Multihull Structure Thoughts

I have spoken before about chine versus round bilge hull lines. The following Buccaneer 33 trimaran hull lines will show the difference. From the sailing I have done there is little difference in performance between the chine and round bilge in cruising mode. The main difference was probably more due to weight of gear on board and how good the sails were. The major difference between the 2 would be at resale time when the foam glass round bilge hull would get many more dollars than the chine ply hull even if the rest of the boat had ply decks, bulkheads etc. The market doesn't care about quality ply builds if they can get some or all foam glass. Slightly higher up front costs for foam glass but better returns on resale. The chine hull plans require you to "print out" the top and bottom and join the 2 parts at the designated line. This is a half plan for the main hull only. I will put up the float lines later.

 

The following 42 foot cat is designed by a famous monohull designer who works mainly in plywood. It is interesting because it shows how a simple plywood boat could achieve a useful cruising cat. The boat is 42 x 22 foot weighing 12000 lbs and displacing 14000 lbs carrying 900 square foot sail area. But please notice 2 details. One the load capacity is only 2000 lbs this is low, although later in the specifications it says it has a 4000 lbs carrying capacity. Even this is a little low for a true cruiser. Next is the wing mast. It will improve the performance but building it is going to take time and it must be done with aircraft quality plywood especially at the mast head unit. The hull shape and general layout of this boat will work well. The guy who designed this really understands how to build plywood monohulls and intended this to be his personal boat. Plans price is available on the web.

 

The following is a Crossbeam spec for the 42 x 22 cat above. You will have to work out which cross beam as there was no detail.

 

The designer of the previous 42 foot cat also designed a plywood 40 foot trimaran. I have no structural specifications for the boat beyond assuming the plywood specs would be close to the 42 foot cat above. I like the boat because of its simple shape and its wide main hull that would be good for a cruiser. This boat would be an easy shape for a flat panel foam glass build. Tomorrow I will show the shape difference between a round bilge and chine float shape for a 33 foot tri that may hint at a simpler shape for this tri's floats. Please notice the bulb type bow on the main hull. Also a simpler mast and rig would also nice.

 

The following is the float options for a Buccaneer 33. The variations of float shapes between hard chine plywood and foam glass appears to be large. But they are very similar in real world performance on boats I have sailed. If I was to rebuild or a big repair on a ply Buc 33 floats, I would rebuild them in foam. They are less rot prone and would provide a slightly softer ride. Ply floats need good ventilation and need to be checked often to ensure minimal leaks etc. that may cause rot. West system hopefully was done at the build stage. A simple option may be make flat panel foam glass panels and make a "chine" version of a foam glass float. Faster build due to less fairing. Reference the Scarab 18 build document a few pages back.

 

Re post #107 & #108, Reuel wrote an article about his 42' cat design for Wooden Boat, don't think one was ever build, instead he started building a 53' mono for his own use.

 

The shuttle 28 is an interesting design. Probably the smallest ocean crossing capable design from shuttleworth. Designed initially in 1998 and upgraded as the first boat was built. More a cruiser racer than a racer. The materials specs again need to be studied as they tell you a lot about how to build a strong and relatively light boat of this size. But be careful as this design, like all shuttleworth designs, depend on you putting the glass layers in the direction specified in the plans. These are very good boats if built well no short cuts allowed. The rig on this boat is powerful and will not be cheap but the boat will be fast.

 

Timber moisture content can vary the strength of timber and effect its stability/size. What this means in the real world is you should not build a boat with excessively wet or excessively dry timber. Wet timber is well understood as it can promote rot and glues may not hold as well etc. Here is the suggested moisture content.
Hardwoods 9–14% moisture content with the average of 11% (AS 2796.1-1999 Timber—hardwood—sawn and milled products—part 1: product specification, Standards Australia 1999)
Softwoods 9–14% moisture content with the average 11% (AS 4785.1-2002 Timber—softwood—sawn and milled products,Standards Australia 2002)
Excessively dry timber also has problems. Its cell structure is not as flexible and weakened by very low moisture content. Dry timber also wants to soak up additional water which can change its dimensions. The next problems are hard to detect. In EG Oregon (douglas fir) there can be sap pockets that weaken the timber. When some trees are cut down and crash to the ground they may fall on logs rocks etc which damages the surface timber which is cut out, but the timber further in the tree can have micro fractures that weaken what looks to be a good piece of timber. Also do not ignore knots, shakes, splits etc. cut them out. West type products can help seal and solve a lot of these problems but they cannot "strengthen" a faulty piece of timber. The only way to improve suspect timber is to laminate it from thinner layers EG instead of 100 x 100 mm timber glue together 8 layers of 100 x 12.5mm to make the 100 x 100 mm. Be wary of buying "second hand" timber from EG a 100 year old house that is in a dry climate. The moisture content of that timber may be less than 5% and may be weaker. If you know what timber you need up front buy it early and stack it well under cover and air dry it for several months prior to West coatings and use. Final timber tends to be "unidirectional" in strength having much higher strength in the grain direction than cross grain. Please use timber strength appropriately EG no cross grain under a high compression area.

 

A design that depends on good timber and plywood is the Buccaneer 33 trimaran 33 x 24 foot 6000 lbs displacement 530 square foot of sail. The basic design uses 6 mm ply skin (hulls and deck) and frame every meter or so with stringers. The structure is strong and can handle more than I can. The cross beam structure is demountable with an aluminum tube and a waterstay arrangement under the beam. Compare this to the Brown (marples) 34 tri 34 x 21 foot 8000 lbs displacement with 540 square foot of sail. Its hull bottom is 18 mm, chine plates 12 mm and sides 9 mm ply with frames every meter or so with stringers. Decks are 9 mm ply. Each boat works well and will sail any where you like. Attached give you the idea of the Bucc 33. The word doc give a study plan of the 34.

