Slender non-planing foil assist catamaran?

Discussion in 'Hydrodynamics and Aerodynamics' started by S V, Dec 28, 2020.

  1. DogCavalry
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    DogCavalry I aim to misbehave.

    Gonna work until sometime on the first day you hit something in the water.

    If you're serious about your requirements, and can let go of the foil fetish, you can succeed with a Hickman Sea Sled.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    If an amateur like me can make and fit a foil to a tunnel hull, that works, but admittedly on an outboard powered boat that is a convenient way to adjust longitudinal trim easily, then it can't be a dark art. I think the problem SV appears keen to inflict on himself, that I didn't, is a very high lift fraction coming from the foil(s). I wouldn't get into that. In any case, it is one thing to guess and tinker cheaply as I did (albeit successfully), another entirely to invest large sums into a project without a solid theoretical basis for thinking it will be successful. Not a game for dilettantes.
     
  3. DogCavalry
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    DogCavalry I aim to misbehave.

    High efficiency foils have angle of attack behaviors that boat people just can't appreciate. I came from aircraft design. The lift an efficient foil gives varies enormously over relatively small changes in angle of attack. Pass over a deep wake where the subsurface motion of water changes the AOA by 3° and the boat may pitch or roll violently. Pick up some trash such that laminar flow separates, and that foils lift falls to zero, rolling the boat enough to possibly injure passengers. Foils are attractive for lifting displacement hulls clear of wavemaking resistance. You'd think every navy on earth would use nothing else. US Navy literally, literally has a budget greater than every other Navy on earth put together, and they aren't interested.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    the boat doesn't have to be a displacement boat, and foils are used successfully on many planing cats, so it isn't a case of blanket rejection, I wonder whether something like this sketch, that would need surfacing foils to give differential lift according to immersion, to maintain transverse stability, might work, and with a single main engine. IMG_0002.jpg
     
  5. DogCavalry
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    DogCavalry I aim to misbehave.

    I know. But the ratio of lift is very low, so flow instabilities are manageable. And nothing can prevent FOD(foreign object damage). They work best in situations where massive reliability is not critical, and foil failure is only a nuisance.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    I would think foreign object damage is a problem in any fast boat (>30 knots) operating in waters with debris, not just with foils. but I think appropriate countermeasures could be devised, with a bit of thought.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Well-raked foils, with nowhere to trap incoming debris, comes to mind.
     
  8. DogCavalry
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    DogCavalry I aim to misbehave.

    Raked foils, where the mean cord centerline is not normal to the fluid flow pay a huge efficiency price. Induced drag goes up enormously. Marchaj lays it out beautifully in Aerohydrodynamics of Sail.
     
  9. Mr Efficiency
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    Oh ? Then why are those admittedly dinky little "hydrofoils" bolted on the outboards, raked ? They haven't heard ?
     
  10. Mr Efficiency
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    I'd be happy to see any relevant info along the lines you describe, DC, if you can point me too it, obviously those
    "toy" hydrofoils are raked so they shed weeds and what not.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    I'd be happy to see any relevant info along the lines you describe, DC, if you can point me too it, obviously those
    "toy" hydrofoils are raked so they shed weeds and what not.
     
  12. DogCavalry
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    DogCavalry I aim to misbehave.

    Aerohydrodynamics of Sail by CA Marchaj, he of blessed memory.
     
  13. Mr Efficiency
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    OK, obviously don't have the book, and don't like the look of the websites it can be downloaded from, infected.
     
  14. DogCavalry
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    DogCavalry I aim to misbehave.

    Yeah. I paid $100 for it, 30 years ago. Must be one in a library. Or Ad Hoc would know what the current equivalent is. It's not out of date, but probably out of print.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Hopefully B McF can clarify this matter, certainly I can't say I have seen many with swept "wings"
     
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