Piggyback: Bilge Keel threads

Discussion in 'Sailboats' started by lewisboats, Jul 17, 2009.

  1. lewisboats
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    lewisboats Obsessed Member

    Ok: hypothetical (well maybe not so)...essaying into fluid dynamics I guess.

    If you had bilge keels, higher aspect ratio when extended fully (1:3 or 4 say...not too radical) that folded up into nacelles which projected only marginally (around 9" deep and angled at 25 deg or so from centerline vertical). If the foils themselves were asymmetric...IE similar to Airfoils or Airplane's wings with a curved surface for lift to windward and a flatter surface on the leeward side...thus creating additional lift to windward. Would it be possible to reduce leeward slip due to the push of the sails to almost nothing at optimum heel angles (25 deg say)? Could the lift of the foil generated in the denser medium overcome the sideways push of the wind in the sails? Finally...if there is slip...how much could you possibly reduce it using foil configuration and toe in of the foil. Of course...beyond a certain point the toe in will induce stall but in normal or rather AVERAGE sailing conditions could the two be brought to near neutral conditions using the pre-condition of a bilge keel configuration and adequate heel to optimize the use of the off center/angled keel foil?
     
  2. Brent Swain
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    Brent Swain Member

    Tried asymetric keels . Didn't make any real difference going to windward, but may stall sooner or reduce directional stability in a quartering sea.
    Brent
     
  3. tspeer
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    tspeer Senior Member

    Yes, but not the way you think. The asymmetric foils do not produce more lift - the lift will be what is required to oppose the side force from the sail rig. However, they will produce the same lift at a different angle of attack. This means the leeway angle will be different when measured between the centerline of the boat and the direction of travel.

    What you will find is the asymmetric foils will result in the bow being pointed more to leeward so the hull is tracking in the direction of travel. The leeway angle has, indeed, been reduced. But the boat will tack through a larger heading change than before.

    This doesn't mean it's any less close-winded, however. Of there is a reduction in drag compared to the conventional design, then the course through the water will be higher.
    Yes, that's the whole point of having a foil in the first place! Whether it's asymmetrical or symmetrical.
    You can change the angle to anything you want. You can even have the bow pointed to leeward somewhat. The real question is, "What's the drag?" And for that you have to look at what the sources of drag are, and how the new keel design stacks up with respect to each of those sources.

    The two factors that matter most is the total wetted area and the maximum depth of the keel(s). You can combine those into one parameter called the wetted aspect ratio. It's the square of the depth divided by the total wetted area (including keel, rudder, hull, etc.). You should compare keels on the basis of their wetted aspect ratio to get a feel for the likely impact to lift/drag ratio.
     
  4. philSweet
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    philSweet Senior Member

    The first boat I built was put together this way except I didn't cant the pivoting bilgeboards. If you're thinking of a high performance dingy, you should worry about the fact that the vertical center of effort of the hull doesn't get smaller as you heel. I occasionally swapped my boards port to starboard if the weather was really puffy, pulling with the windward board. ( This reduced draft on speed trails over the flats as well). When fully retracted, the leading edges were about 2 inches below the hull. This gave stability when running, kept the slots filled, and made centering the boat on the trailer a breeze. And it's oh so nice to be able to inflate a double-bed-sized air mattress in the cuddy. Well worth the trouble of building two CB trunks.

    Regarding performance, I can't offer much help. I built the craft mostly by eye with out a clue to foil shapes or anything else. I still have the boat. And I have some questions about the the stalling and leeway angles for TSpeer and others.

    1) Many centerboards are thinner than ideal in order to keep the slot and trunk reasonable. Wouldn't an asymetrical board be less prone to stall if the thickness was only 5%-6% of chord?

    2) Leeway angle. I've wondered about this one for a long time. Reducing leeway angle while close-hauled reduces the drag but increases the windage on the hull. If the fin is doing the additional work, then the heeling moment is increased as well, and the fin has to be larger or more highly loaded. Large, pure race machines seem to accept about 3 degrees of leeway. I was wondering what more modest craft find as a best hull leeway.
    Obviously, this will depend alot on hull form and the amount of top hamper; but getting leeway to zero isn't the ideal solution unless you are a submarine with a sail. There are plenty of tabbed keel boats out there. Any answers to this for specific boats? I'm thinking in terms of a fin keeled, spade rudder racer-cruiser. The objective being VMG to windward. Perhaps one way to compare different craft in this respect would be to determine at what speed (percentage of hull speed) would induce a 5 degree leeway.
     
  5. tspeer
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    tspeer Senior Member

    You could get some improvement in maximum lift by going to some camber, such as having the board flat on one outside so it still fits in the trunk. In normal sailing, however the board isn't so heavily loaded that it's on the verge of stall - it's really the low-speed maneuvers that are a problem. The bilge board-equipped boat has a big advantage because the board area is doubled when you need it the most.

    I used to own an M-16 scow whose bilge boards were thin aluminum plates maybe 3/16" thick (it's been a long time, so don't quote me on the thickness!). They had a 1"-ish wide chamfer on the inside leading and trailing edges, but were otherwise completely flat. On a scow, one doesn't say, "Ready about," one says, "Board down," before tacking. The crew puts the windward board down, and the boat pivots about on both boards as it tacks. Then the crew pulls up the new windward board as the boat heels up from sheeting in. Doing a 360 deg turn means putting the helm over and trimming the sails as fast as you can as the boat spins in practically one place with both boards down.

    My experience was that getting stalled out on a tack really wasn't a problem with bilge boards because of the double area. The boards can essentially be sized for the boat moving while hard on the wind, rather than being sized for to avoid being stalled while down-speed in a tack.

    Leeway from the sailor's perspective and leeway from the designer's perspective are two different things. The sailor tries to reduce leeway angle because of the drag associated with it. If the leeway is appreciable enough to be noticed by the sailor, the keel is probably stalled.

    The designer, on the other hand, actually increases the steady leeway angle as a way of improving performance. The designer can always reduce leeway to near zero by using enough lateral plane area. A full keel will have a small leeway angle, but has much more wetted area than a fin keel of the same depth. Cutting away at the full keel increases the leeway angle but the decrease in wetted area improves performance. The limit becomes when the area has been reduced to the point that it becomes more difficult to keep the keel from stalling.

    With regard to lift/drag ratio, probably the best single indicator is the wetted aspect ratio. This is the depth squared divided by the sum of all the wetted area (hull, keel(s), rudder(s)). The square of the depth addresses the lift-induced drag and the wetted area addresses the skin friction drag.

    Best Vmg to windward is obtained when the hull Lift/Drag ratio is maximized. Since the lift is determined by the sail trim, that means reducing drag. Leeway doesn't come into it, except indirectly as it affects the drag.
     
  6. lewisboats
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    lewisboats Obsessed Member

    Thanks so much for the info...I am learning more post by post...keep em coming.
     

  7. Brent Swain
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    Brent Swain Member

    I have been told that putting the downward pointing ends on small airplane wings has led to drastic reductions in drag, and thus in fuel consumption . Pilots can tell you more . One of my clients is using such on a 36 ft twin keeler he us building , so it will be interesting. I am thinking of trying this on my own boat at some future date. Such additions must be designed so they don't foul anchor rodes between them.
    When a boat is rolling in a following sea, the deepest keel has the most effect. If that keel is toed in, then it may tend to turn the boat to windward. As she rolls the other way , the then deepest keel may tend to turn her the other way , reducing directional stability downwind, causing her to yaw a lot..
    Brent
     
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