Double keel or single keel ?

Discussion in 'Boat Design' started by Fanie, Dec 28, 2007.

  1. Fanie
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    Fanie Fanie

    If one uses two small keels a distance apart, would they have the same effect / functionality as one with double the area ?
     
  2. Petros
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    Petros Senior Member

    They theoretically would be the same. But it seems to me that one larger one would be more efficient and have less wetted area, and only one tip loss, less complex too (fewer parts to build). One fewer keel to raise when in the shallows.

    However, on a cat having two small ones has the advantage of always having one in the water in chop or when flying a hull. There is also some advantage to redundancy in case of failure, two smaller ones would have less draft too.

    I have thought this back and forth on a cat I want to design and build, but I have not reached any conclusions. It seems there are benefits to both, so take your pick.
     
  3. PAR
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    PAR Yacht Designer/Builder

    Tandem appendages have been tried, but you have more drag, even if the combined total area is the same, the additional leading, trailing edges and vortices mostly counteract any advantage.

    Tandem dagger or centerboards has a lot to offer, especially in helm balance and a fair number of yachts have been fitted this way.
     
  4. masalai
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    masalai masalai

    Fanie, On your new toy, Go fast & you will never experience any lee worries - having too much fun. Fly both windward & centre hull for more drive (lift) and don't worry too much about that (pointing higher) yet.
     
  5. Fanie
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    Fanie Fanie

    I apologize, I should have mentioned this is for the centre hull of a trimaran. One consideration to using two shallow fixed keels would be so the centre hull could stand upright by itself. Another would be shallow draft.

    Thanks Masalai, but I doubt the cat is going to be fast, maybe if I redo the ally mast with GPR and make it longer and fly a larger sail. At the moment I'm just trying to figure out what is functional and what not.
     
  6. masalai
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    masalai masalai

    My cheeky answer still applies, remember KISS. In that vein, single dagger-board is more than adequate & easier to control. The leading edge of the case can be part of the mast step.


    If you must then extend the case & make it swing up, held forward by a rope and your magic cleat. (I suggest against the cleat - rather be aware of the depth than unexpected broach as the keel swings up)
     
  7. Guest625101138

    Guest625101138 Previous Member

    If you have a single board that is 2ft deep and 1ft long replaced by two 1ft square boards then the drag on the boards to achieve the same lift is the single board is close to twice as much. This is based on a NACA2412 foil. Will be similar for others.

    The reason for the dramatically reduced performance is the change in aspect ratio. This increases the induced drag for the low aspect foils.

    If you keep the same aspect ratio then the loss in performance is not so great. You are changing the Re# so the impact will be less significant as the speed goes up.

    Rick W.
     
  8. Brent Swain
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    Brent Swain Member

    Yes. if they were canted out 25 degrees making the leeward one upright when heeled 25 degrees, increasing it's lateral resistance 100% from that of an equal sized one heeled 25 degrees.
    Brent
     
  9. rwatson
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    rwatson Senior Member

    To complicate that effect, the windward side dagger now totally loses resistance because it is 50. to the vertical, so the net gain is negated, but maybe its better to have one board doing all the work under water instead of up in the turbulent windward side?

    Maybe the 25. angle is a bit too big for a MacGregor style craft that have to be sailed like a Laser, (not recomended beyond 10). heel.

    Also, the bigger angles make inroads into the accomodation area.

    My next boat is planned to have dual daggers, so I am interested in the logic here. I have specified them to be vertical.

    I am also interested in the "toe-in" geometry calculations and how performance is affected.
     
  10. Fanie
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    Fanie Fanie

    How about more than one dagger board per hull - one more foreward than the other... you use one at a time only and could use one in one hull at a time. High speed sailing you use the foreward dagger board to improve the rudder leverage, switch to the aft daggerboard to tack faster and decrease the turning circle diameter.
     
  11. Fanie
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    Fanie Fanie

    In my mind toe-in (if any) should be very small. Half a degree. The toe-in angle will increase with speed without adjusting the angle and only appy if more than one dagger board is used. Streight daggerboards should have a sideways bobbing action side to side, but only if the hulls are as streight as well. The slightest deviation from 100% everything in line should veer slightly to one side which is corrected with the rudder without knowing. Assuming still water and no wind.
     
  12. Brent Swain
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    Brent Swain Member

    Want toe in? Sheet her closer and point higher. Same effect.
    Brent
     
  13. rwatson
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    rwatson Senior Member

    Its not that I would want toe-in just to say "Look Ma - I have toe in!"
    Boats on a beat dont sail straight - they crab across the desired direction of travel.
    Many modern day racers have variable attack angles for their boards to customise gain v speed for their boats
    I have been told that there are optimal angles for dual keeled boats.
    These angles usually result in a "toe in" angle to increase lift on a beat, but without creating too much drag under other conditions.
    I have yet to read any theory on this - but someone may know a source.
     
  14. PAR
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    PAR Yacht Designer/Builder

    Fin mounted trim tabs . . .
     

  15. Brent Swain
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    Brent Swain Member

    The toe in theory was developed by tank tests , which propel a boat from a fixed point rather than by sails. The relevant angle is the angle between the keel and the sails, something that propeling a model in a tank from a fixed point doesn't relate to.
    Brent
     
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