Foil-Assist Keelboats

Discussion in 'Sailboats' started by SuperPiper, Jul 27, 2013.

  1. SuperPiper
    Joined: Jan 2003
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    SuperPiper Men With Little Boats . .

    I've been grinding my way through one of the very long foiling threads on this forum. I'm getting consumed by the hype. I now want to put foils on anything that floats.

    My micro-cruiser will never foil. But would foil-assist lift its dragging belly even 1" shallower in draft? It's that giant bow wave that is keeping me from ratcheting maximum speed from 5.6 knots to 5.7 knots.

    I suppose if it was possible, it would have been done by now.
     
  2. Doug Lord
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    Doug Lord Flight Ready

    ===============
    How much does it weigh, including crew and anything else you normally take with you? How long(LWL) and how wide(LWL)? Does the boat plane off the wind?
    In my opinion at this point and with a boat like that something like DSS MIGHT be the best bet because the foil extends to leeward adding RM with vertical lift-which could allow more sail area in light and heavy air.
    They could be inserted into a slot in the side and pinned with the boat heeled upwind to keep the windward foil out of the water, or a trunk could be built to allow a single board to slide side to side. Or there could be two trunks allowing the board(s) to kick back like a centerboard.
    Post or PM the info and I'll let you know what I think based on that information.
    PS- there is a company using DSS on a cruising 50 footer and of course the Quant 28 and 30 raceboats using the technology:

    Pictures, L to R-- 1&2) JK 50, 3) the foil illustrated (at too little heel),4) the Quant 28 DSS(unmatched race record),5&6) Quant 36 DSS-just launched with two independent, retractable foils. Foils shown retracted in #6:
     

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  3. SuperPiper
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    SuperPiper Men With Little Boats . .

    Hi Doug. Thanks for the reply.

    I've read the DSS threads and i'm not sure i understand the concept. I think DSS may be smoke and mirrors.

    Instead of the DSS board being horizontal, it would provide the same righting moment if it was angled downward from the centre of buoyancy. Heck, if it was in the same vertical plane as the keel, the DSS board would provide righting moment. So why not add area and camber to the keel for the same effect?

    For my application, I'm wondering about a foil on the rudder to provide pitch damping and to keep the short waterline from squating on its own bow wave.
     
  4. SuperPiper
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    SuperPiper Men With Little Boats . .

    In fact, i've even confused myself now. The keel is trying to provide lift to windward and the DSS board would be trying to provide lift to leeward. If they were both in the same plane, the boat may stall itself (?!)
     
  5. Doug Lord
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    Doug Lord Flight Ready

    =====================
    DSS is definitely not "smoke and mirrors". It works by providing a substantial increase in RM while at the same time reducing displacement/wetted surface. It has been proven time and again with the most significant proof being the race record of the Quant 28 where it frequently beat many larger boats and almost always was first in it's class. The Quant 28 thread goes into more detail with video.
    --It might provide some RM in this scenario, but it would also increase leeway and not reduce displacement/wetted surface.
     
  6. Gary Baigent
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    Gary Baigent Senior Member

    Seen at Westhaven, Auckland, while walking to festival film; have no information but will research. Any ideas, Doug?
     

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  7. daiquiri
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    The point of the DSS system is that horizontal foils not only provide the RM, but also carry a portion of boat's weight. It means that less weight needs to be carried by the buoyancy and other forces acting on the hull, and hence the associated hull drag (mainly the wave drag) will be smaller.

    At displacement speeds close to and above the so-called "hull speed", the biggest portion of the overall drag is given by the wave train which the boat leaves behind (as you also have noted). The amount of this drag is strongly related to the displacement-to-length ratio (DLR) so, for a given boat length and everything else being the same, the more it weighs the more wave drag it creates.

    It can be demonstrated mathematically that if one can design an additional lift-creating system (such as DSS, but not only DSS) which has a higher Lift/Drag (L/D) ratio than the hull alone, the overall L/D ratio of the hull + "additional system" will be higher than the L/D ratio of the original hull. Meaning that it will carry the same boat weight with a smaller overall drag, hence with a potential of achieving higher speeds than the original hull.

    Besides that, a higher RM (in case of DSS) means the boat can carry more sail area, and hence have a higher propulsive force.

    The real trick, then, is to design an additional lift-generating system which has a L/D ratio higher than the Displ./Rt (Displacement/Total_Resistance) ratio of the original hull. For low displacement speeds it becomes nearly impossible, because bare hulls designed for low speeds generally have Displ./Rt ratios around 120-150. No foil system can achieve that value of efficiency.

    The effective value of boat's low-speed Displ./Rt ratio depends mainly on the displacement (keeping everything else equal). Heavier hulls show a higher Displ./Rt than the light ones. So for heavy hulls designed for low displacement speeds a system like DSS would be of no use because it would just add drag. The increase in RM would imo hardly make any difference in this case.

    Where lift-generating systems could become useful is the case of light-displacement racing hulls designed for high speeds. Light-displacement hulls have much lower Displ./Rt ratios at low speeds (around 25-30), and hence they are the potential target for such lifting systems. However, a Displ./Rt ratio of 25 is not easy to beat with a foil working in proximity of the water surface, due to added wave drag and loss of lift caused by the aeration. Much bigger gain would then come from the increase of the RM than from the decrease (if any) of the overall drag.

    Cheers
     
  8. Doug Lord
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    Doug Lord Flight Ready

    ================
    Gary, a few years ago Hugh built a 27' semi-scale model of a 100 foot DSS boat out of styrofoam. That may be it-I had heard it was in NZ or Australia and doing well racing.
    Other than that no idea.
     
  9. El_Guero

    El_Guero Previous Member

    Why don't they use a 15 to 30 degree wing tip and generate more vertical lift?
     
  10. Doug Lord
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    Doug Lord Flight Ready

    ===============
    Most DSS foils slide into a trunk so it wouldn't be possible.
     
  11. El_Guero

    El_Guero Previous Member

    We could consult.

    :)

    use a constant curve?

    An 8' span with a 9.3' arch above that should give a decent overall foil. Still leaves 4 foot of support in the hull, longer might be needed, extend the arch. Looks like the foils are about 4' in length when deployed.
     
  12. Gary Baigent
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    Gary Baigent Senior Member

    This KiwiDSS? has a very long foil; check the first bow on photograph - and it appears that they sail with the windward section flying (because of the gentle concave shape) and the leeward section curved close to the surface. I don't think they slide the whole length of the foil through to leeward; would be a very long underwater foil, and maybe that large area would be unnecessary, overkill, plus drag. But I know zero abut the boat; checked crew.org but couldn't find anything of relevance.
     
  13. Doug Lord
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    Doug Lord Flight Ready

    -------------
    Too bad-I was fairly well convinced it was the 27-any idea of the length of the boat and beam-I'll ask Hugh. I guess I'll send him the pictures he might get it right off the bat.
     
  14. Gary Baigent
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    Gary Baigent Senior Member

    Looks around 30 to me. Also the flared deck beam and trapezes - was that a Hugh trademark? Thought his designs had just a gentle flare - but the stepped bow has his look. Will find out more later.
     

  15. SuperPiper
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    SuperPiper Men With Little Boats . .


    A foil on the bottom of the keel or the bottom of the rudder would also provide an upward force as long as the boat remained approximately upgright (less than 30 degrees of heel). This might be more conservative than cutting slots into the bilges of my sweetheart.
     
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