Multihull Structure Thoughts

Catsketcher, thank you for your contribution we all learn something from other people's innovation and hard work.

The alternative way of doing a transportable catamaran is represented by the WALLER TC 750 (yes Tim Clissold TC 750 is not the (Trailable Catamaran) TC 750). In Australia you can get day light hours towing permits for boats up to 3.5 meters wide for travel through the majority of areas. The trailer can only be 2.5 meters wide but the boat can be 3.5 meters wide. Check your local state regulations. The TC 750 is 24.5 x 11.3 foot weighing 3000 lbs displacing 3700 lbs with a maximum displacement of 4500 lbs. TC 750 carries a 250 square foot mainsail and 150 square foot head sail.

The TC 750 was designed as an easy to build and sail, low cost cat, designed for a family of 4. The cabin features a full size double berth, a good sized galley with full 6 foot headroom, and a small dinette area around which 3-4 can sit in comfort. There is also a separate toilet compartment with room for a small shower, as well as two large quarter berths aft. The bridge deck has 4.5 foot headroom.

The multi chine hull (flat bottom with 2 chines) is constructed from 9 mm Gaboon (okume) sheet ply over 12 mm plywood bulkheads, longitudinal stringers and with minimal timber framing. The whole boat is covered with 300 gsm cloth and epoxy. The hulls and under wing are built as one unit upside down and then turned over, to complete the cabin decks etc. Construction employs the timber / epoxy, it is strong to stand up to the rigors of trailering, and is simple and straight forward with the entire shell built as a single unit.

The rig is a simple 7/8 design, with non rotating spars and utilizing off the shelf fittings. It can be home built if desired. The rig features an asymmetrical spinnaker set from a bow pole, but a normal symmetrical spinnaker could be used if desired. This yacht has twin dagger boards and rudders.

The boat should be a safe and reliable performer, with an emphasis on good cruising performance and accommodation rather than all out racing. Note that its intended area of use is sheltered and semi sheltered waters, and not open water.

Note that this vessel is wider than the standard road trailing width for most areas. Normally this yacht would live on a trailer frame at the local yacht club (much cheaper than a berth or mooring).

 

We have discussed a few trailable cats and tri’s so I thought we need to sort out what does beam really mean in terms of sailing in “real” world conditions. Unfortunately, this is a mathematical exercise to highlight the theoretical differences and does not take into account the skill of the sailor or the flexibility of the rig. Some sailors can sail a cat that should overturn in 20 knot winds in 25 knots due to there skill to feather the rig at the right time etc. If you have a flexible rig with eg a fat head main, the top of the main can be made to twist off in stronger gusts etc. Also wave action is not included in this calculation. Just be aware according to the things I have read that once the wave height is greater than the beam of the multi, you have a problem, especially if you have under 100 % buoyancy of tri floats.

For demonstration purposes we will compare the Waller TC 670 (22 x 8 foot beam), Waller TC 750 (24.5 x 11.3 foot beam), Tim Clissold TC 750 (24.5 x 18 foot folds to 8.5 foot) and 2 “pure cruisers” Tiki 26 (26 x 15 foot beam), Heavenly twins (26 x 13.5 foot beam). I will add a trimaran Tremolino TGull 2300 (23 x 18 beam folds to 8 foot) for another view. Now all of these boats have different displacements and sail areas so I will add a final feature which is a performance calculation of a Bruce Number and “average speed over 24 hour” calculation based of Richard Bohemers work.

The Waller TC 670 has an 8 ft beam 2400 lbs displacement and can handle a maximum of 19.5 knots wind speed. Its Bruce nbr is 1.17 and can average 7.9 knots over 24 hours.

The Waller TC 750 has an 11.3 ft beam 3800 lbs displacement and can handle a maximum of 20.5 knots wind speed. Its Bruce nbr is 1.28 and can average 8.53 knots over 24 hours.

The Clissold TC 750 has an 18 ft beam 2800 lbs displacement and can handle a maximum of 23 knots wind speed. Its Bruce nbr is 1.42 and can average 9.43knots over 24 hours.

The Tiki 26 has a 15 ft beam 3000 lbs displacement and can handle a maximum of 29.5 knots wind speed. Its Bruce nbr is 1.17 and can average 7.9 knots over 24 hours.

