Center board trunk for 40ft cold molded sailboat

Discussion in 'Boat Design' started by Hampus, Sep 14, 2011.

  1. Hampus
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    Hampus Junior Member

    What different ways of constructing a centerboard trunk are there for a cold molded, 40ft plywood sailboat? My plan would be to put the center board off centered to one side of the keel sa not to have the slot through the keelson.

  2. Gary Baigent
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    Gary Baigent Senior Member

    No problem with that - why don't you make it a daggerboard and take it through the deck? No leaks, easy removal for cleaning, better hydrodynamic shape.
  3. PAR
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    PAR Yacht Designer/Builder

    There are many different ways to address the engineering surrounding a centerboard case. You shouldn't be terribly concerned about a keelson penetration. The loads can be handled, even with a big slot through it.

    Can you refine you question a bit, as a dissertation on centerboard case approaches, seems like killing flies with a shot gun. In other words, do you have some concerns over a set of plans you have or possably the approach taken by the designer? I would think a plywood hull would likely incorporate a plywood case, likely with solid posts at each end and athwart stiffeners as well. This could be bedded or bonded, but certainly isn't the only way to attack this set of issues.

    A daggerboard on a 40' yacht will very likely make a whole lot of damage in a grounding and unless a racer that will be carefully handled, I couldn't recommend one.
  4. Hampus
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    Hampus Junior Member

    Thank you PAR. This would be the designers aproach.

    No, not a daggerboard. Rather a centerboard that swings up into the trunk around a pivot. Much like the Ovni boats. I had imagined a plywood trunk with solid posts fore and aft. Trunk will double as part of a table and have an access hatch from top for servicing of lines. Stainless pipe for support that will go from forward part of trunk up to the deck and be a combined mast support and pipe for the line to raise the center board and the line to fix it while swung down. The lines will be run back to a winch in the cockpit. The line to fix it would be thin enough to break if grounding and let the board swing up. Top of inside of trunk should probably have some sort of chock absorber to take up the forces of an upswinging board, possibly clad with a stainless plate.

    The boat itself is traditionally looking but with modern rig and a rather flat bottom. I imagine her to be used in the Baltic archipelagos. The center board is to give access to the secluded places normally only accessible only to motor boats. She could also be sailed on the dutch and german waddenzee that allows you to dry out in the most fascinating places. Or along the brittsh isles where the shallow draft would allow you to anchor with less concern about the tide and allow for a shorter rode.

    My main concern for the trunk is the inside and possible rot in keelson, posts or plywood sides.

  5. Gary Baigent
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    Gary Baigent Senior Member

    PAR, back in the IOR days, numbers of 26 -39 foot daggerboard designs were built, champions too. There is a lot of utter toss and unfounded fear talked about running aground with daggerboards; they too, will pop up in their cases - but the idea when sailing is not to run aground! And if in shallow water, lift the board a little.
    The great advantage of a dagger is its far superior hydrodynamic shape to a centreboard design, also you haven't got a draggy, gurgling, centreboard slot diminishing your boats ability to sail to windward.
    Hampus, you could even place the board a couple of feet to one side of hull centre line ... and you would feel no difference to helm feel or the boat's windward ability on either tack. The BS that the board has to be absolutely central is exactly that.
    Building a case for a dagger is simple; use the board as a male mould (covered with 3 tight layers of plastic for mould release) and lay up the case in glass.
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    FAST FRED Senior Member

    The weight of the board does vitually nothing for righting momrnt, so weight it so it will stay down at hull speed .

    A single line should be fine to pull it up, no winch or all the rest.

  7. philSweet
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    philSweet Senior Member

    You may want a down haul line if you plan to dry out. Sometimes you have to get that oyster shell unwedged from the slot. An access hole to insert a pipe that you can beat on also works. But, IMHO, the board should be designed to stay down by itself when under way with the downwire free. Can you post a sketch of the proposed boat and one of the board?

  8. Petros
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    Petros Senior Member

    The forces on the center board trunk can be quite large, so make sure your design will handle the side loads imposed on the trunk.

    I have sailed in both center board and dagger board boats, and have struck objects in both. the centerboard trunk suffered damage in once case, only the dagger board was damaged in the other, and easily fixed. I have seen whole centerboards fall free of the hull when the pivot failed, or the center board corroded up or down, making the boat unseaworthy without a haul out and repairs. Not usually true with dagger boards.

    It is not that dagger boards can not cause hull damage, it is just that I do not think there is any inherent safety from impacts in one design or the other. I am going to build nothing but dagger board boats from now on: simple, lightweight, less disruption to the hull structure, easy to build and better performance.

    For a sailboat that large you install two smaller dagger boards off set to either side of the keelson for better performance. You can than pull one up on either tack to improve performance, have redundancy, and still have a symmetrical hull when going downwind.
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