Bilge Keels

Discussion in 'Boat Design' started by Hisflyingtune, Mar 2, 2010.

  1. Hisflyingtune
    Joined: Feb 2010
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    Hisflyingtune Hisflyingtunesmith

    What are the variables in bilge keel design shape, section, and planform?
    Does anyone know a good primer or good source for this? I'd like to replace the centerboard with bilge keels to increase interior space. So, if I do this, what are the hydrodynamic/sailing advantages and disadvantages?
  2. souljour2000
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    souljour2000 Senior Member

    Better tracking with a following sea for one...(a much-over-looked major safety feature when it comes to small boats IMHO)....loss of speed overall due to added wetted-surface friction...better roll stability or dampening...there's alot more pros and cons I'm sure...I've been hoping for a while that there is someone on this site with some real familiarity with these boats who has also sailed "conventional keels" who can really speak to the pros and cons of them overall maybe...there's just a lot of rumor and whispers about them mostly on this side of the pond...
  3. alan white
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    alan white Senior Member

    There may be a need to stiffen the hull by adding floors (transverse frames that go all the way across the bilge keels).
    Any boat that can sit on its keels needs to have adaquate internal strength across the belly.
    Also, the keels need to be long enough fore and aft and the boats fore and aft CG has to fall somewhere in the middle of the keel's length or the boat will not balance and may tip back or forward.
  4. lewisboats
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    lewisboats Obsessed Member

    They are big in GB...the tidal range is such that lots of boats end up sitting in the mud or on the bottom. The bilge keels along with a decent skeg allow the boat to sit upright when the water runs out from under them. The trade offs include increased wetted surface, lessened ability to point to windward, shallower ballast (partially compensated by part of the ballast being to windward when heeled). Personally I like them but they aren't very popular in the States.
  5. Hisflyingtune
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    Hisflyingtune Hisflyingtunesmith

    Thank you, thank you, thank you for your responses! I know that I'm on the right track or "tack." I realize that there are trade offs. I want to be able to write a table and codify the advantages and disadvantages. Low maintenance and more interior space were what first attracted me to them. I'm still very uncertain about which section to use much as aeronautical engineers would use on an airplane's wing. Which section generates the greatest amount of stability when heeled and which section creates the least amount of drag? Are there any studies in this area? If not, it sounds like a thesis which one could use to lock up a master's degree.
    Thanks Again,
    Hisflyingtune (Steve)
  6. uncleralph
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    uncleralph Junior Member

    I owned a boat with bilge keels and sailed it on the Chesapeake. I would never own another one unless I lived in a place where I needed the ability to sit upright when the tide goes out. I felt there was a significant degradation in performance and I am NOT a performance oriented sailor at all. I always felt like I was dragging around an anchor when I owned that boat. Bilge keels make great sense in Great Britain where boats are left on moorings and it dries out at low tide, otherwise I would stay away from them.
  7. Hisflyingtune
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    Hisflyingtune Hisflyingtunesmith

    Hi Uncle Ralph! Many thanks for your post. Before I abandon consideration of bilge keels, I want to know what design variables there are which can, to whatever degree, mitigate some of the performance penalty.

  8. Chris Ostlind

    Chris Ostlind Previous Member

    I know that I'm known for having serious wood for multihull designs, but I have also done a few monohulls as specific, interesting design issues surface.

    One of those interesting design problems has had to do with a bilge keeled, monohull cruiser. I refer to it as the KS Duo. The name comes from a convoluted reference to a solo micro-cruiser on which I was working with a good friend.

    This boat is 17' LOA with a 6' BOA. When sailed aggressively, the windward bilge keel rises to the surface with a reduced wetted surface signature. I have worked around a bit with two versions of the same hull config... a large cockpit area with a focused cabin environment and a reduced cockpit space for sailing and much more generous cabin space. The benefits of both iterations would depend on the interests of a given owner and their needs/interests.

    I drew it up as a strip built hull and deck with marine ply bulkheads. The renderings yield a lot of info as to application of the design study

    I did a lot of research into bilge keel shaping and placement. I also looked at the chine runner thinking of Matt Lehman, who designed the Paradox, solo microcruiser. I felt that there was a place in the design of the bilge keels where a chine runner-ish form, could be morphed with the trad ballasted and hydro shaped keel forms you might see on the Brit boats. A pair of keel forms like this could be a real plus for shallow water applications, while providing satisfactory windward potential.

    The design was never taken beyond what you see in the renderings, so there aren't any built boats to give as a reference to the actual potential of my design approach. I am of the opinion that the concept has real merit, but I have not had the time to pursue that fulfillment with an actual boat on the water.

    If it provides a bit of stimulation towards this thread's intent, it would be gratifying.

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