The Heavenly Twins has a 13.5 ft beam 7000 lbs displacement and can handle a maximum of 34.5 knots wind speed. Its Bruce nbr is 0.84 and can average 6.1 knots over 24 hours.

The Tremolino T Gull has an 18 ft beam 2020 lbs displacement and can handle a maximum of 26.5 knots wind speed. Its Bruce nbr is 1.37 and can average 8.6 knots over 24 hours.

Summary. The greater the beam the “safer” you are. The greater the weight the safer you are. BUT like all things in sailing it is the balance that is important. The heaviest boat is the safest but also the slowest. The Tremolino is the lightest and has the widest beam but has 100 square foot less sail compared to the Clissold TC 750 result not as fast in theory. In reality especially in light winds it would be a close thing. So, the reality is this the TC 670 is a bay sailor, both TC 750’s are coastal capable. The Tiki 26 and Heavenly Twins are ocean capable but slower.

 

The Slatts 22 is another variation on an unusual trailer sailor. This boat needs speed to maintain its stability as it is partially foil stabilised. It is an asymmetrical tacking, partially foil stabilised proa/catamaran day sailor. OK? The boat is 22 x 12 foot weights 390 lbs and carries 228 square foot of sail area on a 28.5 foot aluminium mast. The boat can take 2 crew comfortably. The idea by the designer Joe Slattebo was to create a fast day sailor for gentlemen not requiring hiking out etc. The submerged T foil is a NACA 012 section of 2.5 square foot area has an angle control in the cockpit via a morse cable. The angle control can move the foil plus or minus angle of attack over a 22 degree range. On one tack the foil provides lift on the other tack the angle of the foil is put down and the foil pulls the float down under the crews control. According to a report in a 1989 Multihulls the boat can sail at about the same speed as a Hobie 16 without needing to hike out etc.

The boat is built of 6 mm divinycell foam with unidirectional and bidirectional fabrics in polyester resin. The foil is foam glass. The cross arms and foil support strut are 6061 aluminium.

The Raptor 16, Raptor 17.5 which were later versions of the Slatts 22 were all made the companies associated with John Slattebo. All 3 boats were asymmetric hydrofoil stabilised boats. Further information can be found at:

Sailing 1 http://www.signaldesign.net/slattssailing1.html

 

Now we will take a couple of items to discuss a true ocean crossing (USA to NZ) home built foiling trimaran that was designed and built 50 years ago. Dave Keiper the designer, builder and sailor wrote a book of his experiences “Hydrofoil Voyager”. So you say “so what” modern foiling tri cross oceans now. Do they do it without any foil adjustments, whilst single handed and self steering without any wind vane or electronic aids? This boat was ground breaking. Dave is no longer with us but I will quote him from a variety of sources. The first part is about the boat build and initial trials. The second part will focus on the foils and the initial crossing to Hawaii.

“The tri is now named WILLIWAW and is 31 ft long overall, and I' m expecting a total displacement of 3000 lbs, including two persons and their supplies. In addition to designing a foil system that should give a lift/drag ratio of 14 or 15 at take-off, I had to work out a rigid but lightweight method of construct ion, and an improved sail rig. Calculations indicate that a 13 knot wind will be required to become fully foil borne. Lacking that wind, the boat can be operated as an efficient trimaran by retracting the foils. The abbreviated pontoons are located forward of a mid-ships and serve for initial stability and for structural fastening points for shrouds, bow foil and lateral stabilizing foils. The aluminium foils have a 150 mm chord length, and will be set with minimum dihedral of 30°. The bow foil will span the width of the boat and will thus have a very high aspect ratio. It will be set for a fairly high lift coefficient at the take-off speed of 12 knots.”

The hull construction is curved frame members (every 500 mm) and planking are of 6 mm plywood. Angle blocks spaced every 150 mm along the frames fasten frames and planking together (initially it had no stringers just triangular connecting blocks. Stringers came later). Bottom and transom will be of 12 mm in plywood. There will be a thin fibreglass skin over the whole boat.

“I have less surface area to plank, fibreglass, and paint than a standard trimaran, and also no built-up cabin structure (5 ft headroom). The hull weighs only I,300-1,400 lbs, but is extremely rigid because of its proportions and its doubly-curved plywood. Practically all of its weight contributed structurally, including inside shelves and benches. The living quarters appear spacious, since they run the full length and width.

