Multihull Structure Thoughts

The following table may be of use in trying to calculate the weight of a laminate for your boat. All weights include a wet out resin with a resin to glass ratio by weight I will assume is 1 to 1. Conversions are 1 oz mat = 300 grams, 10 oz cloth = 330 gsm, 18 oz woven rovings = 600 gsm, 17 oz biax = 550 gsm. A 1708 = 17 oz biax (550 gsm) attached to a 3/4 oz CSM (225 gram).

 

The following is a diagram of the elongation characteristics of various materials (rope or wire). Steelwire does not elongate much before it fails. Dyneema for example is much lighter for strength and low elongation. The reason people like fibre for rigging etc.

 

This boat is one mans dream based on a lot of voyaging reality. Its a home designed and partially built Wharram type of ocean crossing cat that is 23 foot long. The web site gives a lot of detail of the logic for the design and the build. But one point to think about. He was sailing a wharram at night decades ago when he hit something. The next morning he saw a ring of indents where a shark had bitten the hull. Fortunately it was on a frame point which limited the bites impact. He designed a thicker skin on this boat, 12 mm ply on a 23 foot boat. An interesting variation. Sailboat Project http://omick.net/adventure/minimus_ii/description/description.html

 

Lets go back in time to 1968 and see how a production Prout catamaran was built. The boat is a Ranger 27 (also extended to a 31 by adding 4 foot to stern). The full bridgedeck cat was 27 x 12.5 foot weighing a claimed 2600 lbs carrying 290 square foot of sail. The hulls and everything else was CSM and polyester. The hulls were female moulded with a basic 6 oz CSM (5 mm thick) then 25 x 37 mm foam stringers at 300 mm centrelines are added. A 3oz CSM layer is laid over the interior and foam stringers. Over the bottom from waterline to waterline internally is another 3 oz CSM layer (total thickness of bottom is 10 mm solid glass). The underwing is from inside 3 oz csm, 18 mm plywood, 3 oz csm, foam stringers 25 x 37 mm at 300 mm centre lines all covered by 3 oz CSM. Mast bulkhead crossbeam is 18 mm ply with 2 layers of 2 oz CSM on either side. The cabin cockpit bulkhead is the same. All other crossbeam bulkheads are 9 mm ply with glass. All internal furniture, floors and wing frames are 9 mm ply. Rear “crossbeam” is timber s 50 mm thick and 600 mm high attached to 9 mm ply. You may now understand why I say the claimed displacement is 2600 lbs. The materials used in the boat add up to more than the claimed displacement unless the builders did a really high glass resin ratio. 1 doubt that. Modern boats have no CSM in them and are often built with Vinylester or epoxy. We have come a long way.

 

Oldmulti: I'm mighty impressed with your contributions to this thread, lots of exciting reading.

I was looking into buying a 33 pinta, designed in Denmark during the 70's by ib pors Nielsen, to refurbish and ship to the Philippines (where I plan to move in 6-7 years together with my wife). But huge import taxes (50-150%) and shipping fees got me thinking about building my own trimaran down there instead. The 33 pinta is a very interesting design, capable of 20+ knots and I will post a few pics here for others to see.

I'm quite a handyman and think I could manage to build one from scratch as long as I have a full set of plans. Your plans on the bucc 33 and 40 are very appealing to me, but it seems a few files are missing (bucc 33 s1, s2, s6) and (bucc 40 plans 1, 2, and possibly more)? Do you have these laying around so you could share them? I would be very thankful!

Best regards, Roger!

 

Roger. I only have a few sheets of the Bucc 40 plans, not the full set. The bucc 33 plans are all I have but a full boat can be built from it. But I would suggest looking at the Team Scarab web site Team Scarab multihulls http://www.teamscarab.com.au/ as the designers sells real full plans for $150 Australian for modern tris ranging from 12 foot to 32 foot. I have a set of his plans for a smaller tri and they are well drawn accurate and easy to follow. The 32 footer design is available on discount at $100 for the next few days. An EG 32 foot tri building photo's are at Scarab 32 http://www.teamscarab.com.au/Scarab%2032/Build.html Attached is a material list for the 32. The Buc 33 is a good boat but more modern designs are easier to build and have a higher resale values.

 

Oldmulti: thanks for your input, I actually ended up buying those plans, the price is very good, now I have the option of choosing between the scarab 32 and bucc 33 when time comes. The s32 looks well designed, but is completely untested though, even the prototype has not yet seen water. But from the design info and specs it looks like it will make a very fast cruiser. I also like that I have the option to build with either foam core or marine plywood. I know marine plywood is readily available, but would probably get a much lighter build if I could source the correct foam over there.

