Bilge keel wing-section lift.

Discussion in 'Boat Design' started by Cedric Oberman, Jun 4, 2022.

  1. Cedric Oberman
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    Cedric Oberman Junior Member

    Hi Everyone,

    I was watching a boat build video on YouTube in which the builder discussed his decision to fit the bilge keels that he's making for his 14ft ocean going sailboat with a wing profile on the inside, made from a foam aerofoil section fibre-glassed onto the flat steel plate from which each of the bilge keels is made. He explained that, when heeled over during sailing, the leeward keel would be roughly vertical because of its canting angle and due to the wing profile combined with the toe in, it would be generating lift to windward which would reduce leeway. Simultaneously, the other keel, which would be angled at the sum of its canting angle plus the boat's angle of heel, would be generating lift that would act to oppose the boat's heel and thus cause it to sail flatter and hence more efficiently. You've probably guessed the part of this that I'm having trouble understanding - if the centre of lift generated by the vertical leeward keel is below the centre of lateral resistance of the entire hull at that angle, wouldn't it counteract the righting effect of the other keel?

    A supplementary question - would this arrangement produce more or less drag when sailed downwind than the much thinner metal sheet alone?

    All the best,

    Cedric
     
  2. bajansailor
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    bajansailor Marine Surveyor

    H Cedric, have you got a link to the video that you saw please?
    I would be a bit dubious about glassing foam on to a steel (probably mild, not stainless) plate.

    Re your question, I think that there would be some counteracting effect of the other keel.

    When going downwind I think there might be slightly more drag with the aerofoil arrangement, but I doubt it would have any noticeable effect on the speed, especially on a 14' boat.
     
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  3. Cedric Oberman
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    Cedric Oberman Junior Member

    Hi Bajansailor

    Here's the link. At least I think it is the one (big fingers small phone)


    Cheers
     
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  4. philSweet
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    philSweet Senior Member

    It doesn't work. Proven over and over again. The drag penalty is always too big. Bilge boards can work, but you retract one of them. One very basic problem is the distortion of the displacement curve is much worse for twin keels. And it doesn't matter if the keels are flat and toed, or parallel and cambered. If they generate different side forces, they are going have higher drag than a single foil generating the same resultant force.
     
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  5. Cedric Oberman
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    Cedric Oberman Junior Member

    Thanks for your answer Phil. I would have thought so too. He rightly mentions the practical advantages of twin keels for towing/beaching/interior space/maintenance etc but I'm confused as to why these are his priorities given that it's not a versatile craft he's building; it's an offshore passage-maker. Surely not much room for compromise and not much scope for beaching or stopping to apply antifoul? Actually, for me and my coastal day-sailing on the English South coast, those benefits might well be worth the penalty of greater drag associated with two keels of low aspect ratio, if I ever build a boat, I'll consider it for sure, but why a wing profiled cross section makes sense still isn't clear to me.
    Based on the scale model he's made, perhaps his reasoning is, (not that I want to speak for him as he doesn't explicitly mentions this) that, with it's 2:1 ish loa to beam ratio and the outboard positioning of the twin keels, the windward keel would be out of the water at even a modest angle of heel, which would hand back the drag advantage. He may be right. But given he's building a trans-oceanic boat, surely the downwind performance is the thing to focus on.
     
  6. Robert Biegler
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    Robert Biegler Senior Member

    I think the proportion of time spent sailing, out of all the time the crew is aboard, is probably less for long distance cruisers than for weekend sailors. Then, this is a small boat, more likely to be taken into shallow water because when you ground unintentionally, you could push it off with a pole or by jumping in. A small boat is less capable of carrying a dinghy, so you might want to be able to ground the boat so you can step off. For a given tidal range, the grounded small boat with its shallower draft spends more time out of the water, making that more practical, so falling dry upright is more important to a small boat. Finally, he once had his son helping out, and the kid looked about six, and there was a girl who looked even younger, presumably his daughter. If the offshore part of the sailing is something he intends to do once the kids are out of the house, and possibly once he has more experience, that could be 12 - 15 years. That is a long time to sail a boat specialised for one job. The twin keels make sense, and so does giving them a profile, rather than making them flat plates. The asymmetry, from what I have read, is unlikely to provide the benefits he expects.
     
