Asymmetric hulls

Discussion in 'Multihulls' started by Trevlyns, Oct 15, 2006.

  1. Jimbo1490
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    Jimbo1490 Senior Member

    The Prindle 19 as well as the Tornado, on which the P19 design is based, have pivoting centerboards rather that traditional daggerboards. They beach quite well and after 17 years of ownership I have never broken a board after literally thousands of beachings. The difference in ease of turning is really amazing. I think everyone learning to sail in a beach catamaran as I did first learns to tack by backwinding the jib to carry you around. Later you learn to tack more properly so as not to lose so much speed in the tack. But in a rough sea (20-25 kts seas 5-8', IIRC) the quartering sea would not even allow my P16 to go safely into the irons. And trying to travel backward the heavy seas almost pitchpoled me backwards twice! Of course you should never go out solo on a P16 in 5-8' seas but for the record, it was only 10-15kts and 3-5 ' when I set out. These things happen with the ocean :rolleyes: A small lake like Lake Murray (50,000 acres) is MUCH safer.

    Since owning the P19, we actually look forward to the rough stuff because the boat handles it so well! I have sailed along for miles in a deep reach with the tip of the lee hull buried with no apparent affrect. The boat does not even seem to slow down. I have never even come CLOSE to a pitchpole, even with severe weight/balance abuse, downwind with spinnaker flying, etc. The other great rough sea beach cat is the Nacra/Inter 20 and of course any of the Supercat/ARC ships. The Tornado would be a great heavy air boat but for the lack of buoyancy forward of the front crossbeam. Not that it's bad, mind you; it's just not outstanding.

    Jimbo
     
  2. Stephen Ditmore
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    Stephen Ditmore Senior Member

    I've discussed this issue with Rudy Choi and with a fella in Gloucester Massachusetts (USA) who built a proa that was dead flat on the leeward sides.

    The proa builder said it was too extreme. The boat would actually climb to windward, he claimed, but he felt the vessel was not as fast as it could have been.

    Rudy Choi would multiply the inside offsets of his catamarans by a constant factor to get the outside, and claimed that some of his designs, when flying a hull, had effective waterlines closer to symmetrical than they would have been had he built symmetrical hulls.

    That raises another possibility.... achieving effective asymmetry by canting symmetrical hulls.

    Today there are analytic tools like Splash CFD that I think should be employed if possible to answer these questions. As with modern foil design, the shaping should be an outgrowth of the flow analysis, where possible.

    I'd love to hear from Tom Speer on this topic.
     
  3. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Here's a link to a paper that might help the discussion along a bit...

    TUCK, E.O. " Can lateral asymmetry of the
    hulls reduce catamaran wave resistance? ",
    20th International Workshop on Water Waves and
    Floating Bodies, Spitzbergen, Norway, 29 May-1 June 2005.
    Proc. ed. J. Grue, University of Oslo.
    http://internal.maths.adelaide.edu.au/people/etuck/pdfiles/vortex04.pdf

    I have verified Ernie Tuck's calculations and agree that asymmetry is likely to have some minor advantages but only when the hulls are not optimally-spaced. If they are at their optimal positions (for a particular speed) then asymmetry is unlikely to lead to any significant advantages.

    All the best,
    Leo.
     
    1 person likes this.
  4. yipster
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    yipster designer

    Leo, thanks for the " Can lateral asymmetry of the hulls reduce catamaran wave resistance? " input
    great study and math on wave resistance yet other factors like stability, windage, seaway,
    construction and amaacces may be more in favor of cambered hulls
     
  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    No argument there. It would be nice to see some actual computations or (repeatable) experiments to verify any performance claims. Advantages in construction and access are, of course, not really comparable to performance.
     
  6. fhrussell
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    fhrussell Boatbuilder

    But if you cant symmetrical hulls, then when heeling, the hull is in a symmetric attitude...correct? Lock Crowther did this early on. The Catana's are also designed this way...if I am following you on this.

    Yeah,...where is Tom?!?!
     
  7. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Affirmative. Conversely, if you don't cant a symmetrical hull it will become asymmetrical as it heels (esp if rocker and waterline half breadths are very different).

    Yipster, buddy.... by "cambered" do you mean rockered, or something else? Perhaps you're saying one could create an asymmetric hull by taking a symmetric hull and plotting the transverse offsets from a cambered axis (the way some asymmetric foils are created from symmetric foils). Do you know of any designer who has done it that way?
     
  8. yipster
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    yipster designer

    Stephen D, good to check my rough language but after a check it is the outstanding camber as in cars frontwheels i mean.
    sitting here with two envellops bottoms out wider, thinking bout how computers determine the uniform car design nowaday's.
    tv meanwhile tells planes structurals are computerised but still need a windtunnel, by the time its all virtual,
    materials, pricing and what else computerised it probably becomes a reversed process where the sum be the input?
    now these envellopes on the desk can also open each "hull" asymetric, sb wider than ps and mirrored making them asymetric right?
    hmm hairspray and color inkt testing next? :D
     
  9. dutch1955
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    dutch1955 New Member

    asymetrical amas?