I have sailed the craft once so far, in a light air, without hydrofoils. She balances and manoeuvers well. However, it is so easy to get confused about wind direction, because it generates its own wind going upwind, and kills its wind downwind. When a Force 3 wind is generated close-hauled, with 380 sq ft of sail, the craft heels about 15°, and the underbelly adjacent to the leeward pontoon begins planning." (Normally, a leeward hydrofoil would prevent such a heel.) Hydrofoils will be fabricated for WILLWAW next month (Jan. '67). The hydrofoils will add 400 lbs to the craft, but this isn't much more than the weight saved by not having large pontoons.”

Now we discuss the floats. The initial floats were short under the theory the foils would be the main righting moment. The buoyancy was 600 lbs. The Floats were then lengthened to have 1200 lbs buoyancy. Before WILLIWAW left to sail across the Pacific the floats buoyancy was increased to 2000 lbs as Dave realized that WILLIWAW at low speeds with little foil lift could capsize. Dave reported in initial trials the following:

"All in all, this 3000 lb craft has the feel of a 10 ton yacht in light winds, except that when coming into a dock one can put a foot to stop the boat without breaking a leg. 1. Had a couple of hair-raising experiences with the boat earlier- once a capsize with no hydrofoils, and once a wild 60 mile ride with a single lateral stabilizing foil. Before any of my hydrofoils were fabricated, I was testing the boat and succeeded in capsizing it. The capsize was not planned, but I learned much from it. lt occurred in a 20-26 knot gust of wind with 340 sq ft of sail up. At the time, the craft weighed about 1600 lbs. The capsize was gentle, the boat capsizing 'backwards' (bow lifting skywards) because of the rather far-forward pontoons. The mast trapped a column of air, and the boat settled at a 100° heel. The boat was righted easily with assistance from a power cruiser. There was no damage. Then I started making calculations of what the righting moments would be with the hydrofoils installed. Lo and behold, it looks as if the craft should be self-righting with the hydrofoils in operating position and the sails aloft. This results from several factors: (I) the low e.g. and 400 lbs weight of the Aluminium hydrofoils, (2) the small pontoons, and (3) the high and rather buoyant wing section connecting the hulls. At any rate, after a capsize, several factors, one or a combination of them, would certainly right the craft: (I) lowering the sails, (2) windage on the skyward pontoon after the boat swings around, and (3) a crew member hiking out on one of hydrofoil ladders. However, I'm not planning any such experiments in these icy waters. I modified the design of the foil units so that they could withstand the maximum possible water force (which amounts to about 200 lbs per sq ft of surface). Now, with good structure, I feel a bit more confident when taking the boat out for tests."

 

The next WILLIWAW section is about the foils and the final trials before the trip to Hawaii. Williwaw sailed a total of 20,000 ocean miles from USA to NZ via Tonga, Cook Islands, Tahiti etc. Williwaw was lost at anchor in Hawaii on the return trip. Williwaw had a 16.3 foot beam. WILLIWAW could do about 15 knots in true winds that didn't get over 10 or 12 knots.

“Here are a few figures on foil areas. Floating foil area projected on the water surface is about 18 sq ft. Actual foil area is about 15 per cent greater, because of dihedral. If you want combined foil and strut areas, you will have to tack on an additional 50 per cent. The take-off area, considered with the craft heeling at 8 degrees and the bow lifted 2 degrees, projected onto the water surface, is 12 sq ft. To get actual areas, tack on the same percentage. At high speed, foil area will decrease as the square of the speed, except that all blading within one chord length of the surface will be rather ineffective. To get efficient lift at high speed in choppy waters requires foils running at a depth of 2 or more chord lengths below the surface. My lift coefficients decrease with speed because the bow comes back down to normal trim. Some more geometry may be of interest: Separation between bow and stern foils is 26 ft. Lateral foil lift is roughly lateral from the sail centre of effort, a distance of 9 ft. The lateral foil is 9 ft aft of the bow foil. I would caution anyone from using these dimensions unless they can get an overall foil lift to drag ratio of at least 13. With lower lift/drag, all spacing’s should be increased. I've noticed that, if my foils are rough, or I've hooked a lot of grass, that WILLIWAW handles poorly. When everything is right, and the wind is non-turbulent, she takes off relatively quietly, like a beautiful bird. In strong turbulent winds, she will labour in taking off, as when you took a ride. Probably, much of the foil vibration results from the non-standard strut sections.

