Trimaran sail area

[Kudzu]

Playing with the idea of a SOF trimaran. I have a pretty good idea of the hull shape. Still refining it but I think I am close. Now I am starting to thing about the sails and what I want and how much and rudders and centerboard.

Main hull is 18' long. I want something fast but I am not going to be racing so not trying to squeeze every bit out of it. I have sailed a fair amount but not well versed in design aspects. So any suggestions would be appreciated.

[Doug Lord]

Looks good-do you have any numbers? Take a look at the "Thread Summary" here, post 97: 18' Trimaran vs F18 Catamaran
Has some numbers that might help a bit. Good luck and please keep us posted. You can use this thread sort of like a "Design, building ,testing" blog if you want to.
Best of luck!
Heres a good site in case you don't know about it: http://smalltrimarans.com/blog/
And another site by the designer of the W17: http://www.smalltridesign.com/

[Corley]

Quite a few beach tri's of this size use a beach cat rig the advantage is you can make the decision on how much sail area you want and they are available at a reasonable cost.

[Percyis]

The type of structure will limit the size of the rig. A typical 18' SOF boat is meant to be light in weight and simply built. The skin has little strength save to keep the water out. I suppose you can make a heavier structured frame with more pieces to accept a larger rig, but why?

Kudzu defined a non-racing/performance design, so a smaller rig will do the trick here. Maybe a schooner rig with self supporting masts?

[Kudzu]

I have some general info but the design is very much a work in progress. I design and build kayaks, not sailing boats so this is new ground... excuse me, new water for me.

This project started with the idea of converting one of my kayaks to sail. In my research and watching videos I realized what a wet ride a kayak was at sailing speed and the hull shape could be improved on. So I started looking at making the hull taller and one thing lead to another. Here is what I ended up with. Of course very subject to change

As it sits it's 18' long. 10' beam. Waterline is 16' 2"' x 1' 8".
Ama's are 14' x 1'.
I am looking at sitting kayak style with foot steering so you hands are free.

[Doug Lord]

What does "SOF" stand for??

[Doug Lord]

Small tri's, unless designed carefully to avoid it, can be very, very wet-no better example than the Weta-and its not just this video-there are many videos of the Weta showing the firehose effect. So if dry is important make sure you anticipate this: http://www.youtube.com/watch?feature...&v=4AIisNuG7CI

[Kudzu]

Skin on frame = SOF

The angle makes this one look ill proportioned but this would be similar to the construction but obviously a little heavier built. Covered in a heavy synthetic skin.

[Corley]

If I was building this style of boat I'd go with a stayed rig, an appropriate structure for carrying loads could be similar to what is used on the supanova off the beach trimaran an aluminium perimeter frame 12' wide with support for the mast step, shrouds and traveller built in you could use a forestay mount on a hull bulkhead or use a horizontal spar running forward from under the perimeter frame to carry the (low) forestay loads. An appropriate rig could come from a small cat like a hobie 14 which would give quite relaxed sailing attributes. You could make the whole thing one piece and easily removable to keep the structure simple.

[Doug Lord]

Originally Posted by Kudzu

Skin on frame = SOF

The angle makes this one look ill proportioned but this would be similar to the construction but obviously a little heavier built. Covered in a heavy synthetic skin.

=====================
Thanks-I was thinking a lot of stuff and had decided it might be Sail Or Fish(!)

[Corley]

lightweight stringer frame was quite common in multihulls built in the past boats like the gougeons proa slingshot are effectively the stringer frame equivalent of a SOF style of construction a very thin skin supported by closely spaced stringers and frames (ring frames in slingshots case).

[cavalier mk2]

I like the skin boat applied to multis, at times I've pondered if skinned foils would work more like natures creations and be adustable with air pressure. For a multichine shape I think glued lap would be faster and dryer,if lined correctly they can add lift and introduce air that helps reduce the skin friction of a solid sheet of water. Spronk cats were often done that way.
I would think you could vary hull shapes from flat to concave by skin tension which really adds a dimension to boat tuning. There was a large skin on frame cat built curragh style that sailed from Europe to South America so it works even in large sizes. The frame needs thought if you want a taught forestay, a una rig would be the way for flexible hulls.

[Petros]

Great idea! I have been thinking about something similar for years. I have used skin on frame construction on about a dozen sea kayaks, several canoe like hulls (including one with a small sail), a 14' sloop sailboat, and on a small tri I built for a contest (we won the sailing part of the race, we were 2nd overall in total points-included build time, materials costs, etc in total score).

What I want to do is make a larger center hull for cargo, and use two kayaks for the outriggers, with a trampoline deck tying it all together. We would sail to our destination and set up camp, and detach the kayaks to go explore local waters.

Sail size is a matter of choice, the larger it is the faster you can go in light wind, but the more difficult it will be to manage in heavy weather. Look at what similar sized cats and tris use, and I suggest go smaller on the sail until you get some sailing time in it and want to go faster later. Just go with a simple main sail, you can always add a jib later if you want. Typically keel and rudders are sized off a ratio of the sail size, like 5-7 percent of the sail area. There are fancy analysis that can be done but that will work for a fun day sailor. Make sure you make the keel, rudder, and all the attachment points very sturdy, the side forces can be quite large. Also, make sure the mast step is very stout, and distributes the loads over a large area of the frame, the forces here can be very large as well, especially in a big blow.

I have sailed small tris with foot pedal rudders....I hated them. I would much rather have a conventional rudder tiller arrangement. It allows you to move around, stretch your legs, etc. which can not do stuck in a narrow hull. Do not go sailing in a kayak, and do not go kayaking in a sailboat. I have used both types of boats extensively since I was a child, and the foot pedal rudder arrangement sucks big time in a sailboat (I do not even like them in a kayak, unnecessary I have found if you know how to use your paddle). Anyway, keep it simple, you can make most of your rigging from inexpensive hardware from a hardware store rather than a Marine supply outlet (unless you want to spend big big bucks on a few pieces of hardware).

Good luck with it, sounds like a fun project. keep us posted.

[Petros]

Here are some photos:

Skin on frame tri with modified junk rig. Cost about $55 in materials to build


this is the 14' sloop I built, my daughter and I on first launch. All of the materials only cost about $40. I salvaged most of it.


Here is the frame under construction


Another skin on frame sail boat


The fun you can have with skin-on-frame construction


sails deployed in our dragon boat. Twin cat rigged self deploying modified junk rig, I designed it myself!

[Kudzu]

LOVE the dragon head!!

I have been looking at this little tri and emailing with it's owner.



He is using sails from a Laser I believe. 120 Sq feet and gets 12mph out his. So I was thinking of going with something along this line. Seems like a good place to start and learn from. If it goes well and is half as much fun as I expect I can see a second one being developed from what I learn.