    Hi my name is Curtis
    I'm looking for a trailerable tri with schoal draft, large cockpit for day cruising with friends. The Corsaire Sprint comes the closest, would asymetrical amas work on a tri? Barnegat bay is my homewaters and sandbars and mud flats surround me.the R22 mono is 2nd choice but I want more choices of where to sail (speed) like going around the island instead of up and downthe bay all day.
    Does such a boat exist? thanks
     
  10. fhrussell
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    fhrussell Boatbuilder

    Asymetrical amas are not unusual. I know Jim Brown had designed them many years ago and the general concensus seems to be positive. Also, Newick has an interesting asymetric ama design that starts as an ellyptical shape. You should look at a Tremolino, too...if that is the size that suits you.
     
  11. mattotoole
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    mattotoole Senior Member

    I raced a Hobie 16 for a few years. I can vouch for their having an upper speed limit beyond which they don't go easily, no matter how well sailed or how strong the wind. My experience was before the days of handheld GPS, but it seems to be around 15-18kt. I'm sure a modern H16 sailor could corroborate this. The H18, OTOH, would easily go faster and faster. This may not be all due to hull symmetery though -- the H16 has bouyancy and trim issues which may cause at least as much drag.

    None of these cats plane. Skinny hulls are just easy to push beyond conventional hull speed.
     
  12. mattotoole
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    mattotoole Senior Member

    You might consider the Windrider 17. The actual cockpit is small, but there's plenty of lounging room on the tramps, and plenty of bouyancy to carry a few folks. Of course it's more exposed than a Corsair/Farrier tri would be.
     
  13. designz
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    designz Senior Member

    Leo,

    I was reading your very interesting discussion here about asymmetrical catamaran hulls and I tried to download the technical paper you mentioned but unfortunately the link no longer works - can you please E-mail me the paper. My E-mail address is as follows;

    designz@floatingpoint.com.au

    Thanking you in advance for your assistance,

    Regards,

    Christopher


     
  14. Derek_9103
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    Derek_9103 Derek

    I read about the original Hobie 16's and Rudy Choy's Hawaiian cats having hulls which acted as leeway-countering foils - I extended the idea to trimaran amas, and a Google search led me here, where somebody has beaten me to the idea.

    The blurb in the thread which best captures my train of thought: "Roy [Seaman] did agree asymmetric hulls are great for cruising, not having to worry about skegs, boards, etc and having a very shallow draft."

    To check how trade-offs play out in real life I'll build a radio-controlled trimaran with foil amas, and "race it" against the same RC trimaran with normal amas.

    I'll play with taking the idea as far as it reasonably goes - if it's a flop, so be it - if there's some benefit but it hits the wall of diminishing marginal returns, I could make up the difference with a small(er than normal) centerboard.

    Hobie 16's are fast enough to be fun, and still sailing decades past their prime, their "foil" hulls can't be all bad.

    I'd guess if I picked a random wing-shaped foil, the center of lift would be closer to the leading edge (farther forward) than a normal ama daggerboard's. So would I choose a foil shape with a center of lift shifted toward the trailing edge (aft) than average, and maybe tweak other factors?

    (It's probably relevant that the angle of attack would be controlled by the degree which the boat makes leeway despite the ama foil's presence)

    Getting around to my question taking this thread further than it went before:
    Along the lines of Stephen Ditmore's comment about CFD analysis...
    I'd guess "the devil is in the details" about what exact foil shape would best implement asymmetry.

    What specific shape should the ama as a foil be, as a foundation?
    Would a NACA foil work?
    Shot in the dark... NACA 4412?


    I ALSO am curious about Tom Speer's thoughts about this, since his name pops up all over foil questions here, and out in the internet.
     
    Last edited: Sep 22, 2020

  15. tspeer
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    tspeer Senior Member

    I believe there's a fundamental problem with using asymmetric hulls to generate lift. The induced drag associated with the lift is inversely proportional to the square of the span. What that means lift from the hull comes with a lot of drag compared to a foil because the foil goes a lot deeper.

    The way I look at it, the total hydrodynamic side force has to equal the side load applied by the sail rig. So the lift from the hull/keel/foils is basically fixed - it has to match the side force from the rig. Drag is the coin with which you buy that lift. You can go with expensive lift from the hull, or you can go with cheap lift from a foil. Cheap lift means better performance.

    If you look at how a Hobie 16 is sailed upwind, it's typically trimmed so most of the side force is coming from the rudders. The lift on the hulls is mostly used to balance the yawing moment. In other words, the hull acts more like a canard than a wing, and the rudders are acting like wings.

    Now it turns out the induced drag associated with the lift also decreases with speed (squared). So if you go fast, you can get away with using the hull to produce lift, as with a Hobie cat reaching up and down the beach.

    As for the shape, a hull is a slender body and the whole notion of wing sections is based on the assumption that the wing has a high aspect ratio. So I don't think a NACA section is a good basis for designing an asymmetrical hull. Unfortunately, I don't know of any inexpensive way of calculating the flow around a lifting hull that you could use as a basis for designing one.
     
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