Trimaran basic questions

I am playing with a design for a lightweight recreational tri. What I have in mind is something around 18' long day sailor and have a few basic questions. My background is designing skin on frame kayaks and small boats, not sail boats.

I started out wanting to alter one of my designs by adding outrigger so it could be sailed. The more I looked into this, the more I realized that it was just to much of a comprise for me. And way too wet a ride for my tastes. So I started from scratch and have a good start but I am am starting to questioning some things.

My goal is a fast boat that is fun to use in higher winds. I am not interested in racing and not looking to squeeze every bit of speed possible out of it either. I designed it with a multi-chine hull to make it more round and keep the wetted surface low. Now I question the advantage over say a shallow Vee hull which is obviously simpler to build. Would a flatter bottom has some advantage in sail powered boat? It would see that a flatter hull would want to plane in the right conditions where a rounded hull wouldn't. But the bottom line is I really don't know if I my thinking is sound or not. So it's better to ask.

I have a couple of other questions but one step at a time.

 

There have been many successful tri's built with flat bottom 'dory' main hulls. As an example Richard Woods Strike 18 has one and seems to perform well. If you're not concerned with ultimate performance I'd simplify as much as possible for an easier build.
http://www.sailingcatamarans.com/Strike 18.htm

 

Kudzu,

The thread basic multihull knowledge may be of help for you as it is following a similar line. in providing answers to design aspects.

Richard woods has a flat bottom trimaran.
I think there may have been a thread on the W 17 by Mike Waters which also is flat bottomed. The thread is W 17 trimaran what do you think?

 

Also check the John Marples' Seaclipper 16.
JT

 

The problem with a SOF sailboat is that the higher speeds will make a bigger load on your flexible skin. When you have a ply skin it will naturally not distort as much.

I suggest a round bottom with narrow regular spacing of the frame will better support the skin and sail faster since you wont have distortion.

No proof, but I have been thinking about the same boat. For myself, my questions are centered around the ability to take the twisting of the hull from the mast step, cross arms, board (center, dagger, lee) and the rudder.
I also am concerned when sailing on a reach, much of the sail load will bury the nose of the boat, causing the hull to bend up.

I just recently measured a Yost SeaTour 15 for bending due to weight in the cockpit. The boat was supported on sawhorses at the #1 and #5 frame. 50# was put in the cockpit about where the seat (mine) would rest. Without skin the hull bent 1/2 inch at the center. With skin the hull bent 1/2-1/16 at the center.
The frame contributed the majority of the stiffness, engineering principles suggest the bend would have been about the same as without the skin without ever assembling the frame. I.E., the stiffness was the same as the bundle of stringers loosely collected together.
Catamarans are under a tremendous load with the bow supporting the entire weight, the mast punching down, and the sailors weight countering the entire load.

My conclusion is that you need more bending strength and stiffness than a typical kayak construction and the skin only helps minimally.
You need more torsion which I don't see how to get easily since there is no wood in that direction.

I am really interested in how you make all this work.

I didn't start out to be negative - I really want to do this my self.

Marc

 

Warwick, I can't find that thread. You can't use a singe letter 'W' in a search and without it, it just pulls up to much. Designers name didn't find anything either. Got a link?


upchurchmr I have the same concerns and have been thinking on this for a while. To get the performance/speed I want I have about decided that plywood makes more sense. I can beef up the frame and add a lot of extra support but I am still concerned about twisting the frame. One way around that might be attaching the outrigger and mast together so that some of the forces are taken by the outrigger instead of all on the frame.

One thought was doing what Plat did on his Geodesic boats and using the Kevlar(?) roving on the frame. Never tried it but I think that would add a lot of stiffness. But then you probably don't have a smooth hull shape at speed. I suspect you have a faceted hull then and that would be a speed killer.

I haven't give up on the idea but I do think that plywood is a better choice. So at this point I am just playing with ideas and seeing what I can come up with

 

W17 trimaran...What do ya think??

 

Thank you for suggesting my Strike 18 trimaran. However you might find the Strike 15 a better choice if you crave real performance.

http://sailingcatamarans.com/Strike 15.htm

Having said that a Strike 18 is faster than a Windrider 17 (see the video below)

http://www.youtube.com/watch?feature=player_embedded&v=Q4iFXdFuihw

I assume you have seen/talked to Dave Gentry?

http://www.gentrycustomboats.com/

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

LOL! I thought the thread was 'w 17 trimaran'. If I had searched for the whole thing I would have found it.

Richard, Dave Gentry and I are acquainted, we hang out in some of the same forums. Here is my web site. www.kudzucraft.com.

 

Kudzu, at least you where able to find it, May be I should have mentioned that it was the thread title.

you may be able to find out more about the W 17, on Michael Waters web site http://www.smalltridesign.com/ I think Michael might be on the members list as well, having responded to the thread.

 

I suggested Dave as he recently built a SOF sailing canoe so has tried a fast, big rig boat, not just kayaks. We were both presenters at the recent Sail OK event where he demonstrated SOF construction and was out sailing a little outrigger canoe he had just finished

Richard Woods

 

I don't know if you like the construction of the new "flat-bottomed" tri of DIY-tris.
Still don't know if I really like it but it kind of intrigued me.

http://www.diy-tris.com/2012/10-24-footer.htm

 

Thanks Hasyb for your post.

It is an interesting way to get a larger radius on a flat bottom.

I wonder if there would be any strength benefits in using a ply, foam, fiberglass construction.

 

I have seen his web site and I like some of his ideas. I like the basic concept of his boats too.

 

What is the difference in performance between flat and rounded bottoms?