Foil assist cruising cat possible?

Discussion in 'Multihulls' started by riskmore, May 12, 2018.

  1. riskmore
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    riskmore Junior Member

    IMG_1689.jpg

    This Dufour 82 with 30 ton displacement has keels and needs more waterline. I'm about to put on sugar scoops but wondered if a foil would help. I see even cargo ships are claiming better fuel efficiency with transom foils so maybe I can lift my transom like this but with T foils .
    Screen Shot 2018-05-12 at 3.33.31 PM.png

    Getting lost in the foil forums so I'm thinking about building a bracket on the transom and folding down the foil while dialing in the fixed angle of attack.

    IMG_1695.jpg

    If this is not a total waist of time then I'm looking for guidance on starting dimensions for stainless steel.

    If this works then I'll next consider this idea. Comments?
    966708f6-0161-4b95-972c-65628b2626a4.jpg IMG_1691.jpg
     
  2. Doug Lord
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    Doug Lord Flight Ready

    Why do you think that lifting the transom will make your boat faster?
    The best way to add foil assist is to use a lifting foil at about the same position a daggerboard would be located*-not at the transom. A rudder T-foil can be used to enhance pitch stability but is not generally needed for foil assist.
    *just forward of the longitudinal CG
    Catana 59 cruising cat with foil assist:
    catana59.jpg

    Catana 59 curved daggerboard foil:
    catana59- curved foil.jpg
     
    Last edited: May 12, 2018
    dsigned likes this.
  3. dsigned
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    dsigned O.R.C. Hunter

    82...
    :eek:

    So if your boat is 30 ton, and you're looking for some sort of assist, that means about 10-20 tons of lift. Maybe less as the hull itself is likely generating some lift. But in any case, I suspect it's going to require a large foil. I'd have to do some more work to figure out how big...
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    And a large foil, especially at low speeds = drag.

    Thus it is the usual lift v drag issue.
     
  5. dsigned
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    dsigned O.R.C. Hunter

    Back of the napkin seems to give a foil area of .8m^2 (for 10 000 kgs of lift at 8m/s -- roughly 15 knots --for a lift coefficient of .4). This seems small to me, but seems to equate to an 8 ft x 1 ft horizontal surface, which I suppose makes sense. I wouldn't be shocked if my approximations are wrong, though
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Well, using your simple numbers

    Lift = Cd x 1/2 x rho x Area x Speed^2 = 0.4 x 1/2 x 1025 x 0.8 x 8^2 = 10496 N = 1070kg.

    But still doesn't take into account the drag of the foil and its supporting structure! Ergo...what is the quid pro quo...?
     
  7. riskmore
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    riskmore Junior Member

    Thanks all for the comments.

    As to why the stern foil first I think the boat is loaded beyond the design weight because the waterline is at the top of the transom at rest and is creates what seams a hole with violent vortexes in the wake. This seems a case for sugar scoops to add waterline and improve the bottom release angle.

    Maybe I should start with a forward foil on the center but I'm not yet convinced there is a benefit to the basic concept to submerged foils making the boat faster at these speed . I've seen power cats with back swept v foils and dihedral in this position but no sailing cats with a center foil. There is a convenient structure to connect a swing down foil below the mast. Experimenting here would be more expensive. Do you think it more likely I would have a results here?
     
  8. DCockey
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    DCockey Senior Member

    My recollection is the idea of a foil at the stern of a ship is to reduce drag by interacting with the wake, not simply to provide lift.
     
  9. Doug Lord
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    Doug Lord Flight Ready

    You should probably have a naval architect(that really knows foils) help you with the design and engineering-or else you're asking for trouble. You definitely don't want a single lifting foil aft!
    This is "Happy Feet"-it uses a unique foil system that slides athwartship and consists of a wand controlled main foil forward and a rudder t-foil aft.

    happy feet sa 3.jpg happy feet- foiler site.jpg
     
  10. dsigned
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    dsigned O.R.C. Hunter

    That was my problem: didn't work out the units! Newtons =/= kilograms.

    So to lift the boat significantly out of the water (10000kg) , you're talking ten times the foil size = 8m^2.

    That's a huge foil area. I would be significantly less worried about the parasitic drag because a) you could potentially make it retractable and b) the difference in drag by making your cat essentially weigh 2/3 of what it does would be substantial.

    That said, I think OP might be better served by attempting to put said cat on a diet. Replace everything that's not a structural part of the hull with a lightweight version of the same. You could also increase the angle of attack of the foil to reduce the size needed.
     

  11. Ad Hoc
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    Ad Hoc Naval Architect

    Exactly!
     
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