Canting keel(Free surface effect)

Discussion in 'Boat Design' started by budner1, Apr 19, 2005.

  1. budner1
    Joined: Apr 2005
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    budner1 Junior Member

    I'm doing a uni project on retro fitting a canting keel to a fixed keel racer. I'm hoping some1 could tell me when the free surface resistance really penalises the canting keel.(ie a keel that can cant 40degress, what angle of heel should the boat start reef?)
    :) Peter
  2. SailDesign
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    SailDesign Old Phart! Stay upwind..

    Not sure where free surface enters into this (usually calculated for large tanks on large ships), but a canter should reef at the same point that a fixed-keel boat should reef - when the skipper decides that it needs to. As far as heel angle for reefing, that is again the skipper's decision, and will not change just because of a canting keel, although the aftermath of an emergency tack can be unsettling :)
    If you are retro-fitting, however, your biggest worry is the rig. Increasing the righting moment will increase the loads in the rig, resulting in heavier shrouds, a mast with more section, and probably an undate on the chainplates, bulkheads, etc. to boot.
    Is this a theoretical project, or hands-on?
    1 person likes this.
  3. Doug Lord

    Doug Lord Guest

    canter/ "free surface"

    I'm wondering if your concern was the canting keel strut itself getting so close to the surface that drag increased dramatically? If the keel is only going to cant 40°(and you take Steves advice) then I don't think you would ever have the problem . From hydrofoils I know that you want to "fly" at 2-2.5 chords below the surface and I would think that would apply here.
    Many high performance canting keel boats cant at 50 - 55° and I just found out that Bethwaites new 79er will have a cant angle of 60°! Those boats probably still wouldn't get too close but you'd have to give it some thought!
    Steve is right about retro canting keels and it
    is a very tricky area: if you increase RM you overload the existing structure; if you decrease ballast she won't float on her lines..
  4. budner1
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    budner1 Junior Member

    Retro Fitting a Canting Keel

    This Project is purely theoretical. I have already considerd the extra loadings on the hull and rigging and am adjusting them to withstand the new higher loading while maintaining the same factor of safety which they carry at the moment. However i am having troubles finding out wheter or not there are guidelines or scantling rules in regards to the changing of the keel. If any1 has any up to date info on A.B.S or Lloyds in regards of the structure supporting the new Canting keel it would be greatly appreciated.
    Thanks again,

  5. billys maverick
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    billys maverick New Member


    PS: I'm just realizing the age of this post. That stated, if anyone (especially yourself) is still interested in this thread, I'd enjoy continuing the discussion.

    My Response:
    I'm interested in your trains of thought on this topic. I'm working on a similar project at another uni. The scope of my project is one of a structural analysis of a mounting plate from which the canting keel may be supported.

    I'd like to reference you to the following paper:

    It discusses the free surface effects generated by the bulb acting near the free surface. And, in typical FriendShip Systems fashion, breaks the bulb design into key shape parameters and offers an approximation model for the bulb design with respect to free surface interactions.

    As for the fin... well, I don't see much (side) force generated by the fins. What force is generated will certainly become less useful as the fin nears the free surface and pressures on the windward face of the foil decrease, perhaps venting in maximum cases. But, thanks to the CBTF appendages, we maintain nearly all of our righting arm. We do have to think of wave pushing UP on our foil acting, of course, negatively to our righting arm.

    That said, a nominal reefing point could be argued to exist when the arc of the bulb's path goes above horizontal, thus decreasing the transverse righting arm of the system.
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