Strip planked boats and saildrives?

Discussion in 'Sailboats' started by Omeron, May 5, 2008.

  1. Omeron
    Joined: Feb 2007
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    Omeron Senior Member

    I am not a NA, therefore please excuse my lack of knowledge from the beginning.
    I have long been thinking about the compatibility of saildrives and strip planked construction.
    The boat in question is about 37 ft in length, cruiser racer,with a fin keel.
    I imagine that any strip planked boat, built upside down over a mail mould, would have a strong laminated keel running from stem to stern to form the backbone of the boat. Whether you have bulkheads, ring frames, outside sheeting of glass fibre, or substantial strips of enough thickness, majority of the longitudinal stiffness must be provided by the keel, let alone carrying the fin keel itself.
    For a boat of this type of construction, i believe installing an inboard diesel engine with a saildrive should be totally out of question. As this would mean cutting a hole right on the centerline,and through the keel, and interrupting the continous load line running from stem to stern.
    Is my assumption correct? Or is there a way of constructing the keel to overcome this problem?
    Are there any boats built this way, and utilising a saildrive as well? (i imagine there must be)
    If anybody can shed some light on this, and in general how hulls are designed to have a substantial gap in a place seemingly least desired, i shall be most grateful.
     
  2. TeddyDiver
    Joined: Dec 2007
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    TeddyDiver Gollywobbler

    Neither am I:cool: But there are basicly two kinds of strip planked hulls.
    The other, more traditional one, is built just like an ordinary wooden boat except the planking.
    The "modern" one, where longitudinal strip planking and directional S-glass are epoxied together is partly like a fiberglass hull and can be constructed without traditional keel and ribs etc premade stucture. Instead the stuctural members are constructed afterwards inside the hull. This type is sometimes called Monocoque.
    In the first case there's usually a saft drive auxiliary and in the latter case it's possible to install also a saildrive.
    BR Teddy
     
  3. catsketcher
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    catsketcher Senior Member

    Load paths

    There is no real reason why you can't cut a hole in the bottom of a boat if and this is a big if, if you give the loads a path to follow.

    If the boat is the second type of construction then it will be monocoque and the load path will not be a straight line down the keel. You can however use unidirectional glass to help feed loads around the cut out. Multihullers have to do this when they cut out windows and walkthroughs through main beams. It is not rocket science but you need to be pretty careful. Some people say you have to add only the weight of glass you omit but this does not take into account the stress concentrations caused by corners. If you do cut a hole in the hull reinforce it with unis that gently curve around the hole. The stress concentration in a notch is inversely proportional to the radius - smaller radius higher stress - that is why you drill a hole to stop a crack in steel or alloy.

    cheers

    Phil Thompson
     
  4. mgpedersen
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    mgpedersen Junior Member

    Actually I don't think it's that big a deal. When you consider the stiffness of the boat, the hull shell contributes far more stiffness to the boat than the keel itself. In fact, it's quite possible (with the right glass reinforcing) to build a strip boat without a keel.

    If you want to get technical, the global stiffness of the boat is a function of the material, the area of the structure, and the distance that this material is from the neutral axis. If you want an equation, the stiffness is given by AY^2, or area times distance from the neutral axis squared. The hull shell is further from the neutral axis and it's also significantly wider than the keel, which are both in favor of the hull contributing more. Suffice it to say that as long as there is good monocoque construction it's not something to lose sleep over. If you're totally paranoid, you can always flare out the keel around the opening and build up additional thickness. But I think you're better off putting that weight into the keel.

    The only time I'd get nervous is if this cutout were close to the aft end of where the keel attaches. In a grounding where the back of the fin keel tries to push up into the hull, you don't want a stress riser/hole.
     
  5. Omeron
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    Omeron Senior Member

    Thank you gentlemen.
    That answers most of my questions.
    But your descriptions and explanations of the so called 'monocoque' construction now begs another question.
    I can understand that the entire hull and scantlings can be designed to carry all the bending and torsional loads imposed on the hull as a total. And that you really do not need a conventional keel as such. But i guess the problem still remains as far as point loadings or rather concentrated loading due to keel and to a lesser degree the engine.
    So what is the best approach in terms of beefing up these areas?
    Internal ring frames around these areas?
    Internal layers of wood?
    Internal layers of composits?
    Increased fibre reinforcement outside?
    Internal structure of metal construction?
    Increased strip thickness towards centerline?
    All of the above?
    None of the above?
    :))
     

  6. TeddyDiver
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    TeddyDiver Gollywobbler

    So what is the best approach in terms of beefing up these areas?
    All depends how the stucture been done
    Internal ring frames around these areas?
    When frames are used in the structure
    Internal layers of wood?
    For a cold molded perhaps
    Internal layers of composits?
    Increased fibre reinforcement outside?
    in a monocoque both above can be used
    Internal structure of metal construction?
    for metal boats
    Increased strip thickness towards centerline?
    Extra lyers of frp is far better (and easier)

    My 5c
     
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