The secret of the success of my fixed 4 foil configuration is that it separates the problems of lateral and longitudinal stability. The bow and stern foil combination handles the problems of sail pitching moment and of changes in water incidence due to wave encounter. The leeward lateral foil opposes the sail heeling moment and the leeway force. The windward lateral foil is
normally out of the water but, if the bow foil gets negative incidence, it re-enters the water and helps the other foils to prevent the craft from re-entering the water at a steep angle. At high speed, the leeward lateral foil carries much of the craft weight, while the bow and stern foil combination acts like a "sensor" to maintain proper angles on the heavily loaded lateral foil.”

Later trials with full foils resulted in: “We tacked for a couple of hours to get out of the Golden Gate with light headwinds. We then met WSW winds, which later picked up in the gusts to 25 to 30 knots, with steep, often confused, breaking seas. We took the wind on the beam, and later more aft as conditions worsened. We reefed the mainsail at 2 p.m. For 10 hours, we averaged better than 10 knots. We were flying occasionally at 18 knots to 20 knots across lumpy sea platforms, but wallowed a bit too long in the troughs, trying to take off. (I later discovered that my bottom was not smooth, which, along with the heavy load, would explain take-off difficulties.) At dusk, we dropped the mainsail completely, and got the boat self-steering at reduced speed. Through the night, we checked occasionally for freighter traffic. At times, I think it must have been blowing a gale. Early in the morning WILLIWAW was laid over to an 80° heel by a monstrous wave. I hung from my bunk with hands and feet. She restored herself quickly as we heard the thunder of the wave breaking . . . We covered as many miles that day as I ever did in NIMBLE, yet on WILLIWAW, I was carrying much less sail area, and had the boat self-steering for 12 of those hours. We could have done better with moderate conditions. Only a huge heavy yacht would have done better than WILLIWAW in comfort. I made the mistake of not setting the windward lateral foil on the first day, since we would be on the same tack for four days. This was an unsuccessful experiment, for on beam reaches in the rough stuff, the windward lateral foil serves a very useful purpose (I later found out) in that it lifts up the windward side when an angry wave comes in to break on the side.”

On a later trial the following happened: “On the bow foil, some Aluminium shear bolts failed from fatigue, and we were treated to the spine-chilling experience of having the bow foil forcefully retract while we were doing 20 knots. My crew member witnessed it, but I was down below and didn't realize what had happened, since WILLIWAW only nosed down a few degrees and re-entered the water like a decorous lady. At the moment, I'm redesigning the foil hardware for easier handling and better fastening. Of course, on this prototype, I couldn't spend much time with such matters, it being most important to test the overall configuration and basic structures. I'm very satisfied with my foil configuration, and we didn't suffer any sort of cracks in the welds of the Aluminium foils, in spite of the heavy conditions. Most of the time, we were self-steering, and doing it without vane gear.”

The Hawaii trip: “We had a 16 day passage in WILLIWAW (Sept. 4-Sept. 20) from Sausalito to Kahului Harbour on the island of Maui, the shortest distance between the two being 2,040 miles. The daily direct distance average thus is 127.1 miles per day. Overall, it was the easiest passage I've ever had, though we had pretty rough weather for the first day out of San Francisco. The hydrofoils contributed significantly to it being an easy passage, assisting in maintaining control in heavy weather, contributing to self steering (we self-steered almost all the way), contributing to comfort (the motion was more like that of gliding than the usual roll, pitch and yaw). The foils contributed to speed the first few days of the voyage where we had consistent strong and moderate winds. After that, we had variable winds, a day or two of calm and a week of very weak Trade Winds. The last few days, we had light to moderate Trade Winds, strong during rain squalls. When the wind died on us, we pulled up the foils but the chop was so bad, I almost got thrown off the deck a couple of times. We re-set the foils for comfort, even though we lost speed that way. When the seas flattened, we retracted the foils again. More important than the speed potential is the potential for comfort, control and self-steering. The fantastic light air performance of the trimaran is preserved by retracting the foils, but in the moderate and heavy stuff, the foils get rid of the pounding, tunnel interference, quick motion and broaching of the trimaran.”

