High performance small tri project

Discussion in 'Multihulls' started by frosh, May 13, 2007.

  1. frosh
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    frosh Senior Member

    Tom I disagree!

    From the looks of the drawing above, I'd say the amas are way too far back - their center of buoyancy needs to be ahead of the center of gravity by an amount that depends on the L/D of the rig -QUOTE- Tom Speer.

    The whole key as to whether my design will work successfully, as I expect it will, even with the amas NOT moved from the position shown in the diagram I posted; or whether your opinion is correct depends mainly on the following provisos.

    Are the trimaran heeling forces generated by the sails on any point of sailing forward of a broad reach going to be counteracted primarily by:
    EITHER (1) The buoyancy and leverage arm of the leeward ama, or
    (2) The weight of the crew sitting far enough to windward to signicantly reduce the load of the leeward ama so that it is not hard pressed into deeper and deeper immersion as the wind strength increases. Even more important, the crew will control with agility and weight transfer to windward, the amount of pressure fairly precisely, and consequently how much downforce will be transferred to the leeward ama.

    Conventional trimarans follow the principle described in (1) above. Your references and own arguments are completeley valid if this is the case.
    In the case of my experimental tri, I am relying hardly at all on principle (1) and almost entirely on principle (2).

    This is not possible on even a Corsair (Farrier) Sprint 750, (a fairly small tri), although placing crew weight to windward as far as possible in a good breeze will undoubtedly add to performance. This is a small tri, and on anything larger, we can discount the effect of crew weight to windward in proportion to increasing hull and rig size more and more as the boat becomes larger. Whether this is in direct proportion or some sort of logarithmic proportion is not highly relevant to my design or this argument at all.

    My Tri is expected to weigh in fully equiped to sail, without crew at around 85 to 90 Kg. This is less than 200lb. for you Americans. Crew weight depending on who is aboard will be around 160kg for two men fully clothed to sail.
    Again this is in the region of 350lb.
    Furthermore the heavier crewman will be fitted witha a trapeze so that he can extend another 1 meter further to windward when necessary.
    The crew weight as a total fraction of sailing displacement is huge for any normal sort of tri, and so is the the proportion of crew weight leverage to sail heeling moment huge, in relation to any normal sort of tri. My tri is fitted with a fairly modest size rig with a reasonably low CE for the sails.
    I don't want to go on with much more technical stuff in this post and turn it into a marathon, but this is the gist of it all.
    There is also the fact that my vaka generates a lot of hydrodynamic lift in the region about 25 to 50% back from the bow. We already know this from real sail testing as a proa, and the forefoot tends to ride a bit above its designed lines already. This was exactly the same trim situation encountered by "Crossbow 2", in its proa incarnation. I will post a photo of Crossbow 2 sailing at speed in this trim situation next time.
  2. frosh
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    frosh Senior Member

    More on planing multihulls

    As promised in the previous posting here is the link to "Crossbow 2". It shows some superb, but old classic photos.

    Also, Tom, I am not sure that you have got Jim Antrim's design philosophy correct about his broad flattened ama stern sections being for windage reduction and not for planing. I couln't find the article re Antrim in your links. Perhaps you can post the link more precisely so we can read Jim's words for ourselves.
  3. CT 249
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    CT 249 Senior Member

    Frosh, your boat looks to be similar in concept to the Bethwaite HSP. That had the floats even further back, and it was a stunning machine to sail.
  4. tspeer
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    tspeer Senior Member

    "The weather ama is always completely exposed to the wind, and at least the top half of the leeward ama is exposed. Considering that 200 percent buoyancy was desired (exactly as much hull above the full load waterline as below), a section shape for minimum wetted surface, and a beam/depth ratio betwen 2 and 2.5, as well as a cross-sectional shape to minimize windage, a circular cross section for tha ama was an obvious choice. This cross section has a Cd of about 1.2 as compared to 1.5 to 1.6 for a more typical oval-shaped cross section (see reference [1]). The circular section is also ideal from a structural point of view both to resist torsion and for strength against hydrostatic pressure. In addition, hull surface area is minimized, and consequently hull weight."

    "The ama bow sections need to be higher than they are wide for obvious reasons. I wanted the stern sections to be wider than they are high, both for dynamic buoyancy and to minimize windage. Consequently the ama cross sections shown in Fig. 8 are elliptical over the full length of the hull. Obviously waves will frequently be sloshing over the low or no-freeboard stern sections; however since the water will en effect be flowing downhill it will develop a slight forward force component that will at least partially compensate for the increased frictional resistance."

    Antrim, James K, "Design of a 40-ft Multihull Sailboat for Offshore Racing," Marine Technology, Vol. 27, No. 5, Sept. 1990, pp. 285-299.
  5. frosh
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    frosh Senior Member

    Hi CT, The Bethwaite HSP was exactly what I was trying to emulate, except I believed their floats were too small and became a liability every time one hit the water. Also the centre hull could easily be further optimised by going narrower . My aim was to turn these two negatives of the HSP into two positives.

    Tom, you are right and I am right about Antrim's design aims for his amas and I quote from your posting.

