Whale-tail hydrofoils on small runabouts

Discussion in 'Powerboats' started by marshmat, Jul 30, 2005.

  1. marshmat
    Joined: Apr 2005
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    Location: Ontario

    marshmat Senior Member

    Anyone know any good plans/sources for whale-tail hydrofoils?

    The boat in question is a 15' x 5' open runabout with a 1994 Johnson J30 longshaft. The hull is a 2'-wide flatbottom on centre and a 24-degree vee to the outside of that. Prop is an OMC 3-blade aluminum, 10x13. Hull weight is 115kg and the motor another 50-60kg or so. The motor is set with the antiventilation plate about 1/2" below the bottom (any higher and it vents in turns) and it runs best at about 5-degrees positive trim.

    Since putting this motor on (upgrading from a 15) the boat now has a couple of quirks:

    1) When running solo or with one passenger sitting aft (CG about 4' forward of transom) a steady porpoise develops at about 18-20mph. Negative trim does not help at all. Putting the passenger in the bow (CG about 5-6' forward of transom) solves the problem and she is then good to 30mph.

    2) When running slightly stern-heavy, there is excessive bow rise (up to 15 degrees) when climbing to plane. Negative trim helps a bit but screws up high-speed handling. Adding weight in the bow solves this.

    Both problems seem to point to a 'whale-tail' hydrofoil bolted to the antiventilation plate. Trouble is, cash is a tad short for this job. So I'm looking for any do-it-yourself plans for such a gadget.... having built the boat myself, I'm comfortable doing the modifications but would rather work from something proven than something I drew up myself... Any help, folks?
     
  2. yokebutt
    Joined: Aug 2004
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    yokebutt Boatbuilder

    Matt,

    How about trim-tabs? I'm thinking used piano-hinges, some stainless sheet-metal, a few pop-rivets, a couple of discarded turnbuckles from the nearest rig-shop and you'll be in business.

    Yoke.
     
  3. Corpus Skipper
    Joined: Oct 2003
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    Corpus Skipper Hopeless Boataholic

    I'm with Yoke on this one. Whale tails are better left to someone with more letters after their name as they employ a foil shape that is probably easy to screw up.
     
  4. marshmat
    Joined: Apr 2005
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    Location: Ontario

    marshmat Senior Member

    Hmm... that might work, ya. Crank 'em down when running tail-heavy, and back to level when the bow's loaded..... how big do you think they'd need to be? (i've never seen tabs on such a small boat before...) 10" wide by 8" long or something? This does seem like the kind of thing I could hack together in the garage and have it work!
     
  5. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    If it is a tiller motor, a piece of PVC pipe extension will let you steer from farther forward.
     
  6. marshmat
    Joined: Apr 2005
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    Location: Ontario

    marshmat Senior Member

    It's a tiller motor, ya... but it's got a few controls in the end of the tiller handle other than the throttle (engine stop, emrg lanyard, throttle friction) and I can't find an extension that doesn't block those.
     
  7. gonzo
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    gonzo Senior Member

    Could you fit a PVC pipe with a hole for the emergency lanyard?
     
  8. Aquatek
    Joined: Jul 2005
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    Aquatek Engineer/Inventor

     
  9. marshmat
    Joined: Apr 2005
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    Location: Ontario

    marshmat Senior Member

    Yeah, that's kind of what I was thinking of, Aquatek. Too much $$ for this project though, unfortunately, at least from the dealers around here who would have to order it in from halfway around the world.

    I'm kind of liking the scrap-metal-trim-tabs idea, believe it or not. I've never seen them on a boat this small though, anyone tried this and can comment on how well it works? Any idea what size the things would have to be? (Boat data in post #1.) Thanks for the ideas everyone.
     

  10. lewisboats
    Joined: Oct 2002
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    Location: Iowa

    lewisboats Obsessed Member

    Ebay!

    Steve
     
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