Planing trimaran as in Bethwaite HSP

Discussion in 'Multihulls' started by Armac, Nov 8, 2004.

  1. Armac
    Joined: Nov 2004
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    Armac Junior Member

    Hello everyone,

    A first post from a new member.

    I'm intrigued and sometimes fascinated by the possibilities of planing tris (and cats) similar to those developed as HSP's by Frank Bethwaite. The cat would seem to develop better numbers, the tri more user friendly and likely better in light air and upwind. Keeping the thing light yet strong enough without being overly expensive seems to be the main challenge.

    My initial thought is of a tri 16' o'all, with 11' amas, 12' beam and 2' "armchair" hiking racks for a crew of two. The main hull would be quite skifflike, with about a 4' chine width. Weight would be 250-300 lb, and depending on heeling arm and crew weight, the Bethwaite SCP would be in the .4 to .6 range. Bigger racks or traps would improve that as would lighter structure.

    My question is whether anyone out there as any experience with this type of beast? It would seem to be a pretty wild time blasting around passing pretty much everything, and yet could be surprisingly capable and durable in rough water!?....

    Armac
     
  2. Doug Lord

    Doug Lord Guest

    Planing tri

    I think Bethwaites boats are pretty neat. I've designed and built two planing tris(one 14' and the other 20') 20+ years ago-they were fast in medium to heavy air.
    Each had planing main hulls and high beam to length ratio ama's ; neither could fly the main hull.
    I think if I were to do it now I would have a high beam to length ratio main hull suited for light to moderate air and planing ama's that would use some sort of variable geometry system to allow them to function without too much drag at low speeds but at high speeds they would use stepped planing hulls. The boat would be designed to fly the main hull and could use a small hydrofoil on the daggerboard to hold the boat DOWN so that it stayed at the right angle for the planing ama's. I would use a wand to hold the main hull at the proper altitude and a rudder t-foil for pitch stability.....
     
    Last edited by a moderator: Nov 9, 2004
  3. Armac
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    Armac Junior Member

    Love been there done that

    Thanks for the perspective Doug, it's all in the details.

    What kind of weight were your boats?

    What "dihedral" angle, or main hull to ama angle did you use? My hope would be to fly both amas as much as possible, especially in light and moderate air. In heavier air, I can see using foils or ama geometry to encourage dynamic lift to leeward to avoid submerging the lee ama.

    Coming from a mono background I have an inbred prejudice toward a tri--rationally of course, but a cat seems to be the quick solution. Envision two linked sailboards with comfortable hiking racks and a huge amount of righting moment, a decent rig, and enough foil area to go upwind. Could be amazing.

    Still, for a more forgiving but still exciting ride, I'm tending toward the tri. Does anyone have some prejudice toward an optimal deadrise angle amidships and aft?

    Kind of a quirky thread but one ride might change everything.

    enjoy

    Armac
     
  4. Doug Lord

    Doug Lord Guest

    Planing tri

    Armac, both boats were designed so that the amas were substantially clear of the water when the boat was level-don't remember the exact angle.
    I love your seating philosophy-might as well be comfotable when going fast as hell!
    What gets me in small fast multihull design is that even though the 60' tris have pushed a square platform most designers (except Dr. Sam Bradfield and a few others) have gone with tri's whose overall beam is substantially less than square. The technology and materials are available to build a small square tri and the righting moment would trully be awesome.
    If you don't intend the ama to be in the water at max speed and you have a 12' beam it seems like you would have the max RM of a boat like the 49er; is that what you figured?
    I would think that if the main hull was designed to be easily driven and fast in light to moderate air then when the boats speed gets to be fast enough that the amas would plane (and you could fly the main hull)you'd have the potential of higher top end speed if you solve the geometry problem so that the amas worked well in the moderate wind range. And you would have over twice the righting moment of the HSP type...
    --------------------
    Armac, you had mentioned cats having good numbers but look at the rigting moment comparison between a typical 16' beach cat and a 16' monster tri with a 16' beam. Whether you used stepped planing hulls,unstepped planing hulls or displacement hulls for the ama's you would have way more power to carry sail than the beach cat .I can visualize using the seat/racks you mentioned on each ama and just really moving.... The major disadvantage is that the carbon cross arms would cost a fortune...In this case for sure-money is power!
    Good luck!
     
    Last edited by a moderator: Nov 9, 2004
  5. 249

    249 Guest

    The HSP was a very different beast to the one you're thinking of, Carmac.

    I don't know about the planing cat theory. It's been tried about 100,000 times before, with no real success. Naval architects say that the problem is the aspect ratio; a high-aspect ratio surface (read short and fat hull) is more efficient at high speeds than a low-aspect one (a long skinny hull like a cat or tri's). There may be also a problem with bow fullness and nosediving.