 

The Hitch Hiker 33 was designed by John Hitch who also designed a 40 and 50 foot version of these boats with a variety of rigs. The boat is 33 x 22 foot displacing 12500 lbs and carrying 550 square foot of basic sail. These boats are plywood skinned timber stringer and framed. The skin is 9 mm ply, doubled on the dory bottom. The stringers are 40 x 25 mm at about 400 mm centre lines with a frame of 9 mm ply and timber edging every meter. Decks are 9 mm ply. Plywood and timber box beams act as cross beams. At least one 33 footer has circumnavigated and many others have travelled the Pacific. When I spoke to the owner of the boat that circumnavigated, she would go again tomorrow in the same boat if she could. The boat reached and ran well but she avoided upwind sailing if she could. The boat would go upwind but pitching was a problem. A horizontal stabiliser on the rudders help reduce pitching, next step was going to be bulb bows. The boat had enough room for her family and could carry the load required in its basic format with a cockpit roof cover, no deck cabin. She said the cockpit was dry most of the time. I have spoken to other Hitch Hiker 40 owners and those who built the boat as planned were happy. The more they modified there boats by adding full headroom bridgedeck cabins the less happy they were. Space is one thing but when galleys are put on the bridgedeck etc, pitching increased sailing qualities declined etc.

 

This a PDF about 25 foot micro multihulls designed in New Zealand. It has many interesting ideas and an interesting bit about hull structures on page 47-48. The symposium was done prior to 2000, so some of the specs are a bit old but the information is still valid.

 

The 14 meter tri shown here is designed by Patrick Balta. A man with a lot of flat panel experience when he worked for Geoff Schoining in Australia. The tri is 46 x 29 foot displacing 12000 lbs and carrying 1100 sq foot of sail area. It is plywood timber construction. I do not know details. further information can be found at Balta Patrick, Architecte naval, motor yacht, catamarans http://www.balta.fr/tri14.html It is an interesting boat.

 

Ehad Hossam. If you are still reading this thread the following document may give further hints on the development of mathematical hull equations. For everyone else this 1980 paper gives the logic behind Hugo Myers design of sailing cats (EG seabird 44 footer that held the transpac record for a time) and multihull safety approaches.

 

Wharram does some very simple boats occasionally. The Amatasi 27 is a simple boat for sail fishing with limited accommodation. It sails relative well for what it is. It was designed for a 2010 design competition which it won. The boat is 27 x 12.8 foot cat displaces 2300 lbs carrying 230 square foot of sail. The boat is built of plywood, timber, some glass, epoxy and rope. The 5 cross beams are round solid wood. The forward mast beam is 125 mm in diameter the remaining 4 beams are 105 mm in diameter. The hulls are 9 mm ply with taped chines. The rig is a very simple wing sail plan and probably a solid wood masts. This was one of the first larger Wharrams that had chine hulls which have been carried forward to his later designs. The attached give more detail.

 

One of the problems of home built boats is the designers specifics a structure but builders want to vary it due to availability of materials etc. One of the best designers for detailed specifications was Ian Farrier and his F series. Example is the F 22. Its now a production only tri but plans were sold initially. The F 22 is 23 x 18 x 1320 lbs weight with 328 sq foot of sail. Its a complex build but if built to plan an excellent tri. Now about home builders. The float of an F 22 has a standard layup of 600 gsm biax e-glass outside 10 mm H 60 foam 400 gsm biax e-glass inside with vinylester or epoxy. Another builder did his F 22 float and deck with inside 295 gsm s glass biax, 10 mm H60 corecell, outside 2 (295 gsm s glass knitted biax) in epoxy. Another boat in Brazil outside 2(280 gsm biax) 10 mm foam and 280 gsm biax inside. A Finnish F 22 builder float has outside 240 gsm carbon kevlar 50/50 mix with 200 gsm carbon next to foam 10 mm corecell foam and on the interior 240 gsm carbon kevlar 50/50 mix in epoxy. One builder preferred 12 mm foam to strengthen his hull and laid additional glass in some spots. A least one builder went all carbon. Ian Farrier was asked to approve all these glass variations and basically some were acceptable but they were at the builders risk. In short not all F 22 are the same and if your buying one second hand check with the original builder to see if Farrier approved of any variations in the build.

 

There is lots of fascinating stuff in this thread but this boat interests me the most (well, apart from the Newick atlantic proas). Realistic expectations and a strong focus on what is important to the owners. I will follow the build and sea trials with interest.

 

Oh my God!

I got a little stale with Boat design net a year or two ago and have not been looking in for a long while and then this thread comes up. What a gem! It is as if I have stumbled on a treasure trove of designer and builder insights straight from the source. Thank you Oldmulti so much. I have started at the beginning and will take a while to get all the way through.

cheers

Phil