    " I wanted the stern sections to be wider than they are high, both for {dynamic buoyancy}, and to minimize windage"

    I already knew this, but wanted you to face up to it also.

    Cheers. Sam
  6. Chris Ostlind

    Chris Ostlind Previous Member

    Antrim's "planing" amas

    Interesting that now, some time after the fact, that Tom Speer posts a suggestion that Antrim's amas were not of the planing type.

    http://www.boatdesign.net/forums/showthread.php?t=14047&highlight=planing trimarans

    In fact, as Tom so purposefully demonstrates in his argument, Jim was going for a serious reduction in windage as opposed to his, so-called by others, suggestion that his amas would plane when pressed. He never made the "it will plane" claim. He only suggested that the forms were conducive to the potential. Big difference.

    Thanks, Tom, for proving my point, even if you were some 8 months out of the loop.

    Chris Ostlind
  7. Doug Lord

    Doug Lord Guest

    Fact vs fiction

    Jim Antrim most certainly does make the claim that his amas plane:
    Under "Ama Shapes"-
    1)-DIRECT QUOTE FROM JIM ANTRIM:" The high buoyancy planing geometry is highly resistant to burying the bow at any speed."
    2) DIRECT QUOTE FROM JIM ANTRIM:"..dynamic lift from the ama hulls."
    Antrim 30+
    Address:http://www.antrimdesign.com/trimarans/erin/ Changed:1:17 PM on Friday, September 30, 2005
    Chris, I think you seriously misunderstood Tom's quote of Jim Antrim-in that quote he refers to "dynamic buoyancy"- another way to describe planing.
  8. Chris Ostlind

    Chris Ostlind Previous Member

    I'm not going to do this dance with you, again, Doug. You're making words where there aren't any present.

    Tom's comments support the argument I made and they come directly from Antrim's SNAME account.

    Perhaps you can take on Tom's description of his take on Antrim's article from SNAME as a means to justify your position?

    My position has been clear from the beginning and shall remain so until a serious set of proofs emerge. You have not provided that information and now, Tom has nicely supported my argument from Antrim's own posting.

    It is rather convenient that Tom has provided this suggestion but when you take a long look at the amas on both Erin and the Antrim 40, "Zephyr", you will see that Jim has been consistent in his design application with regards to form and function.

    Go ahead and argue otherwise, but you can't change what the designer, himself, has written on the topic. This suggestion comes after you have placed such great credence on the work of Farrier and his planing claims and design approach, so it seems only fitting that you would now graciously back away from the argument in light of this published position.

    Or, are you suggesting that Jim Antrim has no idea as to his own design purpose?

    I have merely made observations in the past that Antrim's amas were not of the planing variety and to support that observation, I directed you to Antrim's work on Erin. Now, Tom has further illuminated my position from a well documented paper published by SNAME, dealing with amas with design elements virtually the same... and you want to argue that point with impunity.

    Go ahead... argue away.

  9. frosh
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    frosh Senior Member

    From Jim Antrim's own web site on the 30+

    Brief Description

    Easy handling and high performance sailing in this high quality and ocean rugged racer/cruiser. High buoyancy and dynamic lift from the unique ama design provide an unusual level of safety and control. -QUOTE- JIM ANTRIM

    I think that this own quote from Antrim where the first paragraph emphasizes the design aim of his amas, as providing "High buoyancy and dynamic lift", pretty much settles what Antrim wants to achieve as his highest priority for the amas of his designs.
    It does not seem about "Windage", although I accept that elsewhere he does discuss that keeping the windage down on both the leeward ama topsides, but more significantly the entire windward ama is an important part of his design brief.

    However the fact that the very FIRST paragraph only discusses high buoyancy and dynamic lift is enormously revealing!

    Why continue arguing about this then?

    ALSO - Who is now prepared to stand up and be counted in the camp that
    "Trimaran amas definitely dont plane!" So is anyone able to explain the meaning "providing dynamic lift", as it pertains to boat hulls, if the meaning is really something quite different to planing.

    Remember if you make the statement supporting your contention that amas can never plane, please write your name underneath, so that the rest of us can later remember who you are.
  10. Chris Ostlind

    Chris Ostlind Previous Member



    Don't all hulls provide some degree of dynamic lift as soon as they move forward under provided power?

    Is that the same as planing?

    My original contention which goes back to the hissy fit discussion back in November was: A trimaran does not plane if it is getting substantial weight shift support from the leeward ama under sail UNLESS... that body (the leeward ama) is also planing...In which case, the entire craft could be considered as being on plane.

    Really simple.

    I have never seen a vaka hull being fully supported on plane without the amas taking up substantial load in an immersed, non-planing, condition. The times when I have been shown photos and/or video clips to show a leveraged vaka skimming along the surface, it had substantial amounts of the leeward form fully depressed and not anywhere even close to planing. So, it remains that I take this position.

    Tom Speer has indicated at least once in the past that he has, in fact, scooted across a body of water, on plane, with both amas fully clear of the water. This tiptoe balancing act would meet my description, but alas, Tom has not seen fit to provide anything other than his anecdotal commentary. He has not, apparently, repeated the exercise with decent documentation to provide a measure of support for his claim.