 

The Searunner 34 from the previous post is a strong boat as is indicated in the attached word file construction pictures. The price you pay for this strength is weight. The centerboard case in this boat is very strong but is heavy. The 34's boats total skin weight is literally 50% heavier than a Buc 33. The cross arms are strong. If this boat is built well, it will serve you well. But it is not bullet proof. I know of a 34 that hit shelving rocks on a rugged coast line in Victoria. The boat hit bottom and initially survived but was pushed further up the rock shelve. Result the 9 mm float shell had holes punched in followed by the forward bottom, flooding and a boat that could not be salvaged. The same would have happened to a Buc 33 in the same situation. Even well built strong boat, if sailed incorrectly, will be damaged just as much as a lighter boat if it hits something hard. Remember more weight equals more momentum equals a larger crunch if it hits something. The doc has cross arms, followed by float, followed by center case then the rest of the main hull framing.

 

Trimaran cross beams come in many formats but the 2 simple ones are aluminum tubes or timber builds. Fiberglass beams can be from hard to simple depending on the skill of the designer. The 2 boats below are about the same size and intended purpose. The bucc 33 is 33 x 24 feet x 6000lbs displacement has over 50 boats built. The LC 33 by Bernard Rodrequis is 33 x 20 feet x 6300 lbs displacement had a least 2 boats built. Each has 6 mm ply skins. Bucc 33 multichine. LC 33 round bilge 6 mm from flat sheets of plywood which were slightly tortured or had minimal darts cut in them at strategic points. Bernard built several boats this way up to 40 foot tri this way and understood how to apply conical shapes into a hull to make it look like a round bilge hull. I have only one old article on the technique which I will scan in one day. Back to cross beams. The LC 33 beams are relatively simple box beams with 108 x 48 mm top and bottom laminated mahogany flanges and 12 mm ply webs doubled at some points. the Bucc 33 has E8150 205 x 140 mm aluminum beams with a 65 x 12 mm stainless steel strap underneath to act as a water stay. Each is relatively easy to do but requires excellent building around attachment points to ensure the loads are distributed effectively and leaks are minimized.

 

One of the more original small trimaran designers is Frank Smoot with his Slingshot series of trimarans from 15 to 24 feet. These are day sailors but he has developed very fast simple way of launching and recovering a boat. For one of his designs it only takes 2 minutes to get the boat from the trailer 7.5 foot wide to a 12 foot wide fully rigged full sailing boat. He has youtube video's showing the various boats. His early designs start with 3 mm ply hulls with minimal framing glued together with construction glue. Later designs went to 6 mm ply but the 3 mm worked very well as demonstrated in one video as he hammer the side of one boat. The hammer bounced. Simple aluminium cross arms and masts ranging from 50 to 75 mm with 3 mm walls etc. The boats have simple lee boards and rudders. Original thinking well developed for shallow water bay sailing. You will learn a lot from a deep web search.

 

Material lists can tell you a bit about a designer. Many designers have a minimum structure for a boat. They will not use below a certain weight of fiberglass cloth, foam or plywood irrespective of the boats size. There are often good reasons for this. Availability of materials, some light weight or thinner plywood are often more expensive than the standard 6 mm ply, people often like stiffness under foot (Marples 9 mm decks on 16 foot day sailor tri's) and puncture resistance of slightly thicker materials. A good example of this is Ray Kendricks Scarab tri's. The Scarab 16 x 11 foot displaces 560 lbs, the Scarab 18 x 15 displaces 1350 lbs and the Scarab 650 21 x 18 foot displaces 2000 lbs. Each of the scarab boats above hulls are either 6 mm plywood or 400 gsm 45/45 10 mm klegcell foam 400 gsm 45/45 glass. Other designers do 3 or 4 mm ply on 16 foot tri's EG Woods on his 15 and 16 foot Strike tri's hulls. Marples does 6 mm ply on hulls but does 9 mm ply on deck as he thinks the extra reinforcement required to make 6 mm ply stiff is not worth the extra building required. The Scarab tri's have different structure weights on cross arms, rudders, boards but basically the hulls, deck and bulkheads are similar. Each of the mentioned boats are good boats that sail well. Some boats are very strong for their size. Compare the structure of similar size boats before you add an extra layer of glass or thicker ply. You may be adding unnecessary weight.

 

The Shuttle 31 cat is one of my favorite designs. Its 31 x 22 foot with 600 square foot of sail area. These boats make great cruisers and are capable of good daily averages. The material list tells you the foam glass layup structure of each component of the boat. Just match the letter listed beside the area of a component against the laminate reference panel below. This material list also specifies the strip plank western red cedar and plywood version materials. The strip plank version is 230 kilos heavier but is basically the same boat. The word document is about a high school build project of a Shuttle 31 with some detail.

 

Apologies for putting the wrong material list for the previous post on the shuttle 31 for 12 hours. It now has the correct materials list. Fore beams on cruising catamarans vary from a straight aluminum tube to a composite construction of EG strip plank cedar glass or a carbon fiber tube. The strip plank cedar forebeams are often lighter and can be designed to a specific size and need but generally require more work. The majority have a "seagull" striker (bridle wires) to take the forestay loads with the actual crossbeam tube acting as a compression tube to keep the bows apart. But designers occasionally use the fore beam as a component of the torsional stiffness of the boat. Richard Woods on his 20 to 30 foot cats pushes his forebeams back and just runs his bridle to the bows. If the bow/deck structure is strong enough this works well. Lock Crowther 60 foot Shotover cat took it to a logical extreme by just having 2 beams in the cats with no forebeam. The following gives an idea of the options with the Spindrift plan giving details of a "conventional" tube and bridle approach.