Doulble end vs transom

[Kudzu]

For reference this has to do with the Skin on Frame trimaran I am working on.

Is there a disadvantage to making the main hull double ended instead of having a transom? It appears it would increase the water line of the hull since your not kicking up the back to keep the transom clear of the water.

With my design looks like I could gain close to a foot of water line by making it double ended like a kayak or canoe. Other than a few Cats, I don't think I ever see this done. So it makes me wonder if there is something going on I don't know.

[redreuben]

Simple, cut of transom produces less turbulence over a higher speed difference than double enders.

[peterchech]

transom also reduces pitching no?

[Doug Lord]

It seems to me that if you're not planing on flying the main hull that a double ended ama is probably ok. Some of the big tri's that fly the main hull and use a foil in the ama for lift tend to have flatter and wider sections aft because the ama shape gives the boat its only pitch resistance. But on tris where the main hull stays in the water(and provides most,if not all, of the boats pitch resistance) the key to speed seems to be a high L/B ratio-above 12/1 or so and having a double ended ama or not doesn't matter a lot one way or another. If the transom drags a lot it won't be helpful. Look at the Farrier ama sterns.
On the other hand, a tri like the Osprey has double ended amas for low drag at low speed and because they have no effect on the boats pitch attitude since they aren't in the water at high speed.


Picture: Osprey double-ended amas; Osprey flying:

click on image-

[Kudzu]

Foils just fascinate me. But for this boat I do not anticipate flying the main hull. Maybe down the road but I want to start with a little simpler design aimed more at the average sailor out for a day for fun, fast(er) sailing. Living on a lake surrounded by big hills our sailing conditions are not optimal anyway. Of course there are there days that it blows hard and looks like you are on the ocean.

[Doug Lord]

Originally Posted by Kudzu

Foils just fascinate me. But for this boat I do not anticipate flying the main hull. Maybe down the road but I want to start with a little simpler design aimed more at the average sailor out for a day for fun, fast(er) sailing. Living on a lake surrounded by big hills our sailing conditions are not optimal anyway. Of course there are there days that it blows hard and looks like you are on the ocean.

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Just to be sure: I wasn't trying to suggest you use foils-I don't think they are appropriate for the boat you propose. I was trying to illustrate some of the thinking that can apply to the choice of double ended ama or not. In your case I don't think it will have a great effect one way or another as long as you keep the ama skinny.
Good Luck!

[peterchech]

I think Kudzu is asking about the vaka being double ended, not the amas...

[Doug Lord]

Originally Posted by peterchech

I think Kudzu is asking about the vaka being double ended, not the amas...

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My apologies, Kudzu!! Amazing that I misread that-thanks Peter.

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Kudzu, on small tri's it is sometimes hard to get the main hull skinny enough for a quick displacement hull-it should be L/B= 10/1 at the widest. So some designers make the main hull a planing hull like the Weta which also helps with pitch control. If the main hull were skinny enough to be a quick displacement hull it might not have much pitch resistance at all if it were double ended.

[peterchech]

Interesting, my outrigger canoe is symetrically double ended and I have experienced almost no pitching at all with it despite having a relatively top-heavy balance lug rig and relatively heavy solid wood mast... it has very little rocker, I think 4 inches at the ends on a 24' boat, maybe that stops the pitching?

[Doug Lord]

Originally Posted by peterchech

Interesting, my outrigger canoe is symetrically double ended and I have experienced almost no pitching at all with it despite having a relatively top-heavy balance lug rig and relatively heavy solid wood mast... it has very little rocker, I think 4 inches at the ends on a 24' boat, maybe that stops the pitching?

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Everything is so interrelated: how high the CE, how much sail, L/B? crew position etc. It's hard to generically answer a question with only part of the info.

[peter radclyffe]

http://www.marineinsight.com/sports-...yacht-concept/

[peterchech]

Wow... uh, because its double ended?

Doug I know there are a million factors to be considered, but could flat rocker be a factor that limits pitching? Seems like it would to me...

[Doug Lord]

Originally Posted by peterchech

Wow... uh, because its double ended?

Doug I know there are a million factors to be considered, but could flat rocker be a factor that limits pitching? Seems like it would to me...

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Sure, in combination with other factors.
Two hulls I'm very familiar with:
1) Hobie 16: high rocker, low buoyancy, very low pitch resistance,
2) Weta: dynamic lift forward, flat run aft ,low rocker,planing hull, high pitch resistance(similar to a hull I designed and built in 1971 and to my new boat).

[Kudzu]

Been a little under the weather but still reading. As I (think) I said. Kayak design is my forte', not sailboats so this is a new ground for me.

First off, this is a recreational design, not out to push the limits in any area. Looking at the possibility of a cheap and relatively easy boat to build. Fast on the water, easy and fun to sail. Again, not talking super fast, flying the main hull, all out speed demon. That would be pushing my frame designs and take the fast and cheap construction element out. Not to say that isn't in the back of mind, but that is the not the design goal for this boat.

I see this boat being piloted by new to intermediate sailors. If this boat could get up near 15 mph with a good sailor that would be the fantastic.

With a kayak once you get above a leisurely paddle speed a longer water line can make a lot of difference. I recently was showing a fellow a couple of my boats. We swapped at the turn around and I went to the shorter design and according to the resistance number 'on paper' there isn't that much difference in the two at the speeds we were paddling, but I was amazed going from one to the other at how much different the effort was in the real world. I had never paddled two of my boats side beside and then swapped before.

That lead me thinking that I might squeeze a little more speed out of my design it were double ended. Of course I know when you improve one aspect you typical hurt another.

[peterchech]

getting an 18' sof trimaran to 15 mph is fairly high performance in my book, lots of stresses there not sure how sof will hold up. But I look forward to seeing you actually do it kudzu.