Those bows remind me of the beloved Nacra 20 bows.

Given that they are equal weight, the cat will have more weight to windward which would result in increased righting moment. The tri has the weight spread out more.

JMO
Clayton

I beieve the swiss boats are not tris.. they have a centre pod, but it doesn't touch the water..It is just for structural reasons to reduce costs.

wow thanks for all the responses.... first off for everyone freaking out about the 18sq.... I have NO intention of breaking up my boat. I love the square. If I do this project I will have an either-or arrangement. My reasoning for the interest in the tri is more of a multi-person fast party barge. I often have 2 or 3 tag-alongs when i go sailing and the sq sucks above single-handed. To be honest I'd love to rig a tornado, but cant really afford to have two fully rigged boats. so to all interested in the sq make me an offer. send me a pm and i'll give you all the details of the boat. but anyway.. heres a pic of the sq.

how do i post a pic directly to the post?

Sorry that answer is incorrect.

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


The reasons why tris are build and use have nothing to do with any differences in righting moment.

The real reasons are that a tri (of equal specs) has less wetted surface area in light winds then a catamaran. In the really light stuff the tri can be balanced on its centre hull only while keeping its rig straight up. This is the most effective sail power to wetted surface such a boat can attain. A catamaran can not in any way achieve this. It either heels to sail on only one hull but also heels the rig or the rig is straight up but both hulls are in the water.

The other reason for building tri's is that the largest portions of the rig loads (which are fore-aft) are taking up by the centre hull instead of a beam structure. The first is much easier to build strong and stiff. Building cats at the same size requires more engineering and more use of exceptional material like carbon. In effect LARGE tris could be build cheaper and with less exotic materials. When done right they could also be lighter overall, but with the new material the building of large cats in now really held back by this anymore.

So now we are seeing much more large catamarans build. In the past it used to be trimarans.

For small sailboats there is no advantage is building a tri over a catamaran; here is will be both more expensive and heavier because of the third hull. The advantage of better light wind performance is too rare to really compensate for the drawbacks.

The only reasons to still build racing tri's is that they can be build wider then a catamaran with less effort and stiffness problems. So in the French ORMA we still see tri's as the class rules allow these boats to be as wide as they are long.

Wouter

Make the post and attachment just as you did. Then view your post' attachment and copy its url. Edit the post and use the image function in the instant UBB Code. Paste the copied url in and you are set.

OK my thinking is incorrect, so if more weight further away from the center doesn't help with righting moment, why do sailors hike out or trap out for that matter????

Clayton

I'm trying to get my head around it too, but I can see that trapezing is actually shifting the CG. You are moveable ballast.

The thing to understand here is both trimarans and catamarans are symmetric relative to their centre lines. Meaning one side of the boat is a mirror image of the other side when viewed from the centreline. If weight is moved away from the centre then an equal amount will be moved away in the other direction. The net result is that the centre of gravity remains were it was, on the centreline itself. So nothing changes in the righting moment of the total craft.

Crews on the other hand do not follow this Mirror image principle. They are either on one side or the other and so DO affect the total amount of righting moment.

Wouter

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?

<img src="http://www.catsailor.com/forums/images/graemlins/confused.gif" alt="" /> <img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" /> <img src="http://www.catsailor.com/forums/images/graemlins/blush.gif" alt="" /> <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

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?