Stability of a sailboat turned motorboat

Discussion in 'Stability' started by eddieb74, Apr 28, 2012.

  1. eddieb74
    Joined: Apr 2012
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    Location: Piedmont, Alabama

    eddieb74 New Member

    I've got a 20 foot Ensenada that I'm converting into a fantail launch. I've got the top half off and have the hull down to bare bones, but my question is, should I remove the 550 pound iron swing keel or leave it in?

    I won't be installing a heavy hardtop, rather I'm putting in a lightweight canopy, but I've no idea if the boat would be too unstable without the keel. I'd rather not leave it in - less trailering weight not to mention I'll need to find a way to remount the winch in the now-open cabin - but I hope to take the boat down the ICW so I'd like to be able to trust it won't go belly up.

    I'd be indebted for your collective wisdom. - Eddie
     
  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Your boat is the same hull as the Balboa 20, though with a different deck cap and a substantial weight increase. You've removed in the neighborhood of 300 - 400 pounds worth of deck cap and liner already and now you want to remove another 500?

    The Ensenada version of this boat displaced about 1,600 pounds, so 500 pounds from that is a 31% decrease in displacement, plus the other 18% to 20% that's been removed. She'll float very high and will be quite "tender" without a significant portion of this mass returned to the boat.

    Simply put, she'll be just fine as a powerboat if she's in the 1,300 to 1,600 pound range. Lighter then this and she'll flop around on her bilge turn, pretty good.

    The best advice you could receive is to build her as you like, centerboard removed and all, but reserve space where the centerboard case used to live for some ballast. Cast some 2"x3"x6" lead bricks, which will weigh about 13 to 14 pounds each, depending on how clean you work. You'll need 35 to 55 of these puppies (worst case), which can be arranged on the bottom of the boat to trim her up properly (bow down or up situation) and pull her down to her DWL. Conversely, you could consider water ballast as an option with some trailering benefits. You'll need a ballast tank and a way to fill and discharge it, but this isn't as difficult as it might sound. You'll need in the area of 60 to 100 gallons of water (again worst case) to pull her down, which is a pretty big hunk of water, but also assumes nothing else, like engine weight and anything else you might be installing.

    Naturally, you'll need to keep track of the weight you've removed, the weight you add and subtract this from what you need. Full up, shoot for 1,500 pounds, which will offer some capacity, without burdening the hull.
     
  3. eddieb74
    Joined: Apr 2012
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    Location: Piedmont, Alabama

    eddieb74 New Member

    Water ballast

    Thank you for your reply. In fact, I've been talking with a fellow who's almost finished converting his sailboat into a fantail launch and he installed water ballast tanks as you've described. I'm going to pester him on exactly how he designed them and put them in mine.
    Thanks again - Eddie
     
  4. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Please post what you find here as well. We are a bit thin on the ground with resect to photo's and drawings of actual waterballast installs on small craft.
     

  5. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Come on Phil, we all are very familiar with this concept. The beer goes in, ballast contained, until you have to pee, where the ballast is dumped over the side (repeatedly). Eddie, water ballast wouldn't be my first choice. It's a lot more difficult, has some potential issues and frankly a 1,500 pound boat can be pulled by anything except a sub compact car. The nice thing about metal ballast (lead) is you can move it, which you can't do with water. If trim changes because you're well fed friends are aboard, you can slide a few hundred pounds a foot forward and all is well. If you make an engine change and the CG moves, you can trim the boat out easily by moving weight. Not so with a fixed tank of water. Then there's the hole in the bottom of the boat thing . . .
     
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