tornado/nacra trimaran

Professor Wouter was talking about this:

________
a....b....c

a=b=c=33 l=3
1.5*33+3*33=148,5

________
a.........b
a=b=50 l=3
3*50=150

Google search for Virus boats, they are a French company which makes a 21 foot trimaran. You could buy just the center hull and add your own bigger ama's (Nacra hulls). The only advantage to the Tri (in that size range) is the larger center hull allows you to cary more people and remain drier than out on the hulls/tramp of a cat. It will not be faster due to the weight. On the cg thing, if they built cats as wide as they are long, like a 60'tri (60 feet wide) the righting moment would be the same, right?

But for more speed, you want less drag (wetted surface), ie. sail it (cat or tri) on one hull as much as possible, so in lighter air, you don't want more righting moment, you want the cat to fly a hull as soon as possible. Wider means heavier too, if just in the extra weight of the longer beams.

I once asked Ian Farrier, designer of the Corsair Tri's why the ama's were so small. I figured bigger, fatter ama's would allow the boat to fly the center hull like the French Open 60's do all the time. He said there are two reasons: the smaller amas will go under water at a lower wind stength which -should- wake the crew to the fact that they should be reducing sail area. Also if the ama's were much bigger, they would have trouble getting them to fit on the trailer when folded. Still, I would love to see a F24 with the F28 ama's on it! I wonder if they could do that as a go-fast option?

If I was going to build an 'off the beach' tri, I would have done something like this. (sorry for the quality of the picture). 200% amas and flying two hulls early.

Buildable in strip and glass. You can even build the mast yourself if you want to. Would take the winter, but probably a much better performer than a Tornado or NACRA based project. Notice the sliding seat, neat feature (builder/owner was 65 I believe)

Hello Dirk,

Actually because you rounding off the numbers 33 from 33.333333 etc you still get a difference in righting moment when indeed they should be exactly the same.

It would have been smarted to use a=b=c= 40 for the tri and a=b=c=60 fro the cat. Now you don't have any rounding off effects that produces inequalities were there are none.

In effect :

________
a....b....c

weights a=b=c=40 with width=3
total weight = 120
righting = 1.5*40+3*40= 180

________
a.........b
Weights a=b=60 with width l=3
total weight = 120
3*60 = 180

But your example does show nicely why there can't be a difference in righting moment between a cat and a tri when both share the same overall weight and overall width.

Wouter

check out the L7 http://multimarine.com/L-7/order/order.html i belive the amas are n6.0 hulls modified. I hear its a nice boat. just thoughts

wouter, of course, they are exactly the same, I agree 3x40kg and 2x60kg would have been more wise than just using 100% but just intended to show the principle and assumed everyone would understand that the difference between 148.5 and 150 is only related to the rounding simplification of 33 % instead of 33,3333333 % .

I never stated a tri with the same width as a cat has a higher rightning moment. it wouldn't be wise to make a 16ft wide 18ft cat as the 18sq already suffer from being to wide. my point was that if you want to make a 16ft wide tri, you could increase your rightning moment but it would only increase your performance if your floats are big enough to support that. Of course racks like on the hurricane 6.5 or the swiss lake 27ft catamarans are a more efficient way if it would be only about increasing rightning moment.

You certainly did not, your education at Delft University garantees your understanding of those principles.

My post was intended towards the other readers in this thread that were indeed on the track of believing a tri had better righting moments.

Absolute;y correct, at that time the tri will just become a catamaran with a 3rd hull in the centre. The only exception of course will be in very light winds where the whole boat will be balanced on its centre hull but you don't need to make a tri 16 foot wide to achieve that.

Racks would be a cheaper, lighter and more effective way to go indeed.

Wouter

I realize this is all theory but I've got nothing better to do right now so....

Let's take two equal boats, both 30 feet long, both 20 feet wide. Both weigh exactly the same, say 3,000 lbs. but it really doesn't matter the amount, as long as they are equal. Now let's pretend they also have the exact same mast, sails, daggerboards, rudders, crew, etc. So everything is

equal

except one is configured as cat, only two hulls, one is a tri, with a middle hull that normally sits in the water, not hovers above the water like a pod-cat.

With me so far? Got that picture in your mind? OK, now let's go sailing.

For our discussion let's say at 10 knots of true wind, the cat fly's a hull. Will the Tri also fly the center hull at the same 10 knots? I don't know. I have always wondered though. Both boats are the same width, both have the same sail area and weight. The mast should be exactly the same distance from each outboard hull on both boats; it's 10 feet from the outside hulls to the center. Now in my little mind, I'm thinking the tri might have a little more righting moment because of the added weight of the center hull being lifted out of the water, but if both craft are of equal weight, that means the cat's hulls have more weight in each hull at 1,500 lbs where as the tri's weight is spread evenly between all three identical hulls, at 1,000 lbs. each. (we said the overall wt. was 3,000 lbs., let's pretend the rig is weightless)

So, to fly the center hull, the tri has both other hulls (2,000lbs) as righting moment, but one is on the centerline so not as much of a moment arm as the single 1,500lbs. of the cat hull. And, why do we see the Open 60 tri's built

square

that is, 60 feet long x 60' wide, while most racing cats beam are only 50% of length? Volvo 40's, at 20' wide and the mega cats are 120' x 60'??

So, all you engineers out there: Which has more righting moment? Which will be

faster

through smooth water? And now for the real food-fight starter: Which one is BETTER?

