that it was designed as a sportfisher. It's a big game-fishing boat based upon a previous Crowther design that operated off Tahiti. The deck drawings show fighting chairs aft above each boarding area. The cabintop is extended aft to cover the cockpit. There is a flying bridge above the cockpit. I wonder what this feels like. From my own big-cat sailing experience, I can tell you that larger cats tend to reduce the feel of sailing due to their already impressive height above the DWL. Certainly the view of the rig will be quite different, especially in a wild jibe.

The rig looks very efficient with a relatively high-aspect-ratio mainsail with lots of roach. Compare this to the relatively squatty main of the Bahia 46. Note also that this mast is drawn with prebend. Fishing under sail means no engine fumes wafting back into the cockpit, no engine vibrations and less fatigue due to the droning noise of the engines. Should the wind quit, you could rely upon the twin 100-horsepower engines coupled to saildrives to give this cat a speed in excess of 15 knots. This just might make the Crowther 45 the ultimate motorsailer.

Construction uses rigid PVC foam cores in a variety of thicknesses. Triaxial, biaxial and unidirectional glass is used for hull and deck in addition to structural bulkheads. Sandwich construction is used throughout the interior to keep the weight down.

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In addition, he has worked with Outremer (Outremer 45 and 49) and Marsaudon Composites (TS 42).

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Crowther 42

Crowther 42 is a 41 ′ 11 ″ / 12.8 m catamaran sailboat designed by Lock Crowther and built by Cat Factory (USA) starting in 2000.

Rig and Sails

Auxilary power, accomodations, calculations.

The theoretical maximum speed that a displacement hull can move efficiently through the water is determined by it's waterline length and displacement. It may be unable to reach this speed if the boat is underpowered or heavily loaded, though it may exceed this speed given enough power. Read more.

Classic hull speed formula:

Hull Speed = 1.34 x √LWL

Max Speed/Length ratio = 8.26 ÷ Displacement/Length ratio .311 Hull Speed = Max Speed/Length ratio x √LWL

Sail Area / Displacement Ratio

A measure of the power of the sails relative to the weight of the boat. The higher the number, the higher the performance, but the harder the boat will be to handle. This ratio is a "non-dimensional" value that facilitates comparisons between boats of different types and sizes. Read more.

SA/D = SA ÷ (D ÷ 64) 2/3

• SA : Sail area in square feet, derived by adding the mainsail area to 100% of the foretriangle area (the lateral area above the deck between the mast and the forestay).
• D : Displacement in pounds.

Ballast / Displacement Ratio

A measure of the stability of a boat's hull that suggests how well a monohull will stand up to its sails. The ballast displacement ratio indicates how much of the weight of a boat is placed for maximum stability against capsizing and is an indicator of stiffness and resistance to capsize.

Ballast / Displacement * 100

Displacement / Length Ratio

A measure of the weight of the boat relative to it's length at the waterline. The higher a boat’s D/L ratio, the more easily it will carry a load and the more comfortable its motion will be. The lower a boat's ratio is, the less power it takes to drive the boat to its nominal hull speed or beyond. Read more.

D/L = (D ÷ 2240) ÷ (0.01 x LWL)³

• D: Displacement of the boat in pounds.
• LWL: Waterline length in feet

Comfort Ratio

This ratio assess how quickly and abruptly a boat’s hull reacts to waves in a significant seaway, these being the elements of a boat’s motion most likely to cause seasickness. Read more.

Comfort ratio = D ÷ (.65 x (.7 LWL + .3 LOA) x Beam 1.33 )

• D: Displacement of the boat in pounds
• LOA: Length overall in feet
• Beam: Width of boat at the widest point in feet

Capsize Screening Formula

This formula attempts to indicate whether a given boat might be too wide and light to readily right itself after being overturned in extreme conditions. Read more.