 

The final in the WILLIWAW story. The next foiling business venture for Dave was the design and production of foils for smaller beach catamarans under the name of DAK. Dave only delivered 2 sets of foils for Hobie 14’s before is untimely death. His son carried on the business for a while changing the small cat aluminium foil sets from 50 mm chords to 75 mm chords to allow easier take off. Also their were several larger designs advertised on the following site: DAK HYDROFOILS - NEW DESIGNS http://www.josephoster.com/dakh/newdesigns.html

I do not know how complete the designs were or if any were built. The general trend appears to be wider beams on the 32 foot tri, with a 24 and 20 foot smaller tris for mini cruising. Also DAK foils were “available” for smaller beach cats. I do not know if the business is still a going concern. The jpeg is of Dave and a beach cat foil.

So, a summary. WILLIWAW was way ahead of its time. Even today there are designers trying to achieve what Dave did as “normal”. BUT if your aim is to sail faster think again. WILLIWAW averaged 127 miles/day. A good modern multi of the same size without foils would average that. Next problem is best described by Dave who said after part of his Pacific journey, in over 22 days of sailing he had only 3 days of real foiling. The wind average was often lighter than, a loaded down for cruising WILLIWAW, required for foiling take off. Finally, the ocean is loaded with a lot more trash now than 50 years ago and a boat foiling does not like garbage on its foils.

I could not find any statements about the actual foil shape used by Dave. I can only talk in general terms but there are 3 general sections used in foil design: an Ogival, NACA 63-412 or a NACA 4412 profile. Each of these profiles give different performance and take off characteristics. What is required is a good lift drag ratio on a subcavitating foil section. A book “The Theory of Wing Sections” gives details of the shape of these sections and their characteristics. America’s Cup foils are now specially designed sections that work within a narrow wind speed and boat speed range whereas cruising foilers have to work across a wide wind speed and boat speed range. Foil design requires a real understanding of what you want from a boat, but if you get it right the rewards are spectacular.

 

Mantis IV is regarded as the first large Bruce foiler that was “ocean capable”. Mantis IV was designed and built by David Chinery in 1970. What is a Bruce foiler? Edmund Bruce had a great mind developing boating ideas and had the maths capability to put number to his theories. He realised that a flat angled plate placed at 45 degrees in the position of a eg tri float could be provide lift or downward force on either tack. The lift or downward force would be created by the leeway angle which created an angle of attack to the flat/plate foil. Bruce original work suggested a single plate on one side of the boat would be sufficient as the plate could provide lift or down force. Many people built asymmetric boats to Bruce’s original specs and they worked as specified in minimal seas. The real problem came in larger seas where the foil/plate was pulling down, the foil/plate could pop out the front of a wave face resulting in no righting moment. It happened rarely but swimming practice was required. The solution was to build tri configuration Bruce foilers with a foil on both sides of the main hull.

Mantis IV is 34.3 x 23.5 foot weighing 2150 lbs carrying about 500 square foot of sail in a ketch rig. (Borrowed C class and A class sails and masts). The main hull was built from 4 mm tortured ply with minimal reinforcement which caused problems as it bubbled in the build. Next time same specs but slightly more reinforcement. The build took 560 hours. The float foils were 16 foot long and have a 20:1 Ogival foil shape. The immersed foil area required is in light weather 4% of sail area, in heavy airs the immersed foil area should be 8% of the sail area. Mantis IV started in the Round Britain 1974 race and pulled out due to rig failure. But the boats performance over several trips was not great. It was capable of sailing, it had the stability required for normal and storm sailing but could only achieve 6 knots to windward and 10 to 12 knots on other points of sail. It sails very flat.

Mantis started a small trend with a few being built but the most travelled is Diablo a 35 foot Bruce foiler developed in Australia by Joe Dobler. It was a cruiser that was shipped to Europe, sailed the Mediterranean and was used as a charter boat for years. Joe progressively developed the boat increasing the foil size and volume and improving its capability but it was not fast.

Finally, a guy “patented” the concept in 1975 then produced a 21 foot day sailing version. The patent killed any commercial interest in the concept. Overall this was an interesting deviation along the road of foiling boats which could be developed further, but I suspect conventional modern cats and tris have so much performance capability that this may be an interesting piece of yachting history. The jpegs are of Mantis, Diablo, 21 ft Holtam foiler and the patent.

