Possible to extend LWL by protruding/trailing foil?

Discussion in 'Hydrodynamics and Aerodynamics' started by Niclas Vestman, Jan 29, 2020.

  1. Niclas Vestman
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    Niclas Vestman Senior Member

    Sorry for possibly dense question. But there it goes: hull speed is severly limited for most non planing small boats, due to their size. When nearing 40 to 50+ feet, especially for multies, hullspeed becomes less of an issue (except for total speedjunkies). There fore, even a modest increase in LWL might make a huge difference. The jump in hullspeed from 25' to 36'LWL should be about +20%. Not to shabby for a meagre 9'. So ignoring practicallity... "Would it be possible to extend the theoretical LWL by using either or both protruding/trailing horizontal hydrofoils?" The idea being to make a "dent" in the bow/sternwave and although making the hullwave more komplex in shape (in effect making a longer one by combining the hull wave with with one or several formed by foils, into a larger component one), still getting a longer theoretical LWL.
    trailing ones I belive, allready exist as hullvanes. I would imagine aft extendind poles with L or inverse T foils at the ends. Obviosly heavily raked backwards. Maybe with 2 or 3 foils in a row on each pole.
    Cases. A 30' sailing cat. With decent load carrying capacity for extended cruising. Most designers would struggle to get L/B ratio above 9:1. But if LWL could be extended by about 6' foil-poles. In my head hullspeed increases by about 10%, and L/B ratio to about 11:1. And that in it self should make a significant impact on drag reduction. Maybe boosting performance with another 5%? With small displacement boats around say 16' the difference would be 50% increase in LWL with an 8' extension. Or about +24% hullspeed.

    So ignoring the obvious impracticallity. Would it be possible? And if so, how far apart could foils be spaced in order to make a good composit waveform? (My guess is that such a solution should be designed together with the actual hull, to be able to function well together.)
    Am I way off?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    You don't get much lift out of foils at the speeds you speak of, or if you did, by reason of size or angle of attack, the drag would spoil the party.
     
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  3. Niclas Vestman
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    Niclas Vestman Senior Member

    Or another case. Wanting a kayak with small form factor for easy handling and transportation, but wanting the low drag of a long one. If having a 12' (3,6m) base, with 2 foils extending both forward as well as aft, spaced a foot apart. That would in effect give the kayak a composit LWL of 2+12+2 feet. (If this works). Making it a full length touring one. Well... probably a bad idea. Bit would love to hear your thoughts!
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Foils give lift, but are not affected by waves the way that buoyant hulls are. You can't duplicate a long boat by adding foils fore and aft to a short one.
     
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  5. Niclas Vestman
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    Niclas Vestman Senior Member

    I did also think about that. But then again. I was mostly thinking about small=light vessels, and at relatively "high speeds". Preferably small multies pushing ca 2,5xroot(LWL). For a 25'er, that would be 12+knots, allmost 1.8 times the lift of speed for low speed foilers. Also the length wise, fore most 10% and aft most 10%, probably create very little of the actual displacement on eg 36' cat hulls. Maybe <2,5% of the displacement. For a light 2 ton fully loaded boat, that would mean 50kg lift foil (in case of single foil per pole design). Thats less than half the design load for some windsurf foils.
     
  6. Niclas Vestman
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    Niclas Vestman Senior Member

    I think it is theoretically possible, but not practice. Mostly because allthough foils redirect waterflow, just like sails do with airflow, the effect extends a very short distance aft of the foils along it's cord line. So to make it work, foils would need to be spaced allmost without spacing after another. Thus giving much higher drag than what could be gained.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    I guess if your trailing foil had attention paid to the angle of attack such that when the boat squatted, it was getting the best lift/drag characteristics, and when the boat was running slow, it could be tilted out of the water, you might get a benefit.
     
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  8. Niclas Vestman
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    Niclas Vestman Senior Member

    My idea was to combine it with a semi flexible pole, so that pitch, but also yaw and roll motions would to some degree be converted to forward motion. And also, the foils would at the same time act as stabilizers reducing those motions and hopfully increasing comfort.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    There is a lot of untapped potential in foils, in both sail and power craft, the snag appears to be unwieldy appendages that can be troublesome to keep out of harm's way.
     
  10. Blueknarr
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    Blueknarr Senior Member

    You are thinking way old school.

    100 years ago sailing yachts had very long bow and stern overhangs that lengthened the waterline while healed over. Sitting upright the waterline was significantly shorter but also less wet surface area drag.

    High winds = more power = more healing = more speed.
    Low wind = less power available so less drag to slow the boat.
     
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  11. HJS
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    HJS Member

  12. tlouth7
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    tlouth7 Senior Member

    National 12 sailing dinghies have foils on their rudders that do exactly what you describe; they modify the flow off the stern to flatten the wave and mimic a longer boat.

    [​IMG]
     
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  13. baeckmo
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    baeckmo Hydrodynamics

    I associate Niclas' thoughts to research by the late Peter Payne, who suggested the use of trailing foils to capture the energy stored in the transverse waves; not for lifting purposes but to produce forward thrust. Somewhere i my "sediments" I have a report on the subject; I'll if I can dig it up for you later.
     
  14. HJS
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    HJS Member

    Report on systematic tests.
    JS
     

  15. gonzo
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    gonzo Senior Member

    The usual solution to extending the waterline is to install trim tabs.
     
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