    Now, Tom's a pretty aware guy and he doesn't seem to be the type to wander off the science reservation for rabid claims, so there could be real validity to his commentary. Likewise, because of his serious science bent on all things boaty, I'm entirely surprised that he has never come to the table with just a bit more of what he, himself, would expect from a colleague who put-forth this level of claiming position were it to be within the field of his profession. In short... where's the beef?

    I get the yak-yak about planing trimarans for bar chatter after the sailing day. That part of it is just fine with me as one is supposed to extend one's story telling capacity for friends, especially when they are buying.

    But, for a real scientific claim, I think everyone here would agree that there should be more than pub chat on the topic.

    For the record, I'd love to see a trimaran flashing along on full plane with one, two or three hulls all doing the bad thing, as the case may be according to design application. It's just my position (shared by many others) that an immersed and fully weighted ama on the off side does not qualify the entire boat for the claim, dynamic lift to some degree in the ama design or not. Get that leeward ama up on the top, along with the vaka and you've signed me up as a proponent.

    And this is no, "One Mississippi, Two Mississippi" kind of thing either... it's a substantial sailing rip of many hundreds of yards... let's just say five hundred to keep things clean.

    Your new boat, Sam, just might be able to get the trick done and I hope it happens that way for you. I don't see it happening with any of the trimarans I design, nor do I see it from any Farrier boat, along with dozens of other speedy tris that are out there.

    The slate is clean but we haven't gotten there yet.

    Conventional style tri amas, whatever the source of the design, would have to have lighter hulls, really substantial rigs and amas shaped completely differently than the ones we typically see to provide a planing surface that could dynamically support a good portion of the boat weight, while the vaka supplies the rest.

    So, go ahead and put my name on the list until the design idiom of the typical trimaran ama changes.

    This isn't an emotional thing for me, so don't anyone go and get all mouth breathing snarly about the topic. Six months ago I asked for proofs in the form of a solidly shot video clip, an extended motor drive sequence, etc. and nothing that was shown came anywhere even close to showing a tri with its ama planing while the vaka hull was doing same. Plenty of cool shots of tris hauling ***, which I really got foamy about, but nothing that could stand the argument up with substance.

    Six months and nobody can provide a single, good video clip to support the topic. Now, if you want something telling, that pretty much says it all.

    My Name... Chris Ostlind
  11. Phil Stevo
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    Phil Stevo Junior Member

  12. frosh
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    frosh Senior Member

    Maybe there soon will be a tri that planes.

    Hi Chris and Stevo, and other interested people in this topic.
    The entire purpose of my current tri, and there is no other purpose, is to prove once and for all, that a tri can be designed to fully plane on say two hulls simultaneously, the third hull being airborne. I am talking serious planing here, like an IC (International 10 sq. m Canoe) going flat strap on a reach in 20 knots of wind. No immersion of the leeward ama beyond an inch or two.
    It might not work, but I have a very strong feeling that it will, and maybe even go quicker than an IC for a fairly similar overall length. (18ft. 6 inches, plus bowsprit). It will never be an open water travelling boat, but it is for blasting on our local bit of sailing paradise, The Swan River, in Perth, Western Australia.
  13. frosh
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    frosh Senior Member


    In my post. no 65 the link shows the boat that I refer to, top left photo. It is actually "Slingshot" not "Crossbow 2". Sorry about any confusion.
  14. gggGuest
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    gggGuest ...

    I cannot imagine that anyone could have any serious doubt that a trimaran configuration could be made to plane on centre and lee hulls. Its just a question of having the appropriate shaped hulls. Surely the question is whether such a configuration would have useful performance or other characteristics when compared to a non planing boat.

    The available evidence seems to suggest is that beyond a certain fineness ratio planing starts to lose its value, presumably because the efficiency of displacement sailing increases with the much lower wavedrag, and the efficiency of planing reduces with the decreasing aspect ratio of the hull.

    So Bethwaites HSP was fat enough to plane, but couldn't operate successfully as a true trimaran (which, to be fair, was the idea). Successful trimarans in the past haven't planed. So surely the question is whether there's a sweet spot in the cross over area that will enable a *successful* planing trimaran. Actually making one plane should not be that big a deal. Three IC hulls (a Slurp in the middle, Nethercotts on each side maybe) would make a planing trimaran, but probably not a very good one...

  15. PI Design
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    PI Design Senior Member

    Frosh - the boat must necessarily have some heel to get the leeward ama wet and the windward ama out of the water. Is the central hull optimised to plane at this angle? In my experience even small heel angles really kill the ability to plane on conventional hull shapes.
    Off topic slightly, but somewhere on the net you'll find details of a pentamaran frigate which consists of two amas on each side - one slightly higher out the water than the other. At small roll (heel) angles only one ama touches the water. As the roll angle increases the second one does as well. This has the advantage of not putting more boat in the water than is needed, graduating the benefit of the amas. I've wondered whether this configuration could be applied to sailing tri's.
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