 

Bob Oram is an Australian designer who came to notice when he designed a 23 x 8 foot catamaran that was a “cut down” Geudgon 32. It was original in design and build, was plywood and was fast. Bob then went on to design a series of cats which all had common characteristics. They were long, light and narrow for there length with moderate accommodation. Generally fast sailing boats that were easy to build. Then public demand got to him. Could you give me a bit more accommodation in the same length, I need to have more displacement to carry things. Bob being a good designer managed to still maintain performance as the boats got wider and heavier but they became more complex. Instead of hulls that had 13 : 1 length to beam they became 10 :1 in some designs. To make the boats as easy as possible to build he employs flat panel foam glass building techniques or plywood etc. He also provides plans for things like cleats, turning blocks, composite hinges, dinghy’s, cheap booms etc to lower building costs as he realises the shell costs are only part of the total cost.

Bobs latest creation is a 32 x 21 foot cruising catamaran for a NZ guy. It displaces 8000 lbs and carries 580 square foot of sail in the main and jib. It has near full headroom in the main saloon which has a dinette and full headroom in the hulls which contain the galley loo and 2 double berths aft beside the aft cockpit. There is also a forward cockpit for ease of sail handling etc. This is a nice design. A choice of foam glass or ply construction. Jpegs give the idea.

 

The Searail 19 was conceived over 20 years ago but took at least 10 years to get fully designed and into production. The tri is 19 x 14.5 foot with a folded beam of 7 foot. The tri weighs 700 lbs with sailing gear onboard. The rig is basically a Hobie 18 rig with a 29.9 foot mast carrying a 176 square foot main and an 83 square foot jib. The spinnaker reacher is 278 square foot. The cap shrouds are synthetic. There is a mast raising system. The towing weight is 1100 lbs including a light trailer.

Nigel Irens designed the lines and geometry of this tri which is made of foam glass and vinylester resin. The lightest version has removable carbon fibre cross arms and floats that weigh about 100 lbs that slide as a single unit into sockets on the main hull. There is also a folder version that has carbon cross beams with aluminium lower folding control arms that act as water stay arms for the on water folding version. The folding version is easy to set up requiring 2 bolts /beam to lock it down and the geometry worked out well due to lower folding control arms. The tri had larger floats produced in 2014 with 30% more buoyancy. The folding system on this boat needs to be studied as its one of the best I have seen and could be replicated on other designs. Please look at the Searail web site for a video of the boat being unfolded. The dagger board and rudder are composite construction with an Aluminium rudder case and tiller handle. The small cabin sleeps one at best with a lockable watertight companionway hatch. Only about 20 made but still available at cheap base price. This tri can really perform. 9 knot wind speed produces 13 knots boat speed. The tri has has exceed 20 knots (claimed max 24 knots) on many occasions. In light airs with a light crew, it is capable of out sailing many popular production tris and beach cats. This tri is a very good fast bay sailor and one design racer, not a coastal cruiser.

 

The following is based on fact. I am reluctant to name yacht, designer, builder and skipper details as it is the experience that is important, not the blame game. The boat is a 42 foot bridge deck cabin cat, designed and built by reputable people. This was the initial production boat and was running 3 months late on delivery. The owner had been invited to do final acceptance after the sales person/designer did a final test of the boat in heavy winter weather. During the final test the boat was found to have several leaks through 2 windows and there was a problem with one end of a self tacking jib track which had been provided with an undersized block facility. The owner had arrived with gear expecting to take ownership of the cat and take it to warmer climates. The sales person/designer said there would be a delay to fix the window leaks and several weeks delay for the correct blocks etc for the self tacking jib arrange to be provided from the block company. The owner went home and left the delivery to a professional skipper and a crew who all had time constraints. The leaks were “fixed” in cold weather. The self tacking arrangement was a calculated risk with one end having some undersized equipment. The delivery skipper knew of the problem and knew on one tack he could not push the boat.