  7. DCockey
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    DCockey Senior Member

    Yes, but that is true for all sailboats unless going downwind. The transverse, horizontal hydrodynamic force on the submerged portion of the boat (including keels, centerboards, leeboards, rudder, etc) which is keeping the boat from sliding to leeward combined with the horizontal, transeverse aerodynamic force on the portion of the boat above the water (including sails) causes the boat to heel.
     
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  8. Skyak
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    Skyak Senior Member

    There is another more complex problem of asymmetric bilge keels -dynamic instability. He is talking about angling the flats in slightly and adding curvature to the inside which will add a couple more degrees to the zero lift AOA. By simply moving forward through the water, the keel that is more vertical will exert more lift causing the boat to add to any roll it may experience. All boats have rotational inertia, and all stable boats have buoyancy profiles that act as rotational springs. The result is a driven mass, spring (damper) oscillator and the damping would be less for the shorter keels. This is why I never built a twin keel sailboat, so I don't know from experience. Do you know of anybody that did see this problem in a real bilge keeler?
     
  9. philSweet
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    philSweet Senior Member

    It won't add to the roll. What happens is the leeway angle changes so that the aero/hydro forces cancel, and when this happens, the heeling moment will be the same no matter what the camber and toe of the foil looks like.

    The only reason this wouldn't be the case is if the proportion of sideforce between the hull and the foil changes significantly. But we are assuming here that the hull doesn't add much to the sideforce.
     
  10. Skyak
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    Skyak Senior Member

    I watched some of the early videos from that guy and gave some advice. Nice guy, working with good materials mostly and has some idea about the task but he is getting his direction from social media which is like "design by committee" but with an unlimited large committee with nefarious bias. I couldn't help over the noise and couldn't watch anymore.
     
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  11. Skyak
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    Skyak Senior Member

    The change in leeway IS a heeling moment, because the rotational center of inertia is above the force of the keel -in my mind at least. I will do some diagrams and calculations to see if it changes my mind.
    Edit addition -I am concerned about motoring and downwind sailing -no aero damping counterforce.
    Do you know of bilge keelers with significant toe in that don't have this issue?
     
    Last edited: Jun 11, 2022
  12. Cedric Oberman
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    Cedric Oberman Junior Member

    Skyak, I don't see (although I'm not disputing) why the vertical keel adds more rotational moment than the other one as they have the same angle of attack. Also, and not related, Phil - I think the chine on the hull is expected by the builder to give considerable lateral resistance as well as the keels.
     
  13. messabout
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    messabout Senior Member

    Looking at the video...........This person seems to think that more forward enclosed space will prevent the boat from pitchpoling. All the while, the slab bow with a lot of frontal area, is sure to provide a sudden stop....and maybe a pitchpole, when slamming into a big wave. The rounded corners of the bow will not help enough in sailing in a hard chance.

    A 14 or 15 foot boat at sea? This thing would be scary on a dead run or broad reach while wrestling the boat in a big sea state. ...........I suspect that it might be prudent to go back to the drawing board.
     
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  14. Skyak
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    Skyak Senior Member

    I agree that it is not obvious, complex, and the task is for me to quantify how and why there would be dynamic instability. My designs were likely more susceptible due to higher speed and broader beam with shallower draft.

    The drivers would be
    Difference in lift between the P and S keels with different heal angle
    -more vertical to the flow for the deeper keel
    -shadowing from the hull and opposite keel for the shallower
    -shallower water flow field (high side) giving way to loss of pressure and ultimately loss of wetted area

    Differences in lift due to heal angle rate of change
    -deeper keel sees flow more from tip toward the base endplate, while rising side flow goes off the open tip
    -The major stabilizing moment would be the change in AOA, proportional to the angle rate times distance from roll center to lift center divided by the boat velocity
     

  15. Skyak
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    Skyak Senior Member

    There is a distinct "pro-ignorance" bias in social media; As in "educated people all told me not to do this but I showed them!" is a dominant theme in highly ranked videos.

    I think talk about preventing pitchpoling is just to imply that this boat will go fast enough to pitchpole. The only way it would ever go fast enough is falling off the face of a steep wave and even then the proportion of keel area to rudder area limits broaching. I have never seen any of these guys discuss what happens when the waves are coming at (or breaking over) that big wide flat bow.
     
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