Adding stability to a sail sportsboat

Discussion in 'Boat Design' started by Team AJ, Jul 7, 2011.

  1. Team AJ
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    Team AJ New Member

    I want to modify my sportsboat so it can be sailed shorthanded. The boat has a carbon keel fin with a leed bulb. The boat weight is 900kg including the bulb with weight aprox 300kg. I want a ballast ratio around 50%. The question is: Make a new and heavyer bulb, or add length to the fin?

    What do you think?
  2. messabout
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    messabout Senior Member

    Increasing the ballast ratio by adding weight to the bulb may violate the design lines of the hull. Lengthening the fin will increase righting moment at the expense of deeper draft. Deeper draft will demand that you select your water with care. At 900kg it would seem that your boat might depend on shifting crew weight.

    If you want to sail single handed, simply reduce sail and/or shorten the mast height. If the boat is from a competant designer, do not mess with the underwater appendages. Depending on how you figure ratios, you already have 50% ballast. The bulb at 300kg and the all up weight of 900kg means that the hull and rig is 600kg. 300/600=0.50 or 50%. Shorten sail, have fun single handing.
  3. Team AJ
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    Team AJ New Member

    Thanks for the reply. The draft is no problem. I can increase the draft by 0.5m without any problems. The question is how much this would increase the righting moment? And what will it do to the sailing performance?
  4. Petros
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    Petros Senior Member

    You will also increase the stress on the hull and keel if you make them longer, both the righting moment and the stress on the keep and hull would go up directly proportional to the length of the keel. Add 50 percent more length and you add 50 percent more righting moment, and stress on the hull and fin. Unless you have good guidance on doing it I would not. Either reduce the sail area or mast height as suggested, or just add ballast to the bilge for single handing. You can always remove it easy if you want to add crew.
  5. Doug Lord
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    Doug Lord Flight Ready

    Consider this:
    Hugh Welbourn could help you determine how to go about it within the parameters of your boat. You might not have to beef up anything if the foil
    is designed specifically to allow shorthanded sailing.
    -You could also consider adding "plug-in"(removable) carbon racks to allow you to sit further outboard.
    -Both these options could be designed to work with the existing structure by not exceeding the righting moment(RM) of the boat with the full crew. Neither would add much weight and both could be removed when fully crewed and either one or both would add performance because RM would be the same as with less weight. Either one or both would have to be carefully designed to work well.....

    pix-Quant 28 with racks and DSS-click on image:

    Attached Files:

  6. Raggi_Thor
    Joined: Jan 2004
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    Raggi_Thor Nav.arch/Designer/Builder

    A heavier bulb or a deeper keel doesn't improve stability much before you heel quite a lot.
    I think 100kg or so of movable ballast would be better.
    Maybe you need some batteries anyway? Why not move them from side to side with a simple rope arrangement?
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I think adding additional ballast on the end of the fin, will test the breaking strength of every fitting in the rig in puffy or gusty conditions. Knowing that most sport boat tolerances are fairly tight, this is a very likely situation.

    Your butt is the best ballast you have, so as has been mentioned shorten sail or drive partly stalled to keep her on her feet when over pressed. Optionally you can consider a bigger feed bag and put on some 40 pounds of extra, adjustable ballast.

  8. jim lee
    Joined: Feb 2007
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    jim lee Senior Member

    Making the keel longer isn't going to change your ballast ratio. Where did the 50% number come from anyway?

    -jim lee
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