On the delivery trip the cat hit a winter storm with big seas. The cat’s self tacking track bad end equipment failed not allowing the cat under much reduced jib and main to power over sea’s on one tack. A rouge sea larger than others arrived, the cat was on the wrong tack and did not have the power to climb over the sea and was pushed backward down the wave. A rudder was bent and lock into an angled position as the cat was pushed backward down the wave. The cat could only sail in circles after that in very bad conditions. As the boat was only 70 miles of the coast, the skipper called for help, the crew were taken off the boat and the cat was abandoned. The rudder system on this boat was over built and specified by a famous steering system company. The cat did not have any substantial tools required to fix the rudder system, EG full size wrenches, hammers etc. due to the short preparation time available to the owner and delivery skipper.

The lessons. Do not go serious sailing in a boat that is not fully prepared to take on any sailing conditions. Assume the worst and take a full tool kit to at least attempt to fix any issues that may occur. Do not go sailing in difficult weather conditions with a time constrained crew who “must” be somewhere by a certain date. The delivery skipper was very experienced and was reluctant to push on but crew constraints limited his options. Have the equipment and knowledge to park a cat in a storm EG a parachute or drogues to slow the boat through the really bad weather.

Finally, the rudder system. The rudders were over specified and over built compared to any 42 foot cat I have sailed on. But a rudder shaft was bent locking the rudder into a position. This can happen to MANY cats. I know one very famous designer who deliberately designed his rudders to bend if the helm was thrown hard over at a boat speed in excess of 25 knots. It was better to bend a rudder than capsize the boat by cartwheeling it out of control down a wave face. Also, there is a limit to how strong you make a rudder system before it is to thick/big and heavy that it effects the boats performance. On this boat the tiller arm was 25 x 100 mm by 600 mm long solid stainless steel. Absurdly strong for a 42 foot cat.

I have been on a 37 foot cat that had to tack on a wave front and reversed a rudder as it was pushed back down a wave. We had the crew and equipment to quickly fix the reversed rudder and sailed home with no problem. A boat sailing in stormy conditions must be prepared for the worst and capable of looking after itself as others should not have to risk their lives to save yours. The jpegs are samples of bent rudders (not related to this cat).

 

Some of those rudder stocks shown in the pics look thin. Using solid 30mm diameter shaft gives a moment of inertia of 40 000mm^4. Using a 50mm 4mm wall shaft gives a moment of 154 000mm^4 with less weight. Even a 40mm solid shaft only gets you 125 000mm^4.

The rudders shown here look as though the builders have used thinner solid shafts. This is not clever engineering. Go with the largest shafts you can - for a 42 ft cat this should be around 50 mm minimum. Change your rudder profile if necessary.

I think the idea of getting a rudder to fold at 25 knots is very dodgy engineering and design. It is a little like an aeronautical engineer having the elevators fall off a plane when the G rating gets high. Losing control is the last thing anyone would want in a fast moving vehicle. Trust the sailor and get the boat to do what they want.

The 42 ft cat that bent its rudder had 1.5 inch rudder posts ( 38mm) which gives it a moment of inertia of 102 000mm^4. The builders could have had a lighter and stronger shaft with a 50mm hollow shaft.

In the instance Old Multi and I are talking about the large steering arm did not fail, the failure occurred in the attachment of the arm to the rudder shaft. This slipped and was held with bolts - no custom end or square fit to the top of a machined shaft - just a friction fit onto a circular shaft. The designer actually said that this was to help slip if overloaded. It did get overloaded when moving backwards, then bent the undersize shaft and then led to the crew abandoning the boat.

So make your rudders nice and strong, use cassettes if you want a failsafe and remember about moments of inertia.

Some links for those interested

ABANDONING BE GOOD TOO: The Skipper Responds to the Builder's Response - Sailfeed https://www.sailfeed.com/2014/07/abandoning-be-good-too-the-skipper-responds-to-the-builders-response/

ABANDONING BE GOOD TOO: The Builder Responds - Wave Train https://wavetrain.net/2014/05/16/abandoning-be-good-too-the-builder-responds/

 

This boat is for those who have grand ambitions but like wood. It was designed 40 years ago by Frank Pelin who did a series of cats from day sailors to this full blown cruiser. The majority of his cats sailed well and were structurally sound if built to plan. Do not underestimate the build time for a boat this size. Unless you have serious help we are talking 5 to 10 years of 50 hour weeks. Free Spirit is a centre cockpit bridge deck cat of 49 x 24 foot displacing (probably weight) 17300 lbs with a 57 foot mast and a cutter rig carrying a 514 square foot main, 500 square foot jib and a 237 square foot staysail. The underwing clearance is 2.5 foot which is just enough if the boat is not overloaded. The hulls have a length to beam ratio of 12.8 : 1.

