Daggerboard Dreaming

Discussion in 'Multihulls' started by perry2222, Mar 9, 2021.

  1. perry2222
    Joined: Sep 2019
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    Location: Sydney Australia

    perry2222 Junior Member

    The threads I have been reading about dagger boards and trunks really interested me, but are a bit old, so if I may, I am using the option of a new thread to ask this question' re Turbulence.

    If the board is aerofoil and the trunk case is rectangular could it be possible to:
    Seal the trunk exit around the board:
    Wrap or coat the top part of the board with something that would allow it to 'release';
    Pour a mixture that would harden into the top of the case.
    Thus forming a shape around the board that would reduce turbulence?
    I guess cleaning the existing slot would be crucial to allow a good bond and might well be the main weak point in the idea.
    Or is this just silly as it sounds?
     
  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    I wonder how much effect the turbulence really has?

    One suggestion - it might be feasible to have a rubber gasket on the underside of the trunk, in similar fashion to the rubber
    gasket on an engine saildrive unit where it goes through the hull?
     
  3. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    Plenty of boats use a rectangular case where only the bottom plate (the exit hole) is airfoil shaped. The boards top end remains rectangular and insures it does not rattle into the case. If the case is high enough this works even when the board is fully raised.

    Yes you can use pour in things to fill the void, for example foaming epoxy. Even pour in PU foam works, but it gets waterlogged over time. Wrap the board in saran wrap or something similar, you need some gap between foam and board.
    You can also use XPS or another rigid closed cell foam, cut or sand the board profile as a negative into two pieces of XPS with room to spare for a light layer of glass and some antifouling paint. Assemble the two pieces around the board, cut the outside to the case dimensions. Now you can simply insert the whole affair into the existing case and close the bottom and top with shaped plates (or directly fiberglass over the foam), that will keep it from sliding out. The foam also acts as a shock absorber in case of impacts.
     
  4. wet feet
    Joined: Nov 2004
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    wet feet Senior Member

    One thing some Moths did in the past was to laminate thin ply to conform to the aerofoil shape -plus a little for clearance-and take the foil section right to the top of the board.Less turbulence and a lighter board.
     
  5. perry2222
    Joined: Sep 2019
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    Location: Sydney Australia

    perry2222 Junior Member

    Oh some interesting ideas here, thanks. I agree, 'how much turbulence'? I don't know. I haven't sailed the boat. Bought it last year, just before Covid hit. Its in Europe up on the hard. Hence the 'dreaming' in the title. Previous owner complained about turbulence and had done some really nasty fibreglassing around the exit slots to combat it, but really a complete failure and made more work. Fortunately he didn't do a lot of maintenance to it. He also complained about getting them up and down. I am inclined to think that he never really cleaned the trunks out properly, but as I don't know and don't know what they are made of they might be flexing under load. Thought they looked pretty substantial.
    The boards need a bit of work, when fully down the tops are below the trunk opening, so I envisage either making new longer ones or scarfing in 3 0r 4 pieces that would give it a full rectangular profile in the trunk to spread the load in a bang - and which might do well with the gasket idea. It would also make the boards longer. Supposed to be 1.8 m depth. I don't understand why more cats parent made with wider (fwd aft) boards, this one could easily have managed 50% more within the internal set up. My assumption is that the more daggerboard you have the better. dag board Port top.jpg dag brd repair starbd .jpg
    The other idea of using some sort of 'foam', creating a negative to fit around the board is pretty interesting too. Foaming epoxy was my first thought but thought that there might be other things I didnt know about. And laminating light pieces together to form a board is something I didnt know. I shall poke around a bit more re materials, but thanks for your very good suggestions, really appreciated.
     
  6. wet feet
    Joined: Nov 2004
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    wet feet Senior Member

    To be clear,the Moth sailors I referred to were using ply,laminated over the boards themselves,to form daggerboard case sides that allowed the board to move and didn't have square ends promoting turbulence.Plywood boards and rudders are a bad idea as they normally snap at the keel line or bottom of the rudder stock.
     

  7. perry2222
    Joined: Sep 2019
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    Location: Sydney Australia

    perry2222 Junior Member

    Ok I see. Thanks Wet Feet
     
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