CSV = Beam ÷ ³√(D / 64)

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The Bay Breeze is a 45’ Lock Crowther designed catamaran. At 23’ wide, the Bay Breeze is a very stable platform for sailing. Come out on a 2 hour public afternoon or sunset sail, or charter the boat privately with up to 25 of your closest friends and family. The Bay Breeze is a Coast Guard inspected and certified vessel. We sail out of Hyannis Inner Harbor into Lewis Bay and Nantucket Sound. The boat picks you up at 147 Lewis Bay Rd in Hyannis. You can google that address or “hospital boat ramp”. There are only 4 parking spaces there. If you want to try and get one, give yourself plenty of time to repark somewhere else in case they are taken.  If you park at 180 Ocean st, it’s an eight minute walk. You must use the pay at pay stations at Ocean st. It is usually always cooler on the water so please dress accordingly. You don’t need to print ticket. We will have your name in our reservation book.  You are welcome to bring a small cooler with drinks and snacks.

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Incat Crowther’s holistic approach to vessel procurement centres around an operation-specific design developed in partnership with the operator - Your Vision, Our Innovation. We perform an iterative process to communicate the project to stakeholders and allow the freedom to develop the project within set parameters. A functional design is then performed to define the vessel for certification.

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Catamaran Hull Design

• Post author By Rick
• Post date June 29, 2010
• 2 Comments on Catamaran Hull Design

Part 1: Notes from Richard Woods

Since the America’s Cup experimented with going multihull, there’s been a lot of interest in catamaran performance and the catamaran hull designs that define performance. Many guys are investigating whether to buy a catamaran or design and build their dream boat. Let it be said here that building a large catamaran is not for the faint of heart. People begin building 100s of boats a year, yet few are ever completed, as life always seems to have a way of interfering with a good boat build. 

Never the less, since the rest of this website is about selecting and buying a boat , it only seems fair to have at least one webpage that covers catamaran design. This page contains notes on boat hull design goals and an accompanying page from Terho Halme has mathematical formulas used in actual catamaran hull design. It has become a popular research stop and an important reference to the catamaran design community.

The content of this page was reproduced from the maestro of Catamaran designs, renown British naval architect, Richard Woods, who not only designs catamarans, he sails them across oceans…. repeatedly. He has a lot to say on the subject of catamaran hull design.

“…When it’ all said and done, the performance of a sailing catamaran is dependent on three primary specs: length, sail area and weight. If the boat is longer it generally means it’ a faster boat. If she has more sail area, it means she’ a faster boat and if she’ light it means she’ a faster boat.  Of course, there are limits: Too much sail area capsizes the boat in brisk winds. If the boat is designed too light, she will not take any kind of punishment. Too slim a hull design and the boat becomes a large Hobie Cat capable of only carrying your lunch. Of course, too long and large and you’d have to be Bill Gates to afford one. Then there are lot of additional and very important factors like underwater hull shape, aspect ratios of boards and sails, wet deck clearance, rotating or fixed rigging and so on….” Richard Woods

All Catamarans are not equal, but all sailboats have two things in common: They travel on water and they’re wind powered, so the Catamaran design equations in the 2nd part should apply to every catamaran from a heavy cruising Cat to a true ocean racer.

Richard Wood’s comments on catamaran design:

We all know that multihulls can be made faster by making them longer or lighter or by adding more sail. Those factors are the most important and why they are used as the basis of most rating rules. However using just those figures is a bit like determining a cars performance just by its hp and curbside weight. It would also imply that a Tornado would sail as fast forwards as backwards (OK, I know I just wrote that a Catalac went faster backwards than forwards)

So what next?? Weight and length can be combined into the Slenderness Ratio (SLR). But since most multihulls have similar Depth/WL beam ratios you can pretty much say the SLR equates to the LWL/BWL ratio. Typically this will be 8-10:1 for a slow cruising catamaran (or the main hull of most trimarans), 12-14:1 for a performance cruiser and 20:1 for an extreme racer.

