Small Tri's under 20', any mention of foils is banned..

Weta

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Did you mean Weta 14, or have they got a new boat? I haven't sailed a Weta 14 but I've been right next to one sailing in about 12-15 and it appears that it is a very wet boat but probably a blast to sail.

 

Weta and Dudley Dix

Weta's do 'go over' - couple of Youtube videos on them tripping over their ama's when pushed too hard. Lovely boats but seem to give a real good soaking. Great fun - but I think requires a wife who doesn't mind being soaked on a windy day. Ideal for Australian and hot climates. Not so sure about Northern Hemisphere sailing. (Author now ducking in case of retaliatory comments from die hard all weather sailors )

Dudley Dix trimaran is a lovely looking boat - had forgotten about it. Thanks for that post.

 

My mistake I do apologise I meant the Weta 14. I guess the point I was trying to make is many times we try to reinvent the wheel on this forum when there are on many occasions perfectly good boats out there already.

 

Weta

The plus of the Weta is that if they go over you can flood an ama to help get it back up again. Gather it seems to work. Wouldn't like to try that trick on a bigger trimaran though....

 

If you make a tri wider and with more ama flotation, it doesn't seem that unreasonable to try to flip it over the front or back. Especially if it's square or oversquare, although those kind of boats are rather rare.

 

I have a 24 that is 18.5 feet wide at least, maybe 19 (it is actually harder to measure than it is worth it). So far seems really great. It has very large amas to boot.

 

I think everything you seem to want can be done more easily with existing sytems. These include Tortured ply/Stressform; Stressform with keel section; Constant Camber, and with keel insertion; Cylinder Molding; Kelsall Swift Sandwich.

 

Kurt hughes has done a lot of tortured ply boats. Some are actual tortured ply, and others are CM which is supposed to allow fatter hulls. I would think he is the world leader on this stuff and he has several smaller boat plans. I think tortured ply is far and away the cheapest and fastest way to build amas in this size (almost even in comparison to simple square ply shapes). For main hulls I think the same if you want a racing tri, but for cruising it doesn't allow fat enough sections in normal practice, on small boats. There are way of gusseting in material, I think that maybe Selway had a design like that. I haven't seen one done that way that created a real bubble shape, it tends to look more flat sided.

 

For all the talk of ply, aren't we forgetting that we have moved on a bit since wooden boats and there are a number of methods using alternative materials such as foam and glass and carbon.

I'm not convinced that there is much labour saving in tortured ply, yes it was a good system devised to create curves with the available materials of the time, but that was 40 odd years ago.

Modern foam and glass boats have the curves, have the resale value, have the longevity that ply just doesn't have. I know I will be shouted at by the ply convertees on its merits but I also wonder if there maybe merit also in thinking along the lines of John Lindahls horizontal foam being formed in female formers or the likes of the Farrier method of vertical foam set in lined female formers.

I guess there are those also who love to work with wood, it does have that nice feel about it that foam and glass never seems to have.

 

Sail Area

To change the subject a bit, the sail plans that seem to be appearing on the boats that have been listed on this thread, all seem relatively weedy to say the least.

I know sail area isn't everything but say we compare the new Pulse 600 at 20 ft and 4.5 metres wide, with its 17sqm main and 7.5sqm jib with a spinny of 21sqm then that matches up nicely with the 18ft F18 Cat at only 2.6m wide ( yes the trap will extend that somewhat ) but surely we should with the extra width be able to carry a lot more, or am I missing something here.

 

