Designing twin keels

Discussion in 'Boat Design' started by Mik the stick, Oct 25, 2015.

  1. Mik the stick
    Joined: Dec 2012
    Posts: 189
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    Location: Devon

    Mik the stick Senior Member

    I could find no info on how to design twin keels so I decided to invent my own method. My design purpose is to decide what size and shape they should be.
    Requirements: What I want to do is design fins to raise the hull 6" of the beach when at rest. In addition I would like them to provide a decent amount of roll damping. Shape is triangular with the top cut of, base length about twice the top length.
    There is a paper on the net which describes the size fins need to be in order give roll damping equivalent to flopper stoppers. This would be ideal for sizing but I lost it when my computer crashed.
    I found that a rule of thumb for dagger boards is 5% of sail area gives board area. but the boat I am trying to design is a powerboat.
    Solution: An average offshore cruiser has a SA/D of 15-16 A value of 15 on a Diesel Duck (the boat i compare all others to) gives a sail area of 957 sqft therefore each fin would be 24squft or more. I don't know how much roll damping you would get, but I think it would be a significant improvement. Drag would be increased costing about 2hp at 9kts. Not a problem for an engine 80hp engine which probably draws around 40hp at 7knots cruise.
  2. tom28571
    Joined: Dec 2001
    Posts: 2,474
    Likes: 116, Points: 63, Legacy Rep: 1728
    Location: Oriental, NC

    tom28571 Senior Member

    Mik, That keel sounds like an excellent net and pot warp catcher. what is the reason for not having some forward slope on the leading edge? Understand the wish for weight down low but the ballast ratio can be kept the same with other than lead on the leading edge.
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