Keel conversion fin to bulb

Discussion in 'Sailboats' started by gregzore, Sep 3, 2011.

  1. gregzore
    Joined: Sep 2011
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    gregzore New Member

    I have an Elvstrom 717 sailboat I am working on improving for single handed sailing. The boat is 7.2 meters long and 3.2 meters wide, weighs around 1200 kg with about 500 kg in a 1.25 meter deep steel keel. The boat has essentially the same rig as a mini 650 prototype boat. The issue being that the righting moment is worse than half of that of the mini 650 prototypes.

    The wind where I sail is generally under 15 knots and most often under 10 knots so I would like to be able to carry as much sail as possible. Currently when sailing singlehanded the I change from genoa to jib around 7 knots and take a reef at 10 in order to keep the rudder in the water. For comparison, with a crew of five, we don't reef until it is blowing upwards of 20 knots.

    Since purchasing the boat I have replaced and strengthened the entire interior structure and installed a 2.7 meter retractable bowsprit for sailing with a-symmetrical spinnakers. This winter I am hoping to convert the rig to have swept spreaders (I did the structural work with the interior rebuild).

    Next winter I would like to change the keel. The goal would be to replace the steel clump with an equally deep or slightly deeper foil with a bulb while trying to maintain the same or slightly improve the boat's righting moment. This will lighten the boat by about 200 kg which would do wonders in light wind. Eventually I will add ballast tanks.

    I am wondering what the options are for attaching the keel. The current keel is simply bolted to the hull which does not seem to be the best idea structurally. I am thinking of building a box structure about 40 cm up from the hull and tying it into the rest of the structure similar to in a mini 650:
    http://www.opensailingusa.com/News/wp-content/uploads/2010/03/Interior_02.jpg

    Are there other options that may be better?
    Thanks for the help, I am a newbie here.
     
  2. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    You've probably seen this but for those that haven't : http://samst.se/?page_id=33
    Sounds to me like you should probably consult with a naval architect on the keel mods unless ,maybe, there are other 717 owners that have successfully done what you want to do. Still, a consultation shouldn't cost much and might save you a world of trouble. Good Luck!



    click on image:
     

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  3. gregzore
    Joined: Sep 2011
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    gregzore New Member

    There are a couple others as well such as this guy who is installing a canting keel:
    http://www.jebl.blogspot.com/

    Another guy I have spoken with used a melges 24 keel and built a box around it. I was thinking of following his example, but am curious what other options are.
     
  4. Eric Sponberg
    Joined: Dec 2001
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    Eric Sponberg Senior Member

    Greg,

    The most common options are bolt on, such as you have now, and the mortise-tenon idea where the top of the keel slides into a box built into the boat, and is then pinned with at least two pins athwartships. Keels are pretty simple, really, so there aren't too many options available. Going to a swinging or lifting keel brings about a few other structural and design options, but I am assuming here that you want to stick with a fixed keel.

    One area that you have to be concerned about is that if you increase the righting moment of the boat with the new keel, you will be increasing the load on your new rigging and new chainplates, particularly so if you are going to add ballast tanks. Are they strong enough to take the extra load? Worth a check.

    Eric
     
  5. Perm. Stress
    Joined: Jan 2008
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    Perm. Stress Junior Member

    For so long as ballast tank (I assume side tanks here) contain ~350-400 l of water -an equivalent of 4 husky guys and you not use tanks and hiking crew simultaneusly, you will remain within design righting moment.
     

  6. gregzore
    Joined: Sep 2011
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    gregzore New Member

    Thanks for the replies guys, they are really helpful. The boat is 'normally' raced with 4 to 5 people on the railing so the goal of the modifications is to make it possible to have an equal amount of 'meat' on the rail, but only one or two crew. Downwind the boat already goes pretty well with two and noticeably slower with four, so it should be nice to get rid of a couple hundred kilos through changing the keel.
     
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