The cat is plywood wood design that will have to be built of the lightest materials you can EG Okume ply and not overloaded with any unnecessary gear or stores etc. It has a simple a dory bottom hulls built of 2 layers of 7.5 mm ply over 100 x 38 mm sawn frames and 50 x 25 mm stringers. The bulkheads are 12 mm ply. The cross beam bulkheads are ply timber box structures. The decks and cabins are 2 layers of 6 mm ply.

3 improvements could be done. Built it with epoxy glues and change the rudders from transom hung to under the hulls with a spade or skeg hung rudder. Frank also did not do centre boards or keels on many of his designs depending on the hull shape to provide lateral resistance. A chat to a current designer should provide a plan for a fixed keel or daggerboard which will improve the cats performance.

 

Just in case it has not been said clearly enough there is also another reason for not making a rudder too strong. Which would you prefer? A bent rudder or a hole in the back of your boat where your very strong rudder survived but the the hull it was attached to has been torn apart by the strong rudder. This is about balance and trade offs again. As Kurt Hughes reported when a big charter cat was trying to berth the skipper had the helm hard over and the skipper push the throttles down hard to swing the stern. The 260 HP desil provided enough propeller thrust on the rudder to bend the rudder shaft on one side. Kurt strengthen the rudders for that design. Kurt is a very good designer and I don't think anyone would have seen that one coming.

 

I'm with Phil here, a larger diameter hollow shaft is the way to go. Dick Newick would double the thickness at the area where the shaft exits the hull and enters the rudder by inserting another hollow shaft inside the main shaft to handle the zone that gets the max bending loads.

A kickup section or cassette means in a worse case scenario you still have a "transom" forward of the rudder zone if there is a fail with the kickup.

 

I think that is the way to go Cav. Either a cassette or a small bulkhead in front of the rudder. That way you get the best of all worlds and putting in a small bulkhead, even if it not is not totally full height is good insurance.

The problem to me is that rudder loads are really hard to work out. To tell the truth, any loads are really hard to work out. On my cassettes I had to play around heaps until I found something that would fuse properly - in the end I use 15mm plastic hot water pipe. It goes just when I want - hitting something, but not surfing or hitting 18 knots. But it took a lot of experimentation to get it right. I am sure that you cannot design a rudder to fold just before the hull gets impacted.

Even cassettes are problematic. They are designed to rotate when going forwards - most of the time you do that. But in shallow water, you go slow, picking your way through the shoals, or you drag anchor and you go sideways. Or like I did, you don't compute the tide drop and come back to find your cat has sat on one of its rudders and you can see a slight bend in it, but it works just fine and so you keep going on your cruise. Glad it was strong enough to take the weight of the boat on an uneven bottom.

To think that we know the loads on our boats gets a little conceited. Dinghies alongside, running into a coral bommie, bouncing off a mooring pile, pulling the boat from the masthead (monos only), bouncing on the bottom as you ground in a chop, going aground sideways in a gale with the rudders down, jacking your boat on one point only (with lots of plates) - these are loads that I have put on boats. I think that these are major stresses our boat and are not calculated by marine engineers. Its almost impossible to do so.

That is why the evolution of our boats is driven mostly by observant designers and builders who can see the issues that some designs have and how other designs cater properly with these same issues. The diversity of boats allows us to observe the way different traits handle their jobs. Successful traits are propagated in the next generation. Design is a lot about observing and incorporating traits that work in a new design. You shouldn't do a lot of design from first principles.

It is not that hard to find a middle ground here. If you really are worried about putting a hole in your boat there is a proven way to stop this - its a skeg. Searunners and lots of other great boats use skeg rudders to hold the rudder with great strength. Going skegless, cassette less and bulkhead free is a road you can take but there seems so little reason to do so when proven methods exist to stop you from ripping a big hole anywhere.

In fact I don't get the new Outremers that have skeg free rudders and diesel legs with centreboards. How do the things dry out in the sand? I couldn't cruise such a boat. I couldn't handle not being able to hang in Percy Lagoon for a week when I wanted to. For me this a really dumb trait.