Trimaran sail area

Kudzu,

Can you show the frame only view of your tri concept? I have been thinking about the same thing and am afraid of the actual shape of the hull when I finished. Actually I started thinking about a round bottomed boat, but to get it really round seemed to require too many stringers.

Do you have a link to the last boat you showed? looks very interesting but I had not seen it before.

Thanks,

Marc

 

No, don't have one. This is all I have at the moment.

 

If you build a multihull as a structure of two or three hulls linked by two main cross beams (as opposed to linking the hulls with a massive and effectively rigid central cabin structure) then the part of each hull which is between the main cross beams can be subjected to substantial torsion loads. I am not sure how to explain this without diagrams, but considering sailing on a broard reach, the bow of the lee hull will become more deeply immersed than the other bow(s), the bouyancy force on the lee bow will be greater than on the other bow(s) and the structure will tend to distort such that if you view the craft from ahead or behind, the two cross beams will no longer appear paralel. This causes torsional load on the part of the each hull between the two main cross beams. This is probably not an issue for the small sail assisted kayak kind of multihul, but for a multihull designed for maximum performance under sail the torsional loading on the hulls can be significant. For amusement, I recently did some FEA analysis on the stress distributions around large rectangular hatches fitted in the hulls of such a craft and concluded that if I were to build such a craft I would need some reinforcement of the skin around the hatches, particularly at the corners.

For a SOF structure having a fabric covering fastened only to transverse and longitudinal framing, torsional load will cause the skin to be subjected to tension diagonally accross each quadrilateral cell of the structure. This could cause diagonal wrinkles, which are probably not at all desireable, or could cause either failure of the fabric itself or stretching of the fabric to the extent that the joints in the framwork will work and perhaps start to fall apart. To avoid these problems you might consider the geodesic kind of SOF structure that includes diagonal framing - a good example would be the fusilage of the 2ndWW Wellington bomber - the Wikipedia artical for this aircraft has some pictures of this kind of structure.

 

Another illustration would be the Aerolite boats by Platt Monford (sp?). He uses a Kevlar tape on approximately 45degrees. The problem would be to be sure the kevlar was well bonded at each point it crosses the frame, or the torsional loads would just cause the kevlar to slip around. The main hull would be specially loaded this way.

John, I assume you considered rounding the corners of the "large rectangular hatches" to minimize the amount of buildup required. Of course the largest possible radius should be used at the corners which results in a circular or at least oval hatch - or just beef it up.

Marc

 

I think you are going to have to think about rig forces in your quest for rig size. Are your cross beams going to handle these forces or are you going to use water stays. Tris don't heel and spill wind so the force on rig can get large. Big rig lots of compression and upward force. Will your skined out construction handle that?

 

Kudzu,

Any progress on this idea?
I am particularly interested in the structure you are thinking about for the ama's. How wide were you thinking about?

Recently looked at your web site and picked out a kayak I liked, possibly my next build, if I don't do a trimaran similar to Smoot's folding tri.

 

No, I am working on some recreational rowing designs right now. I am currently building 19' long 24" rowing .... boat. I want to call it a shell but I think that is a bit of stretch. The idea was a shell with some stability for the recreational rower.

Far from given up on the trimaran idea. It's never far from my mind and still have plans to build it. Just decided there were others designs to purse for now. Things I could develop with less work and get the plans on the market.

As I said this is still a very much on the list. It's just going to take a lot more work and development. The sail and rigging are my big concerns. To do what I really want it needs to stayed and that introduces a lot of new issues. Probably a some failures before I get it to where I want. So I decided to use the time wisely. I still have a big rendering on the shop wall and look it often.