tornado/nacra trimaran

Ok I know i'll catch of a lot of it for this post but i'm curious to build a trimaran from a tornado hull and two nacra 18sq meter hulls. I'm currently sailing a fully restored '81 sqare which has the light hulls.. no foam core, just glass and longitudinal ribs for stiffness. I also have a tornado hull mold and thought the combo may make a decent tri. I was thinking of a 14-16 ft beam with the outter hulls canted and offset vertically about 2 ft...anyone have any thoughts on this one?

Yeah...
sell us the Sq and choose another boat for the amas.

Jay
18sq Cat.1
tami
18sq Cat.II

I rode a tri very similar to what you're proposing, a few years ago on Lake Minnetonka, MN. He was a budding naval architect, and custom built the center hull, either 20 or 22', quite low profile, very similar to a tornado design. He used 2 Nacra hulls, I'm not sure which ones. Round aluminum crossbeams, maybe 18' beam. The rig was from the Nacra, if I recall, and as such seemed a bit undersized, until we took it out. I was quite impressed, mostly with the whole different feel than the cats I've experienced. Very stable, not twitchy, accelerated easily, little heel (not sure I'd ultimately like a boat that didn't fly a hull) . It seemed deceptively fast, even with what I thought was an undersized rig.
Only got that one ride, an hour in maybe 12 to 15 kts of wind, less than a 1' lake chop. I remember it well, I was intrigued. I think you should give it a go, and keep us posted.

Dave
SC20
Flight Risk

How about getting a Nacra 6.0 hull as mainhull? We justed switched beams between a 1989 Nacra 5.5.18 sq and a 1992 6.0. The 6.0 is now 3.3 m wide and the 5.5 down to 2.6 m.

Not sure about the non-sandwich 5.5 you have, but the later sandwich models have the same distance between front and rearbeam than the 6.0 making it a very easy operation as even the tramps did not need any modifications.

Regarding your trimaran project, the overall appearance of the tri would look much more harmonious than what you achieve with a tornado shaped mainhull.

Maybe you can find a damaged 6.0 hull so it would safe quite some money and time.

Carbon tubes could be a good idea to join the three hulls.

Can't really comment on the proposes angles and height differences. If you go with a 6.0 mainhull (and the distance between front and rearbeam is identical with the 18sq), would probably connect the three hulls with long aluminium tubes and go for a ride. This experience would probably give you a good feeling how much you want to cant the outside hulls.

How much money do you want to spend?

If you have the cash, I'd do a foam core, carbon tornado main hull, (using your tornado mold) with a couple of Acat foam core carbon ama's. The whole package would be much lighter and stiffer than some old hulls. It would probably have better resale value as well.

Forte mast, he can make a pear shaped mast in sections and you could glue it together. He could ship it to you in two parts.

Forte carbon beams

Thats the way I'd go....

But I've been accused of having a carbon fetish.

Bill

As long as theres a layer of kevlar in there somewhere, its perfectly fine to have such a disorder <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

I agree with the A-hulls, but do not know where you will find a "carbon" main hull since the last time that I read the Tornado Rules and specs, I do not believe carbon hulls are allowed yet.

The main problem with what you are designing is the rudder controls, beam boxes, assembly time, and insurance. I saw a one off tri similar to what you are speaking in SF Bay in the late 1970s. It was a Sailcraft T hull with two 5.2 Nacra hulls. external beam boxes had to be adapted due to the experimental issue asd the fact that two boats were used. The rigging position became an issue [Tornado tapered non-sealed aluminum mast section] as well as holding the front of the 5.2 hulls in alignment. Heavy whisker stays helped with the alignment. The rudder controls kept breaking as well. The assembly time were excessive... anywhere from 3 hours to 5 hours. That's what stopped him from proceding with the development. He grew tired of spending so much time on the beach and not on the water. Also no insurance was to be had at that time... He eventually reassembled both boats.

fair winds,

thom

thanks for everyones input.....so assuming i can handle all the design and building of the associated parts, hull alignment etc and the appearance of the different hull designs doesnt bother me, and I can get the sail area correct... how does everyone feel the performance would be? any other comments on my proposed vertical offset and canting of the amas? beam width?

oh and does anyone have or can tell me where to get a 3-D surface model for the nacra 5.5 hull and the tornado hull? I can handle most formats but would prefer a pro-e, mastercam x, or an iges file.

Worse than building a catamaran of similar specification. If I am wrong, then I think we will see a number of A-Class trimarans.

The thing about trimarans is that you always have two hulls in the water and you have three hulls. Both of these factors will make the trimaran slower, I think.

Why Trimaran? Is there some other reason why you prefer the tri over the cat?

sparky is right, don't break up that 18square!

Bishop, I'm serious. Contact us. Sell us your Sq. We'll work out some hull trading or something. I'm working on building our local fleet and have some interest...

I seem to recall that the designer's take on this is that if you want the overall best performance without worrying about cost, build a tri. If you want the best performance for a certain budget build a cat. This is in respect to the big class of boats.

Depending on the design of the tri, the main hull may not in the water, or in other designs may plane (Farrier), depending on conditions.

A-cats being restricted to a 7.5' beam would preclude any benefit from doing a tri.

the limit of your performance are the 5.5 hulls. the advantage of the tri versus the cat is the rightning moment. early tris were known for sinking the floats before they could fly the mainhull. perfect turtle machines. but only when you fly the mainhull free a tri would significantly profit from the increased rightning power.

modern tris like the 35ft division tris sailed on lake geneva have a very tiny mainhull and large floats.

if you want a fast boat, build large floats. unfortunately the 5.5 hulls cant carry much weight. i assume the performance of your tri with 6.0 floats would increase significantly.

I both a tri and a cat have the same overall weight and are of the same overall width, then which one has the largest righting moment ?

Beware this is a trick question.

Wouter

another div 35 pic

well, tris are mostly wider than cats... and heavier.

I like tris... when they capsize it looks pretty sexy! LOL

So why not build the cat wider and heavier as well ?

That should not be a difficult thing to do.

Wouter

That's not actually a trimaran. It's a cat with a center stiffening pod to help stiffen up the rigging. From what I read of the design, the center pod is not intended to provide any flotation in the water.

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?

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.