Alternative Keel Attachment Method

Discussion in 'Boatbuilding' started by Sea Jay, Jan 3, 2008.

  1. Sea Jay
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    Sea Jay Doug Brown

    OK, here’s the concept…I have a 46’ composite hull which was designed for a typical bolt-on fin keel. I would like to modify the design somewhat so that the top 30” of the keel form would be a GRP stub keel with space for a large battery bank. This stub keel would be permanently bonded to the underbody of the hull. (Unfortunately it is not molded into the original hull.) The lower portion of the keel will be a lead filled steel structure that would attach to the stub but could be dropped for easier truck transportation. The trick I am looking for is a connection method that would allow the keel be detached from the outside, rather than from the inside bottom of the stub as typical. I would like the freedom to install batteries, engine, tankage, etc., inside and on top of the keel stub without concern about accessing and inspecting keel bolts.

    Has anybody seen anything like this?

    Best Regards to All
  2. alan white
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    alan white Senior Member

    Better to call the upper hollow part the keel and the lower part the ballast. What you're talking about might be successfully done with a longitudinal dovetail joint and an iron ballast. Everything would have to be massively built.
    I doubt anything so complicated would be worth doing. The cost would probably far outweigh any benefit. This is a 46 ft boat!

  3. PAR
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    PAR Yacht Designer/Builder

    Sea Jay, I've engineered many oddities over the years, in order to satisfy a client's needs. This presents some interesting problems, but not insurmountable I would suspect.

    In a 46' sailing yacht, I'd be quite surprised if you could fit all you're hoping for in a keel stub, but a fair amount of it could live there, though I wouldn't recommend many of the items you've listed for several reasons.

    What is the make, model and year of your yacht? This information would help greatly in accessing the amount of volume your current appendage has and the general arrangements of the machinery spaces.
  4. Sea Jay
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    Sea Jay Doug Brown

    Alan, Par...sorry for the late reply. The power has been off here in Sacramento for the last three days and was just restored an hour or so ago.

    In response to your comments...

    I've thought of a "dovetail" arrangement as well but agree about the complexity and cost.


    Just to clarify, I only intend to install batteries directly in the "keel" (using Alan's terminology). Tankage, engine, other equipment might only be located above traditional keel bolts so as to interfere with their easy removal. The hull I have purchase is a Turner 46 (Ontario Canada). I don't think you can find much on the web as they were only in business a short time.

    The method that looks most promising is a variation on a Dudley Dix detail. Let me see if I can provide a simplified description. Picture 2 identical heavy steel plate "shoes", in the shape of the foil cross section. The bottom shoe is drilled for the keel bolts. Then sections of steel "C" channel are welded between the plates and athwartship at each keel bolt "station". The open face of the channel is facing down and is centered over the boltholes. Both of the plates are embedded in and securely anchored to the bottom of the keel, however, the ends of the C channel are open to the air. If standing alongside the keel, you could look through these channels to the other side of the keel. The ballast, with embedded keel bolts, is then fitted, with the tops of the keel bolts protruding through the bottom shoe and into the open channels. The channels are large enough to get a socket wrench and a nut on top of the bolt. In Dix's design, the channels are then closed with plywood, permanently epoxied, and faired into the side of the hull. My variation would be to put a release agent on the nuts and bolts, foam the channel void, and then glass over, and carefully mark, the openings to the channel ends. When it was necessary to drop the ballast, a hole saw could open up the channel ends and the tops of the bolts would be accessible. I've used "C" channel for the purpose of illustration but a stronger, more complex weldment, could easily be made which would simply leave pockets around each bolt head that could be accessed from the side of the keel.
  5. rwatson
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    rwatson Senior Member

    I would be interested in a web site address for the Dix method.
    The concept is innovative and I think very valid. Hower, after a brief think, although you would prefer to not have to worry about keel bolt placement using this technique, you might save yourself a lot of aggro by staying 'standard' and designing stuff around the keel bolts.
    On a boat this size, the engineering calcs alone to satisfy your insurers are going to be a big headache, if you can get anyone to take on the math and responsibility. Dont forget your own peace of mind in rough waters 50 miles offshore.
    Have you approached the original builders? They might be prepared to provide some advice on the concept.
    Good luck with the boat.

  6. Sea Jay
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    Sea Jay Doug Brown


    Sorry, but there is no web link to Dix's detail. He sent it directly to me as an addition to a set of plans I purchased, and I'm not sure he would want me passing it on without permission. Dudley is a pretty helpful guy and if you are interested, I suggest you drop him a note and see what he says.

    There would be no change to the number, diameter, or location of the keel bolts in this design. The only real difference is that the nuts can be accessed from the side. It may be possible to avoid interference of the bolts with a traditional installation and that will certainly be my first choice. However, I am trying to think of a plan B just in case.

    On page 84 of Professional Boat Builder Aug/Sep 2005 there is a photo of a keel designed by Eric Spongberg that shows a similar detail. - good article
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