You have to be very careful of getting too much beam and too much sail area on a trimaran not equipped properly for it or you're asking for an easy pitchpole on a bearaway. That's where f***s can make a huge difference in the design and sailing of a powerful tri.There has to be some way(besides crew movement) to stabilize a powerful tri off the wind.
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CG of platforms at 1/2 the beam These figures are close approximations.
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Twin trapezes on an F18 account for about 70% of the total RM for the boat*
*F18
a-crew=2@175 lb at about 11'=3850ft.lb.
b-platform=400@ 4'=1600ft.lb.
c-Total= about 5450 ft.lb
d- crew percentage of total RM =70.6%
------
18' X 18' tri
a. crew in center plus 3'=2@175 at about 12'=4200ft.lb.
b. platform=450@9'=3600ft.lb.
c. Total=7800 ftlb. or about 1.24 times the F18
d. crew percentage of total RM=54%
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18' X 18' Tri with crew on trapezes off windward ama:
a.crew=2@175lb at about 21'=7350ft.lb.
b. platform=450@9'=3600ft.lb.
c.Total =10950ft.lb. or 2 times the maximum F18 righting moment
d. crew percentage of total RM=67%
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You can clearly see the dramatic increase in righting moment that is possible with the same crew as an F18 just sitting a little to windward of a center cockpit.
Put those guys on trapezes and RM goes sky high BUT what hasn't changed is the resistance to pitching
. So what you have is a boat with tremendous sail carrying ability but without the design factors to allow that power to be safely used.
That's where modern technology can make a tremendous difference.

 

Doug, how is the crew at 11 feet on an 8 foot tri, and the hulls are only at 4 feet? Sounds like the hulls are referenced from the centerline and the crew referenced from the leeward hull. Also, the weight of a single hull is 200 lbs not 400.

RM of the windward hull should be (400/2)*8, which, as luck would have it, still gives you 1600, so it doesn't change your answer, but the explanation is confusing.

 

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Re-look at the numbers-nowhere did I quote the crew at 11' on an 8' tri-thats the approximate position of the CG of the crew on twin trapezes on an F18 catamaran. All the figures are close approximations meant to illustrate the potential power to carry sail of a trimaran with a crew sitting just off a center cockpit and with a crew trapping off the windward ama.
CG of all three boats is counted as being at the center of the boat. CG of the two crew on the two trimarans are: 1)-3' to weather of the center line sitting on the windward side of a center cockpit,2) 18' +3' for the two crew on trapezes. It's very slightly less when you account for the actual athwartship center of buoyancy of both the cat and 2 tri's. And the rig reduces "hull" RM slightly due to heel.

 

Wayne, if kept coated with epoxy wood out performs everything for fatigue resistance except carbon fiber. Glass actually has a shocking fatigue curve, the foam is providing the separation for 2 load bearing skins, as those skins age there won't be underlying strength like say for cedar. Old fashioned view I know but data does support it. Wet rotten wood of course has nothing but if holes for hardware are coated and the membrane kept intact it does stay dry.
Plus it is fun to work with and nice to look at.

 

Pitch poling

Its interesting that the pitch poling nugget has been brought up. Hull shapes have changed a lot over recent decades to the point that pitch poling is now far less significant than it once was. A couple of years ago I sailed a Hobie 16 for the first time, I pitch poled it in about the first 5 minutes, something that I hadn't done for the previous 5 years on my F16. It happened so fast and easily that I sailed the boat back to shore and gave it back to the owner.

What was the difference between the Hobie and the F16, one has very narrow pointy hulls the other modern almost wave piercing hulls with lots of reserve flotation down low in the forward part of the hull.

Yes I have seen some of the designs here on this thread and gone ooh thats one that will bury the foward leeward hull pretty easy and then looked at the non faired in beams and the pointy little bow and decided that is one design that hasn't learnt much from the last 20 years of design nous, but with respect they have designed in small sail areas to compensate.

Yes I have pitch poled my A Class, but it has a very tall high aspect main only and relatively small volume hulls ( to keep the weight down to 75kgs all up ) and in the bare away it can often be pretty dicey to say the least in strong winds.

Compare that to the F16 which has an almost identical main sail, but has a very large assymetric spinny out front and quite large volume hulls to cope, its very easy to bare away and once the spinny is up on its long and high pole, pitch poling is a non entity, its more having the Cajones to get out on the trapeze and drive the boat even harder as the faster you go, the more uplift from the spinny and planing of the hulls.

So what I'm trying to say is that good design of Amas of a modern shape and volume are probably the first part of the design to prevent pitch poling, yes well designed addiitonal f**** will aid, but for the moment here on this thread, lets talk about good Ama design together with good sail plans together with skippers who can understand about good sail management, and pitch poling will not be an issue.