Beachcat vs. Beachtri?

[frosh]

Hi Robin, apart from the structural issues of relatively huge crossbeams supporting the mast base and the rig loads, there is another reason, why cats are roughly half the width compared to overall length. Longitudinal stability will be compromised before lateral stability. Would you rather prevent a capsize over the leeward hull by sitting out, or prevent a pitchpole over the leeward bow by hanging out over the rudders?

[Robin Larsson]

Hi Frosh, yeah, I see what you mean, but dont the trimarans have same problems? Okey, the crossbeams probably get a bit stronger in the connection to the mainhull, and maybe they dont see the same amount of twisting loads, due to the mainhull taking much of the rigloads, am I right?

But the pitchpoling problem must be the same? Or is there a fundamental difference in the shape of a tris amas and the hulls of a cat? Maybe the tri have fuller bowsections or something? But if so, why?

//Robin

[frosh]

Hi Robin, I am sure that you already understand the rig loads and crossbeam situation already. Do not forget the loads generated by the forestay during upwind sailing as well on a catamaran, which is much more easily absorbed by the large central tri hull. Then there is the asymmetric spinnaker set off a bowsprit as well. I would rather accomodate that in a tri.

The big question is why a tri is built almost square and can cope with longitudinal stability which becomes somewhat problematic only once we are flying the main hull, so near the limit of heeling stability as well.
To be truthfull I do not know all the details but will give an opinion, and also invite other forum members to give their views.
It is not to do with the bouyancy of the ama bows being greater than the catamaran bows. In fact for the same overall length ama hulls are much less bouyant throughout their length, than a cat hull in virtually every case.
One needs to study a graph of rig heeling force versus, hull resistance to heeling to get a clearer picture. It is more sudden on a cat when the hull reaches near its limit of lateral stability, and then things can happen very quickly if the rig forces increase even a small amount. On a tri the whole lateral heeling situation is more progressive and power can be reduced or maintained with more of a safety factor in reserve. I will leave it there as this is not my area of specialty.
Regards, Sam

[Hobiestoke]

Hi Robin,
im new here but have been quietly watching your thread.
Here is my 14ft. plywood/fiberglass beachcat I designed and built a few years back:
www.geocities.com/proteus_catamaran
its plenty fast & weighs approx 160lbs fully rigged. a beachable deep symetrical design i wanted to be similar to a dragoon or prindle 15. It uses "wave" type skegs and no centerboards. performs really great. Best part is assembles and dissasembles in an hour and takes up very little space in the garage. recently extended mast to 22ft. for larger performance sail setup and Hobie 14 Shroud/forestay setup. should be approx 150 sq ft of sail when shes done w/ modifications. Getting new sails ready will post new pics in a few weeks when I have time. It was hands down one of the most satisfying projects ive ever designed & completed (O:
Good luck to you!

[xarax]

If one solves the structural problems associated with the mast foot, (or if one uses a bi-mast configuration like "Team Phillips»), I see no reason why the square cat would not be faster than the square tri. It would be cheaper, too...

Xarax, I'm not so sure about that. The tri can use one main hull with two very small ama's that together don't equal the size and weight of the main hull (or one cat hull) and can effectively sail in an OVER square configuration.
Interesting stats on two 60 footers:
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Parliers Cat- LOA 60' Beam 49' weight 11000lb--
two partially lifting foils, two rudder t-foils,planing hulls
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L'hydroptere - LOA 59' Beam 80' weight 11975lb
three lifting foils(designed to fly)/movable ballast system.
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[Robin Larsson]

But Doug, if a tri that sails on mostly one ama, why can that ama be smaller than a comparable cats hull?

Look at this "cat" http://www.decision.ch/ANG/default.htm find the Décision 35 there. Its said to be the fastest cat on Lake Geneva.
It uses a mainhull to cope with rigloads, but it never touches the water

Regarding my own build, we´ll see what it will be, but most likely a wide trimaran, cause I like them more. Since I already have a rather big mono that I´m not 100% sure I will keep for very long once its built, I think the tri build will have to wait a little while.
If I keep the Wasa 55 I´m building right now for a few years I´ll probably build a small tri, if I sell it I´ll probably buy a bigger trimaran, or build one, not sure. We´ll see that when I sell the Wasa.
It really all depends on how much I love the Wasa 55 when its in the water
If I´ll keep it, I need atleast a small tri

Thanks for all help guys!

But hey, if there will be no Tall Ships Race for me this summer, and somethings goes bad with the Wasa, I might just start building something like Chris´s 14foot tri

Hobiestoke, thanks for the info on your Proteus, nice boat

//Robin

Originally Posted by Robin Larsson

But Doug, if a tri that sails on mostly one ama, why can that ama be smaller than a comparable cats hull?

//Robin

Because with a boat using foils/planing amas for lift the hull doesn't have to support the weight
in the normal way-buoyancy- except in an emergency.So the total buoyancy of the ama can be used for that purpose. This is the theory behind my 18 idea and it has been proven in models-and to some extent on L'hydroptere. I think using this philosophy on beachcatkiller tri would produce a very fast boat with greater pitch stability than a cat(using the system in the url below) and, of course, we know that l'hydroptere is fast. But it is not a viable system for a cruising tri-at least not now.
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Ultimate 18' Tri ? - Boat Design Forums
Address:Ultimate 18' Tri ?