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Hey Tim,

You are confusing yourself with the details.

The basic answer to your question simply is (as was stated earlier) :

Both boats are symmetical relative to their centerline and as such the centre of gravity lies at the same point with both designs, namely on the centreline, and because they are also equally heavy and equally wide both design will have exactly the same righting moment.

I the rigs are the same in all important aspects (height, area, shape, stuff like that), Yes !

In science it is not unusual to feel differently about something then what the numbers are actually saying. Still just like any good pilot you need to learn to trust your instruments (numbers) our you feelings as feelings (just as your senses) can easily be tricked by the circumstances.

Afterall, house hold items like Television and pictures dependent on this ease of tricking the senses. An even stronger example is the professional flight simulator that can simulate utterly believable rapid decelleration while being almost as good as motionless itself. I'm sure you are familiar with the latter example Tim.

Wouter

So the math is this: with the Cat, there is only one 1,500 lb hull in the water, and one being lifted, moment arm of 20 feet, or 30,000 ft. lbs. And with the tri , we have one hull at 1,000 lbs, with a 20 foot arm and one at 1,000 lbs with a 10' righting arm. So the tri has 1,000 x 10' (10,000 ft. lbs) plus 1,000 lbs. x 20' (20,000 ft. lbs) which also equals 30,000 ft. lbs, correct?

BUT....the lone 1,000 lb. tri hull still in the water is now supporting the 2,000lbs. of two hulls up, which should make it sink lower in the water, where as the single cat hull, is only supporting 1,500 lbs. which should make it float a little higher, correct? And do we assume the cat's hulls are a little bigger, since they each weigh 50% more than the tri hulls? And since we just decided the two are equal, why do they build the tri's wider than the cats? Or should I be asking why the cats aren't as wide as the tri's? Why is a 60' racing cat not 60' wide, like a racing tri?

This thread has gone completely to the technical or should I say mathematical side of the equation, instead of the basic question of should he build it or not. I say build it. As to the earlier statement that A's are out because of their 7'6

BEAM. That is not applicable here, for you would be using other beams. I know of a few sets of A class hulls that could be had for this project. All of them need some work, but that is about it. I also possess two T-Hulls that could be had for the right price.

What I like about tri's beyond their inherent stability is the fact that they point higher than most cats do. I built my trimaran

Skyhook V" with my partner Ken King, formerly of Navtec and launched 1994. 39’10”LOL and 28’4” on the beam. Built of western red cedar with e-glass skins and carbon fiber at the main stress points. The design was one that we began and Chris White finalized. This is a 20 design of his to look at for ideas. http://www.chriswhitedesigns.com/discovery20/index.php
http://www.2hulls.com/usedtrimaran-2001/skyhook.html
Fast boat, but built more for cruising than racing, without some of the penalties of these heavier bathtub cruising cat properties.

Build it and goes have some fun.

The direct reason for using science and mathematics in projects like this is to establish what can be expected. It is the whole reason why 80 % of science was developped in the first place. Without it we'll be back at our gut feelings and 5 senses, both of which are easily tricked into believing things that are simply not true.

From point of view if you want to spend a considerable amount of many developping something then you better know before hand whether it is a reasonable change of succes. If the underlying principle has a fundamental problem preventing you from achieving succes then going ahead with the problem will be very risky financially speaking. Miracles do happen but sadly alot less often then people think or hope.

The original poster did ask about performance expectations and I think the math did give him a better idea of what can be achieved.

How can the tri by more stable then a cat of the same dimensions considering their righting moments. In fact a tri build for waters with waves can not have all three hulls in the water at the same time. That would negate most of the

advantages

of the tri design and make it take alot more punishment from the waves. Ift one hull is lifted out (the luff hulls) and this happens at relatively light winds already (where stability considerations are meaningless) then the stability of the tri is determined ONLY by the distance between its leeward ama and the centre hull. If this distance is less then halve the width of the catamaran then the tri will have LESS righting moment then the tri and therefor have LESS sideways stability.

The fact that a given tri can point higher can only be explained by its slower speed.

Again the math discussed earlier in this thread explains all this. It wasn't math for the sake of math, it dircetly answered some of the questions asked.

So basically I expect the 16 foot wide trimaran made out of a 6.0 or tornado hull and two A-cat hulls to perform worse then the a 6.0 or tornado catamaran and even worse then any 8 foot wide catamaran with comparable sailarea. With the possible exception of very light winds where the tri can be balanced by the crew on the centre hull only.

Wouter

thanks again to all who have contributed here... i'm learing a lot about cats in general that i've not known....but back to my original post... there's no a-cat hulls in this project.. no 6.0 hulls. I own an 18sq, and own a tornado mold.. so if this project happens it will be with those hulls.

I'm also coming across two distict types of tris here. A pod style where the center hull is smaller than the amas and is designed to fly both hulls, and a larger center hull/smaller amas that is designed to stay in the water. it seems to me that on the later design, the center hull carries the weight of the movable balast, (me), and the amas carries the load of the sail. this seems to more along the lines of what i'm proposing. I do realize that it will be slower than a cat, but should be reasonably fast with a larger crew which is my goal. is that a fair statement?

Yes, I think so.

You'll beat any monohull out there.

Wouter

You should do this to the tri:

http://www.sailingscuttlebutt.com/media/06/0814/

Doug

I want one, the smaller version looks like a blast.