So by and large faster boats have finer hulls. But the wetted surface area (WSA) increases proportionately as fineness increases (for a given displacement the half orange shape gives the least WSA) so fine hulls tend to be slower in low wind speeds.

The most important catamaran design hull shape factor, is the Prismatic Coefficient (Cp). This is a measure of the fullness of the ends of the hull. Instinctively you might think that fine ends would be faster as they would “cut through the water better”. But in fact you want a high Cp for high speeds. However everything is interrelated. If you have fine hulls you can use a lower Cp. Most monohulls have a Cp of 0.55- 0.57. And that is about right for displacement speeds.

However the key to Catamaran design is you need a higher Cp if you want to sail fast. So a multihull should be at least 0.61 and a heavy displacement multihull a bit higher still. It is difficult to get much over 0.67 without a very distorted hull shape or one with excessive WSA. So all multihulls should have a Cp between 0.61 and 0.65. None of this is very special or new. It has been well known by naval architects for at least 50 years.

There are various ways of achieving a high Cp. You could fit bulb bows (as Lock Crowther did). Note this bow is a bit different from those seen on ships (which work at very specific hull speeds – which are very low for their LOA). But one problem with them is that these tend to slam in a seaway. 

Another way is to have a very wide planing aft section. But that can increase WSA and leads to other problems I’ll mention in a minute. Finally you can flatten out the hull rocker (the keel shape seen from the side) and add a bustle aft. That is the approach I use, in part because that adds displacement aft, just where it is most needed.

I agree that a high Cp increases drag at low speeds. But at speeds over hull speed drag decreases dramatically on a high Cp boat relative to one with a low Cp. With the correct Cp drag can be reduced by over 10%. In other words you will go 10% faster (and that is a lot!) in the same wind and with the same sails as a boat with a unfavorable Cp. In light winds it is easy to overcome the extra drag because you have lots of stability and so can fly extra light weather sails.

The time you really need a high Cp boat is when beating to windward in a big sea. Then you don’t have the stability and really want to get to your destination fast. At least I do, I don’t mind slowly drifting along in a calm. But I hate “windward bashing”

But when you sail to windward the boat pitches. The sea isn’t like a test tank or a computer program. And here I agree with Evan. Immersed transoms will slow you down (that is why I use a narrower transom than most designers).

I also agree with Evan (and why not, he knows more about Volvo 60 design than nearly anyone else on the planet) in that I don’t think you should compare a catamaran hull to a monohull, even a racing one. Why chose a Volvo 60/Vendee boat with an immersed transom? Why not chose a 60ft Americas Cup boat with a narrow out of the water transom?? 

To be honest I haven’t use Michelet so cannot really comment. But I have tested model catamarans in a big test tank and I know how inaccurate tank test results can be. I cannot believe that a computer program will be better.

It would be easy to prove one way or the other though. A catamaran hull is much like a frigate hull (similar SLR, L/B ratios and Froude numbers) and there is plenty of data available for those. There is also a lot of data for the round bilge narrow non planing motorboats popular in the 1930’-50’s which again are similar to a single multihull hull.

One of the key findings I discovered with my tank test work was just how great the drag was due to wave interference between the hulls. Even a catamaran with a modern wide hull spacing had a drag increase of up to 20 % when compared to hulls at infinite spacing. One reason why just flying a hull is fast (the Cp increases when you do as well, which also helps). So you cannot just double the drag of a single hull and expect to get accurate results. And any speed prediction formula must include a windage factor if it is to give meaningful results.About 25 years ago we sailed two identical 24ft Striders next to each other. They were the same speed. Then we moved the crew of one boat to the bow. That boat IMMEDIATELY went ½ knot faster. That is why I now arrange the deck layout of my racing boats so that the crew can stay in front of the mast at all times, even when tacking or using the spinnaker.

I once raced against a bridge deck cabin catamaran whose skipper kept the 5 crew on the forward netting beam the whole race. He won.

Richard Woods of Woods Designs www.sailingcatamarans.com

• Tags Buying Advice , Catamaran Designers

Owner of a Catalac 8M and Catamaransite webmaster.