    I think it can be put as bluntly as this - since Herreshoff, people have been designing cats. There have been many, many attempts to get them to plane. they don't (or even if some of them develop some dynamic lift, it's very, very insignificant). I know guys who have designed C class and won world champs n A Class cats. These guys are not exactly idiots, if it worked they would have done it.

    The A class guys drop the WINDWARD hull in the water in severe winds, because the lee hull is getting pressed down so hard as to be slow and dangerous. If they were planing, the hull would not be so low. You only have to look at the As (which have fairly flat but very narrow U hulls) to see that the sailing waterline is under the stopped waterline - so they are not planing.

    I think the 60' tri hulls are flattish on the bottom for volume/wsa reasons, not so much for dynbamic lift but I'm not sure. Then again, Farrier's tris seem to plane so maybe the planing tri would work, because a tri hull tends to be beamier and therefore doesn't run into the AR problem.

    But your tri (which could be a fun dayboat) is very different to the HSP concept.

    There's a huge difference between your 11' amas on a 16' LOA, and Frank Bethwaite's amas of around 4'6" or so on a 18'-ish hull. The amas were NOT there to be used in normal sailing. They were strictly "training wheels" to hold the boat upright when things went wrong. In this role, the flex of the crossbeams worked quite well; when a float hit the beams would bend and the float would take some load but rarely get driven under. A vaguely similar concept was followed by the winning Formula 40 tri by the Gugeon Brothers.

    A 4' chine width hull is quite beamy by today's standards IIRC; I think the Bethwaite B 14 has a chine beam of around 3', and the newer Bethwaites are narrower. Don't take that as gospel, I have yet to measure a B-boat and I'm just going on the way a B14 trolley fits my Int. Canoe. The Vanguard Vector skiff also fits a canoe trolley pretty well, and the Canoe is just 3' wide overall.

    The HSP main hull would have been...hmmmm 2' in overall beam? It was round-bilged at the bow and had a chine aft.

    The whole device was )(*&^%$#@! wonderful to sail, a real buzz of a boat. However, it wasn't an enormous advance in many ways from an International Canoe; a bit lighter IIRC, a bit less hull beam, a MUCH bigger rig (no class rules) and much more beam (ditto). The little planing floats actually acted much like the Canoe plank; when you drop the plank into windward it just bounces and planes along (it's a great planing surface) until you recover - just like the HSP float did.

    The HSP could trip over the lee float where a Canoe can't, so in some ways the HSP was harder; but tacking was much easier. The point is, though, that the HSP is comparable in sailing difficulty with an International Canoe, so it's MUCH more sensitive than a cat. It's a full-on ride, steering and trimming like a skiff to keep the floats out of the water.
     
  6. Doug Lord

    Doug Lord Guest

    Planing Cats

    249, Yves parlier's cat certainly does plane -it hasn't shown that it is faster than the displacement hulled tri's in racing but plane it does.
    They attempt to solve the planing surface aspect ratio problem by using stepped planing hulls and they themselves predict that in under 10 with their varable geometry solution to making the drag of the step disappear the boat should be ok. And at over 20 boat speed- just sailing on the step- is when they predict the biggest breakthru speed-wise since their tests showed that drag at those speeds is just 20% of the drag of a displacement multi hull. It's the range between 10k and 20k where the boat is real draggy.
    I think a planing hull(on the ama's only) on a wide "beach tri" could have benefits at the top end of the speed range if the low end problems were solved-which I'm convinced they can be--time will tell.
    --------------------------------
    One additional thing-and a clarification: in my previous description of a planing(on the amas) beach tri I mentioned the use of rudder t-foils and a small foil on the main daggerboard. The reason for this is NOT primarily to hold the boat DOWN as I mentioned-which is a beneficial side effect; it is to enhance the pitch stability of the boat when flying the main hull and sailing just on the stepped ama. Stepped hulls are unstable in pitch so on Parliers boat the hulls each have rudder t-foils. On this boat the rudder t-foil is necessary but so is a foil on the daggerboard. Otherwise the whole thing could still pivot about an axis between the stepped surface of the ama and the rudder t-foil.
     
    Last edited by a moderator: Nov 10, 2004
  7. Armac
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    Armac Junior Member

    One of the tough things about named catagories of things is that they tend to stereotype visions. In using the word "cat" I'm only referring to two hulls not necessarily two narrow hulls. The "cat" version of what I'm thinking of would be like two sailboard hulls linked with a set of beams that would allow some freedom of athwartships movement-to keep leeward hull flat in the water while the windward one flew. The hulls would be shaped as planing hulls all the way, and if say of 12' x 8' o'all l/b with racks of 3'-4', that would give a righting arm of 10' or 11', against an all up weight (as a singlehander) of about 350 lb. yielding Bethwaites SCP ratio in the mid to high 60% range. This can be increased by building lighter or wider, both more $.

