Trimaran Design

Hello everyone...

I have been putting together some lines for a 33 foot trimaran and have just finished the hull...yup, the learning curve was very steep, damn thing was probably vertical instead of a curve. Anyway, I used Carlsons free software and finished the hull and amas then I put it on AutoCadLight and began to put it all together.

I come here because I need some advise on the design so I can tweek it and hopefully complete it. If it comes out nautically aesthetic enough and seaworthy enough, I plan to give it away to whomever HELPS with some advise, direction and criticism.

Oh, I think I put my e-mail in incorrectly so if you try use BlWtrSail instead sorry. Can anyone tell me how to correct that?
Thank's

 

Contour34

This is a Bob Perry review that may help you if you haven't already seen it:
http://www.sailnet.com/sailing/97/bobmay97.htm

 

Hello Lorsail...

Very nice boats those Contours. I have seen that review actually. I have the same idea of folding the amas, but only so that the boat is berthable...12.5 feet on my design if I remember correctly. In order to trailer, I have to demount the hinges and the amas would lay on arms attached to the trailer that would swing the amas up above the deck. I'm starting to work out the design of the trailer now and it looks like its going to be around 9 to 10 feet of height. Seems awful high.

Thank's for the reference.

 

tri

Oh, well.. Just out of curiosity, do you use the same princible the Contour guys do in moving the CB forward as the boat heels?

 

Hello Lorsail...

I'm not exactly sure how they move the CB forward as the boat heels. The way that I designed the Tri to do that is in the amas and not in the main hull although I suspect that there would be some forward movement of the CB in the main.
The amas are 200% with a max L/B of 14.1 @ full mainhull displacement and greater at any less displacement. Theoretically, I can fly the main and the windward ama and still not submerge the leeward hull, but that will always remain a theory because I have no intention of testing it. The profile is torpedo like?, high and fat in the front with the keel swinging up to the stern very quickly which moves the CB forward as the ama is pressed. The profile is elliptical.
Downwind, the CB really moves forward. Oh yeah, the amas keel is modified from v at the bow to progressively flat at the stern which helps it to plane in light winds. The coefficient ratio is a little low for the amas at about 60. The coefficient for the main is 54...wish I could have 64 or 65 but then I would have reduced the L/B to almost 7.1 and I wanted no less than 8, so I left it alone and got an 8.9 to 9.2 depending on trim, heel and load. This of course is all theoretical in my design although proven mathematically or in software or in other designs. I avoided designing her at oppossite extremes and instead chose the in between as much as possible unless the extreme was absolutely necessary.

 

tri

On the Contour ,as I understand it, both hulls have a CB at around 52-53% aft but the ama is shorter so as the boat heels and the ama carries more weight the CB(of the boat) shifts forward. So the wholeboat CB as measured relative to the main hull length moves forward to about 45% more or less.So there is a natural resistance to pitchpole as the boat heels...
I doubt that the Contour is designed to fly the main hull but that sure would be spectacular like it is in the Open 60's.

 

Hello Lorsail...

This is interesting. I dont think that I measured the distances of the CB as a percentage. I designed the main so that it is just aft of center (not sure of the %) but the ama CB is not lined up with the main. Come to think about it, it might be close...I have to check it. I'm not sure if it should be as I wanted to keep a bow up attitude as much as possible in the main and the ama. I forgot to mention that there are assymetrical daggerboard foils in the amas for lift and to improve pointing, and the amas are at main displacement at about 8 degrees of heel. I have to check this %'age thing out and I'll get back to you with the numbers.

 

Hello Lorsail...

The CB for the main is at .5757 of the LWL static. No heel, no pitch. The ama is at .5450 of its LWL static. These are both seperate measurements. The ama is set 6 inches aft of the main bow. Hmmm, I wonder how the ama dagger foils will act on this? Now I'm thinking that the foils may fight the CB moving forward quickly due to the lift...or maybe it could just keep the CB right where it is and maintain a level trim which might mean attaching the ama perfectly?

I guess that I can always add slight negative pitch to the ama before it is attached to the structure if I need to move the CB forward. But then I would be most concerned with a possible trip because of the nose down attitude. I think I can make the amas shorter, but then she might squat. It looks like a model will reveal whether its back to the drawing board or leave it alone. What do you think?

I thought for sure that there would be a lot more help in here...its so quiet that you can hear a mouse piss on cotton.

