Sunday, November 16, 2008
Why don't we see a tri like this anywhere? Seems like if the technology that has been applied to Orma Tri's, Banque Populaire,the DOG tri and Stealth beach cats was applied to,say, an 18 foot tri- cats would no longer rule 20' and under. Why isn't someone doing it-or are they?
What’s the point?
More speed. You would have nearly twice the righting moment and 1.7 times the SA for any given length if you went all out. Should be faster in almost any condition than a cat of the same length.
Came up with this a couple of years ago and thought I'd mention the concept since I can't afford to patent it-and because I think it has a great deal of potential.
I was impressed by Yves Parlier's l'hydraplaneur which for those that don't know is an Open 60 multi with a difference: it is a cat with two stepped planing hulls looking somewhat like seaplane hulls.In the original article in Seahorse the boat was supposed to have some form of "variable geometry" to mitigate the negative effects of the steps at low speed. The article said the area of highest drag for the boat was between about eight knots and twenty knots with drag above twenty dropping to a fraction of the drag of a "normal" displacement multihull at that speed. In sailing against the ORMA tri's the boat showed flashes of speed but was basically crushed in the two races I'm familiar with.
Since I've designed and sailed numerous multihulls both full size and radio control the idea kind of interested me but I wanted a way to eliminate problems with the step at low and moderate speeds.What I came up with is this:
I envisioned a trimaran test boat 18' LOA with an 18' beam. The main hull would have a hydrofoil on the daggerboard and one on the rudder. But the amas are the key to this concept: they ROTATE! At slow-moderate speeds the ama is an 18-20/1 displacement hull but as the boat approaches a still to be determined "rotational" speed the windward ama is rotated 180°; the boat then tacks and the other ama is rotated .The bottom of the rotated ama is a stepped hull designed to reduce drag(up to 80% compared to a displacement high L/B hull) above a certain speed. The two foils are designed both to lift the mainhull early AND to possibly add to RM as max speed is approached.They also function to assure pitch stability given the massive sail carrying power of the boat.There is a lot of room for experimentation with the foils since it is possible that their area could be kept quite small. The test boat would be designed to utilize two crew trapezing off the windward ama.
I've built a small balsa model just to work out the geometry and it works out quite nicely.It is a weird looking hull with the displacement hull deck the bottom of the stepped planing hull! Next thing would be a fully functional rc model but that is quite a ways off. I think it is a beachcat killer with real high potential speed.Great applications on larger versions as well. But right now it's just an idea.... So what do you think?
Rotation would be accomplished from the cockpit using a belt drive crank(wheel) with a separate actuator for a carbon pin. The boat would be engineered by a naval architect and a smaller test version would be built first. The foiler I am building now will have removable amas(to replace the pods) so that rotation can be experimented with fairly soon-6mos to a year from now, I hope). Not just rotation but also the variable angle of attack of the planing surface using two foils for pitch control.
I don't think a small trimaran has been done yet that incorporates the power this thing has-except maybe Exploder or Reynolds early tri. The boat is designed to fly the main hull(like an ORMA 60) from a boat speed of 5 knots and up with the foils used PRIMARILY for pitch control.
A few more notes:
1)The rotation is only done at the as yet to be determined crossover point-not necessarily when the main hull flys(which will be at about 5-6knots boat speed max.) but when the planing hulls will work optimally.
2) Rotation will be manual and very simple and quick-it would not be frequently done at most venues.
3) Shrouds ,if any, will go outboard almost max(or where the NA says). I would like to look very carefuly at an unstayed highspeed rig like a Moth type(w/o the stays) as well as a wing rig for a speed version depending on weight. My realistic guestimate is that a partially stayed rig will be required.
4) Estimated upwind SA for this boat is 380sq.ft. compared to 227 for the Hobie Tiger. The Max RM for the 18 tri would be 10512 ft.lb vs 5197 ft.lb for the tiger-over TWICE the Hobie RM for the 18.
This is based on two crew of 160lb. each on trapezes with the 18 just flying the main hull and the cat just flying the windward hull.
5) The 18 would have 2lb/sq.ft. sail loading vs 3.15 for the Tiger.
6) The Hobie weighs 397 lb. and is fiberglass/foam sandwich and this boat would be 100% carbon foam/honeycomb and a very rough estimate of weight would be all up at 447lb.
7)The boat could probably be "toned down" substantially and still beat most beach cats. The idea here is to illustrate what is possible with the trimaran configuration. One of the central themes of the 18 are the small hydrofoils on the daggerboard and rudder-FOR PITCH CONTROL ONLY. This,and the fact that the center of buoyancy moves forward when the ama's are planing will give the boat extraordinary resistance to pitchpoling.
8) The concept allows substantially smaller ama's and lighter all up weight than would be required without the foils and planing hulls.