2 replies on “Catamaran Hull Design”

I totally agree with what you say. But Uli only talk sailing catamarans.

If only solar power. You need the very best. As limited watts. Hp.

The closer to 1-20 the better.

Closing the hulls to fit in cheaper marina berth. ?

You say not too close. But is that for sailing only.

Any comment is greatly appreciated

Kind regards Jeppe

Superb article

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Pittwater Online News

September 14 - 20, 2014: Issue 180

    Lock Crowther Multihull Regatta

Lock Crowther Multihull Regatta

by David Bishop and Alan Brand

Over the upcoming October long weekend, the Multihull Division of the Royal Motor Yacht Club at Newport will be hosting the annual Lock Crowther Multihull Regatta. 

This regatta is dedicated to the memory of Lachlan (Lock) Crowther - one of Australia's foremost, and a world renowned, multihull designer. 

Hosted by the RMYC since 1996, this year’s regatta includes an ocean race for suitably equipped boats on Friday 3 October as well as four inshore races on Saturday and Sunday 4 & 5 October.

The Regatta typically attracts many of Australia’s fastest multihulls, as well as cruisers, for three days of fabulous fun both on and off the water. Sponsors’ prizes are awarded at a sumptuous Regatta dinner for places in Racing, Cruising and OMR divisions. Arguably Australia's most friendly, fun Regatta! And this year we’ve issued an invitation to “off the beach” multihulls such as Hobie cats to compete as well. 

For the ninth year running, APC logistics, an Australian based international freight forwarding company, has signed on as principal and title regatta sponsor with Multihull Central/Seawind Catamarans providing major support for the cruising and social activities. Click HERE for a full list of regatta supporters. 

__________________________________

Lock Crowther was an all-round multihull designer who enjoyed great respect among his peers. His design career spanned more than 30 years and during that time more than 2500 Crowther Multihulls were launched. His first trimaran was designed and built in 1958. It was an 18 foot day sailor which Lock campaigned on Victoria’s Gippsland Lakes. 

The late 1960s saw the first of Lock’s Ocean racing and performance cruising trimarans known as Kraken, Zephyr, Tempest and Impala series ranging from 25 to 55 feet. His designs were very advanced in comparison with other designers of the time. During the early 1970s Lock further developed his ocean going trimarans and introduced the popular Buccaneer series ranging from the 24 foot bay racer to the open ocean 40 footer. Many Buccaneers are still racing today.

The first of his catamaran designs were introduced in the early 1970s, with the Spindrift 37 and 45 footers. During the 1980s and early 1990s Lock’s catamaran designs continued to develop and many of Australia’s well known catamarans such as Bagatelle, D-Flawless, XL2, John West, Windswept, Shotover, Top Gun and Room with a View were being campaigned on the Australian racing circuit. 

RMYC member Jim Geddes’ “Te Arawa”, a Crowther Spindrift 45.

His Catana design was produced as a production boat in France in 1987. Among his innovations was the introduction of drag reducing bulbous bows to catamarans.

During the 1970s and the 80s his trimaran development continued with the then radical Twiggy series. During the mid to late 80s the famous 40 footer Bullfrog was campaigned in over 30 ocean races throughout the pacific between the west coast of the USA and the east coast of Australia. 

Lock was also a great sailor campaigning many of his own designs. He was the winner of the 1969 New York to Bermuda race on his Kraken 40 design Ringo sailing the first 500 miles in 36 hours. In 1972 he won the Brisbane to Gladstone on the Kraken 40 design Captain Bligh which sailed through Cyclone Emily.

Lock Crowther died in 1993 at the age of 53 doing what he liked most - sailing his boat Deguello. 

Lock’s last boat “Deguello”

The first “Memorial” regatta in 1996 attracted 22 participants with 12 of the fleet being Crowther designs. The racing division was fittingly won by a Lock Crowther 40 foot catamaran Mighty Manfred owned by David Bishop and the Cruising division by a Crowther Seafire 33 owned by Horst Anna.