 

tri

Definitely don't create any nose down attitude for the ama!
In "Princibles of Yacht Design" by Larsson and Eliasson(p81) they talk about a fairly narrow range for the CB of any hull- something like 52.5 to 53.8% aft for lowest resistance. You might want to research that some more.
You might get more help(or not) on a multihulls specific forum. Are you familiar with the steam radio multihulls list? If not e-mail me at : lorsail@webtv.net and I'll send you the latest copy...
Sounds like(I haven't laid it out) that on your boat the boat CG is behind the CB of the ama when the boat is heeled and that is good.
Have you read Shuttleworth's tutorial on multihull seaworthiness?
The daggerboards, if they are canted or banana boards(like Open 60's) can have a profound effect on how the boat performs but their location has got to be part of the complete performance equation of the boat so ,for instance, they are not behind the ama CB; so that they don't adversely affect balance and so that they are not too far forward. And finally and most important of all that they don't create too much lift since they are ,in essence, a hydrofoil w/o an altitude control system. A bad day can occur if the boat is flying the mainhull sailing on the ama and banana board and the board generates so much lift that the ama comes free of the water: the boat can tilt forward (or back) with potentilly bad results. A t-foil on the rudder has been used on at least a couple Open60's but that still allows the boat to pivot(catastrophically) on an axis between the t -foil rudder and the banana board-sort of diagonally. You've got an exciting project and I wish you well!

 

Hello Lorsail...

Nah...no nose down on the amas.

Wow...52.5 to 53.8%. How in stone is that written? If I change the hull to that then the CLR of the sail will not have enough lead on the CLA of the hull. Is that for any hull?

Yes, the Cg is definitely behind...but doesn't the CB move and line up in all the axis once the hull trims out on the wet?

The boards are canted inboard 25 degrees so that a final angle of 32 degrees exists when the ama is pressed to full main displacement of 3500#'s and the ama is vertical. I think that 8 degrees is plenty on the heel for the beam as I want the windward ama to be clear of the water for about 12 to 18 inches and I don't want to spill too much wind up top. I checked the math and the numbers say that the lift should not be sufficient to lift the ama out of the water. Shoot...you can forget me trying to fly any hull. I have no intention of even thinking of incorporating such an idea.

Lor, I don't know...I think I might have laid her out pretty good as she is right now. I have been all over this thing for about a year now and the numbers seem to add up. I tried to step back from the math and tried to look at it as a whole and it just seems right. She looks balanced and seems aerodynamic (flush deck) and her windage is not at 8% like a racer but it isn't 21% like those brochure boats...she sits at 15% light. She has a very high SDR (42 light and 31 at full displacement with genny and main) for light wind but she has great beam (24') and her CE is not high (17') because her mast (39') is only at 1.2 x LOA (33.72'). If I'm correct, I would have to reef at force 5 if she were light...that is a lot of stability, and I could always go to the jib or a blade and drop the genny.

I thank you for your well wishes. Do you think that the radio list will help me to make some comparisons or help me to put the model together in order to test it? I will definitely ask you for a copy. How would I test a scale model for accurancy in extrapolating information into real size numbers? I think the wind part would be very difficult. Back to the learning vertical!

Thank's

 

tri

When the boat heels and the CG is behind the ama CB it creates a bow up moment to oppose the rigs bow down moment; the design skill comes in getting the right separation.
I've had a business for the last 15 years or so building rc multihulls(among others) see: www.microsail.com
If you have questions about the model I may be able to help. The trick is building it big enough that it will actually work as a scale model.Such things as foil area are less reliable but on my hydrofoils they scale directly from full size foils with no area change for scale effect.Foil t/c ratio on small models does not scale at all; model foils -generally- are thin-t/c ratios of a max of 7%. You can learn about balance, behaviour as the boat heels ect. Parlier built a two+ meter RC model of his planing cat...

 

Hello there Lorsail...

I took a look at the site for your business. Very, very nice. I can't believe how those things come off the water. I can imagine being in one of those scared half to death with a serious case of bolemia supplemented by diahrrea because of the hightened amount of uncontrollable adrenaline trapped in my skull that has no way of being dissipated due to the frozen state of my body.

I can imagine hearing the uncontrollable demonic screams of a maniacal mind attempting to escape my skull and run away, and realizing they're mine without being able to do anything about it.

We must have some kind of "big bang" suicide gene just waiting to be awakened "to cleanse the earth of us" like all the other lost civilizations that got too big or went too fast. Holy mackerel. Not me. If one of those foils go or hits one of those little things that sends you scrambling to panic stations when they go bump in the middle of the night, thats curtains. If you're lucky it will be swift instead of laying there looking at your entrails or looking at one of your eyes looking back at you for hours before rescue. Those boats must keep people in a constant state of agita stress city.

Why don't they just get on a...what do they call those things...a, ground effect wing? While were at it, just strap a rocket on the hull and lets see what happens to the ground, water, torpedo dive, wing. No worse can come of it than the possibility that exists already.

I wonder if some of these things are thought up under the influence or the lack thereof of appropriate medication.

 

reaction

You had that reaction looking at a site for rc sailboats?!! You best not go to: www.monofoiler.com

 

Nope...had that reaction looking at the "real photos" of those boats.