Submerged block in daggerboard case.

Discussion in 'Multihulls' started by elhix, Mar 6, 2010.

  1. elhix
    Joined: Feb 2005
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    elhix Junior Member

    Hi Folks,

    The daggerboard case on my almost finished cat has a downhaul block inside it that is below the waterline and so will be generally submerged. I foresee problems with fouling and wondered if there might be a better solution... Would it be possible to have a stationary block made of some super slippery material that wouldn't cause excessive friction OR be troubled by marine growth?
     
  2. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Not a common problem

    Usually designers put grooves in the daggerboard to let the downhaul and uphaul lines run well. Have a look at some other designs and see if you can alter yours. Unless there is a very good reason you shouldn't put a block below the waterline.

    cheers

    Phil
     
  3. gonzo
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    gonzo Senior Member

    I think that it depends on the design. Is it a simple block?
     
  4. sandy daugherty
    Joined: Jun 2008
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    sandy daugherty Senior Member

    If ABSOLUTELY necessary you could turn a large Delrin sheave to fit between the walls of the trunk, spinning on a large diameter captive bronze axle, inserted from one side and covered with a screwed down water-tight lid. but there are much better ways to control a board, with the blocks well above the water line. Even they suffer when the hydrodynamics force seawater up the trunk.
    The Moxie 37/38 hung lift blocks from the shrouds! Good thinking. Unfortunately, the down haul line could pull the board below the deck molding, The deck edge turned down inside the trunk and could trap the top of the board down when there was a side load

    If you go with the Delrin sheave, try to mold the trunk so that the sides stand higher (proud) of the shoulders of the sheave to reduce the likelihood of material being forced between the face of the trunk and the side of the sheave, =>||<=
     
  5. elhix
    Joined: Feb 2005
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    elhix Junior Member

    Just to elaborate:

    The daggerboard trunk is 76mm wide but has an extra 25mm channel that runs from top to bottom to accept the 12mm downhaul line and the block itself. When the 2.4m daggerboard is in the fully down position only the top 25% of it is in the case* which is why the block needs to be so low.

    * These dimensions are approximate as I've not got my plans to hand.
     
  6. sandy daugherty
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    sandy daugherty Senior Member

    I like all control lines to be accessible. If you dead end the line on one side of the board and lead it over the top, and down to a block mounted high in the trunk, you can fix things that go pop in the night. You could mold a channel from the block to the winch so it wouldn't be a trip hazard.
     
  7. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Make the board taller

    25% is very low. I am not sure what the Farrier's are but I would be surprised if they were that short in the case when submerged. I would be very worried about hitting anything with the board at any speed. The less bury you have the greater the force (torque really). Why not just increase the board length? It would be more normal. I would suggest the board should end at the settee or bench height inside. I would guess that this would be half the freeboard.

    I am not even sure of the channel all the way down. Sounds like a good catchall for barnacles.

    Don't mean to be mean but I would worry personally.

    Cheers

    Phil
     

  8. elhix
    Joined: Feb 2005
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    elhix Junior Member

    Thanks for the input everyone, you've given me some things to think about...

    The board is indeed short in the case but my boat is a Shuttleworth and the board and case are built exactly to plan. I trust John's design 100% - in this case the structure is specifically and heavily reinforced.

    I'm still not sure about the block though!
     
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