    Going with Lorsails planing ama on a tri and flying the main hull is probably the best combination of speed and seaworthiness (I know I'm gonna get blasted for that word) for the bigger size range of these beasts (15'-20'). The downside is expense and increased driver skill.

    The HSP type as described by Bethwaite and the last guy, has the advantage of minimum structure for a tri and good protection from that dreaded weather roll, but I think I can envision sticking that tiny lee ama in a wave and triggering a wet and costly sequence.

    What I think I'm tending toward is between Lorsails idea and the HSP. Bigger amas that could fly the main hull if thats what was happening, but would mostly fly free or pick up some righting moment. As for square proportions, yeah, that would bulk out the righting moment and the cost of beams. Wider racks would also help the RM. Kind of where I'd like to go with this is a doublehanded sailing toy, that is quick, comfortable, and not too fragile or expensive, and also doesn't need a champion skiff driver to survive. Kind of an old guy's revenge: load up a nice woman and a few beers, go out when it's blowing stink, sit back and play in the waves and pass most everything out there. High comfort to speed ratio.
     
  8. Doug Lord

    Doug Lord Guest

    Super tri

    Armac, the "high comfort to speed ratio" is great! In my opinion thats one of the advantages of the planing ama tri: Coupled with the foils for pitch stability and altiude(heeling)control the planning ama hull will be far less likely to pitchpole! You should have high speed and relatively solid control-much better than a Hobie 16 with two on the wire.
     
  9. Andy P
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    Andy P Junior Member

    There was a 'speed sailing' planing cat in UK in late 90's - Bootiful
    Featuring variable geometry hull to crossbeams-so the leeward hull could be advanced by 1 crew pedalling -this was also the 'steering' method by moving centre of effort. Sailsheeting also done by pedals and bike chains etc by the other 2 crew.
    see link http://www.ussc.co.uk/ not updated for a while....never went very fast.

    The 1/4 scale model with sailboard hulls was promising, ( but slow vs sailboard it was based on ) and when scaled up it didn't work very well. Too heavy, too much wetted surface I expect, and a problem to steer.
     
  10. 249

    249 Guest

    Sorry Doug, I should have said that the cats don't plane WELL.

    Armac, I understood the concept; I've sailed on smaller cats that had exactly the same concept as "two sailboard hulls", although they were not independently linked. In fact, one or two of these boats could be dissasembled and the hulls could be sailed as windsurfers.

    So they were sailboard-type planing hulls, and they still didn't work. They were dog slow in almost all conditions, and they suffered from nosediving.

    A board is different because you can trim it aft quite easily to lift the bow and change the aspect ratio of the planing surface; in fact, you HAVE to do that or you get catapulted. Furthermore, they are so light that moving the crew weight aft works very well.

    Finally, the planing-hull longboards like the Olympic Mistral IMCO are slower than the older round-bottomed "displacement" hull boards (like the old Olympic Lechner D2) around a course until the wind is blowing 14-16 knots. That's info directly from a guy who has a world championship win in one of those classes, and an Olympic medal in the other class.

    Considering how effective boards are at planing, the fact that the "displacement" shape is normally faster than the planing shape seems rather significant.

    You can also look at Moths, the most advanced of all dinghies. Whether they plane or not is a moot point, with some of the very best designers/sailors (like Andy P) but they are certainly not a standard planing hull and they are incredibly fast - competitive with the world's best Flying Dutchman!
     
  11. grob
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    grob www.windknife.com

  12. 249

    249 Guest

    I've gotta bag it the boat in that link. It's just another windsurfer/cat; the same as the Itza Cat from about 1983, and others.

    The designer says he's "taken what I considered to be the best aspects in contemporary boat design from the world of dinghies, multihulls and windsurfers and combined them in the NJS-125."

    Well, I don't know where the "best" of dinghy design comes into this beast. Nor exactly where the "best" of cat design comes in. After all, it's a very different concept to the best cats - A class, F 18s p'raps, Tornadoes, C Class. None of them have planing hulls with loose connections and a rig that's about 10' off the water. All of them have wing masts, and displacement-shaped hulls.