Winner of the first Lock Crowther Regatta in 1996, David Bishop’s “Mighty Manfred” a Crowther 40.

Over the years the regatta has attracted over 500 multihulls of all shapes and sizes from all over Australia – including Sean Langman’s ORMA 60 trimaran now named BOQ Team Australia . 

Sean Langman’s ORMA 60 trimaran “BOQ Team Australia”

Come down to the RMYC over the weekend for a closeup of some of these amazing sailing craft. Next week we’ll run a “bio” style article on Lock Crowther the man, including some interesting anecdotes to provide a more personal perspective of this amazing Australian multihull designer. 

Finally, a 5 minute highlights video of action in the 2011 Lock Crowther Regatta and full details and Entries on our Website

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Key Features

• Length: 16.46m/54ft
• Beam: 7.32m
• Draft: 1.25m
• Keel: Daggerboards
• Hull: Aluminium
• Location: Sydney
• Masthead sloop- 3 spreaders
• Mast: Aluminum 18m approx
• Lazy Jack Boom Bag
• Make: Volvo MD22L
• Horsepower: 50HP
• Drive: Sail Drive
• Fuel Type: Diesel
• Fuel Tank: 800L + 160L

Exterior Hull & Deck

• Winches: 8 Harken (2 Electric)
• Anchor Winch: Electric
• Gas Bottles 2 x 9.5kg
• Deck Wash: bow and stern
• Deck Shower- hot and cold
• Plotter: Raymarine + Simrad
• Solar Panels: 6 x 120W
• Wind Generator
• Shore Power

Interior Features

• Stove: 3 burner gas + grill + oven
• Sink: Double bowl S/S + single bowl in cockpit
• Water Tanks: 650L
• Fridge: Upright Fridge/Freezer in galley, chest fridge/freezer in cockpit
• Ice Maker in cockpit
• Berths: 3 Queen Berths, 1 single  + saloon converts to a double
• Toilet: Two electric
• Holding tank
• Air Conditioning in saloon
• Washing Machine

Additional Information

• Tender: Caribe 3.4m
• Outboard: 9.9HP Tohatsu
• Solid cockpit hardtop
• Roland digital piano in saloon

• Length: 15.24m (50ft)
• Draft: 2'10"
• Hull: Composite
• Location: Horizon Shores, QLD
• Mast: Aluminum 60ft
• Hydraulic in mast furler
• New jib furler
• Mainsail: Dacron, in mast furled
• Genoa: Dacron, furling
• Brand: Nisson/Diecon
• Horsepower: 33HP
• Drive: Shaft Drive
• Fuel Tank: 500L
• Winches: 6 Arco
• BBQ: Stainless Steel
• Gas Bottles: Yes
• Sounder: new
• Plotter: new
• Autopilot: new
• Solar Panels: 4
• Stove: 2 burner + oven
• Sink: Stainless Steel Twin
• Water Tanks: 500L
• Fridge: 12V Eutectic custom made
• Freezer: 12V Eutectic custom made
• Berths: 2 Queen Berths + 1 Double

Lock Crowther’s most successful off shore design, this proven ocean cruising cat has 3 double cabins; beautiful Tasmanian oak interior fit out; large saloon for entertaining with custom timber table; well appointed galley includes oven and stove large custom fridge & freezer. Twin 33 hp shaft drive diesels gives 8 knots when cruising. Hydraulic furling mainsail, furling genoa plus spinnaker; Dagger boards for good windward performance a must when doing long ocean passages. Ideal HUGE family cruiser set up comfortable, easy to handle ocean sailing.

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Trimaran Projects and Multihull News

Trimaran project is intended to showcase multihull news with a focus on racing, build projects, launchings, interesting ideas past and present and also updates on my own trimaran projects. Lots of content and updates to come so keep checking back.

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Crowther Design 85 MK2

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