    Windsurfers? Well, if you sailed boards in a variety of conditons around a triangle with equivalent sails, you may well still find that the U sections D2s are the fastest; they are certainly faster than the Mistral type planing longboards this chap seems to have used for his inspiration. And he didn't seem to ask boardsailers what one of their problems is - it's that hulls capsize at speed under the centreboard or skeg sideforce. With a loosely-mounted hull with a big centreboard and a rudder you're definitely going to have hull-capsize problems, as this guy found out.

    And the rig? Well, maybe he has asked different people. But sailmakers/designers I know who are fairly leading-edge in boards and boats stress that they have very different characteristics (drag etc) and therefore almost all boats need very different sails. Modern boardsails are gutless as all get-out, but they are very very low drag. So you need massive sail area and either pumping or not sailing in light winds. Not to say that the big rigs are quite low aspect, unlike this one.

    Nor, it seems, to cats and dinghies share similar sail-shape needs according to the sailmakers I;'ve spoken to who know both. It's a bit like taking a motorbike engine and shoving it into a rally car; different beast with different "torque" required.

    When I write this sort of stuff, people say I'm disrespectful and conservative. But surely it is more disrespectful to just say, in effect "current cat design is all wrong, only I know the way it should be done and only I have been able to work it out".

    Surely a respectful way would be to walk into the AYRS or some library with a bunch of back issues of sailing mags and see that your "new" idea is in fact as old as the hills. Then perhaps you could think that the guys who have spent their lives designing current dinghy rigs and cats are not, perhaps, ****** - maybe they are as smart and as innovative as oneself? In that case if they haven't done it, there's probably a bloody good reason.

    Some people say that this attitude doesn't create innovations. I beg to differ. Look at Uffa Fox, he looked and learned about Pengilly designs and Morgan Giles designs and others before he created "Avenger". Bruce Farr learned from people and designed conservative small boats before he tried to make something new. So did the Bethwaites. When CSK designed the ancestor of modern offshore cats, they spent hours studying old Polynesian shapes. Nigel Irens of modern tri fame knows heaps about old luggers, and he learns all the time.
     
  13. Doug Lord

    Doug Lord Guest

    Planing Tri

    Armac, it seems to me-cost aside for the moment- that everything you want: blistering speed, ease of handling and a high comfort quotient could be achieved in a square(or wider) tri 16-18' long.
    I think the planing hull has real potential on this type of boat if used on the ama's because with a little ingenuity and variable geometry you would skip the draggy part of the speed range that affects most planning hulls.
    The hydrofoils(main hull only) mentioned earlier would not be used for lift up only for lift down but their real contribution is for stability in pitch and to a smaller extent in roll-the crew would provide the vast majority of righting moment. The foils would make this thing easy to handle-at least after you got over being scared to death!
    As to cost: the most expensive part would probably be the carbon tubes for the cross arms- (and engineering to get them right)and perhaps the hinges to fold the thing for trailering like the old Sailbird tri.There are other ways to do it besides hinges but that is a nice tidy way of doing it. One good thing from an experimental point of view is that all(99%) the money is tied up inboard of the amas so you could probably afford to experiment with the ama shapes to get them just right...Sounds interesting and just might break new ground with small multi's-think about it a bit......
     
  14. Armac
    Joined: Nov 2004
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    Armac Junior Member

    Pulling it together

    249,
    I'm starting to appreciate your positive negativity, and take what you say about the specific development that makes individual things work. I got caught in my own language trap in referring to two linked sailboards as an example of a *good* direction for planing cats, mostly I was referring to the righting arm of a cat config vs a tri and in the form of planing vs displacement hulls-or better hulls that could transition. What should make something like that work is the earlier transition to a planing mode than a course style sailboard due to a bigger rig and more righting moment. Anyway, the solutions I've seen to date admittedly bite. It is also grating to see the "walter wallet" approach with the overhyped future results.

    Doug,
    You've pretty well nailed where I am with this. Last go around was a mini tri, main hull about 18' x 3.5', square or nearly so, with 14' amas, asymmetric and planing, and a simple set of detachable carbon/glass/foam beams. The thing would be designed to be adjustable and adaptable as a development platform. If this thing gets built, it will be out of parts, pieces, and overruns from previous projects, and hopefully only a surgical infusion of cash.

    This seems to be a tough place (small planing tris) in terms of scale, lack of much prior art, and the vast range of other existing solutions to the high perf play boat and their devoted advocates. Whatever, I'm not giving up my armchair racks.

    Armac
     

  15. Doug Lord

    Doug Lord Guest

    armchair racks

    Armac, couldn't agree more about the armchair racks! Mono or multi I like the idea...
    Good luck and I hope you get to do this boat. Think about a main hull that is very narrow and low resistance so in lighter air drag is a minimum and then you can work on planing amas for when the main hull flys...
     
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