Sailboat water ballast

Discussion in 'Sailboats' started by razor, Jan 5, 2006.

  1. razor
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    razor Junior Member

    How would you go about calculating the reduction in heal by adding water ballast to a boat? What are the measurements and variables do I need to understand to make the calculation (distance above waterline, distance from centerline etc..)?

    Thank you
     
  2. RHough
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    RHough Retro Dude

    Ballast is ballast. Doesn't matter if it is lead or water or crew.

    1 ton of ballast 1 furlong from the centre will create 1 ton furlong of moment.
     
  3. safewalrus
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    safewalrus Ancient Marriner

    keep pouring it in until you stop healing - or sink! :rolleyes: Hmm! not very scientific is it? :p some wouldn't like it as they have to have lots of maths to prove how clever they are. :( Works tho'
     
  4. Skippy
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    Skippy Senior Member

    RHough: 1 ton of ballast 1 furlong from the centre will create 1 ton furlong of moment.

    And if your boat moves 10,000 furlongs in 1 fortnight, then your average boatspeed is 10,000 furlongs per fortnight. :p

    btw, with a 1-furlong ballast lever arm, be VERY careful around shoals. :)
     
  5. Doug Lord

    Doug Lord Guest

    Water Ballast

    For what it's worth and slightly off topic: canting keels have generally proved faster than water ballast at least in big boats.
    Some boats put tanks as far outboard as possible even using a flared hull to get the tank as far to weather as possible. Some boats use water ballast just filling a central tank (very inefficient)and others use it just for pitch trim. For use as righting moment the further to windward you can get it the better. But then you have to figure out how to get it from one side to the other and how to lose it when you don't need it.
     
  6. razor
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    razor Junior Member

    I was thinking of a retro-fit application by flaring the hull, it would seam to me to be the least expensive approach, PVC tubes glassed over on the sides of the hull.
     
  7. sharpii2
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    sharpii2 Senior Member

    You really can't. All you can calculate is how far it moves the ahwartship center of gravity. But this can give you a reasonable estimate of enhanced sail carrying power.

    To do this, you need to know two things.

    1.) the sailing weight of the boat with all its usual stores and equipment aboard, and
    2.) the weight and center of gravity of the water ballast in its assigned place on the boat.

    You first add the two weights together (the boat in its usual form plus the new water ballast).

    You then multiply the distance that the Center of Gravity of the water ballast is from the centerline of the boat times the wieght of that ballast. This gives you its moment.

    Then finally, you divide the the moment of the water ballast by the total weight of the boat.


    This merely gives you the amount you have extended the righting arm of your boat. The original righting arm could be anywhere from 10 to 15% of the boat's over all beam. If your boat is fine ended and deep in section, the number is closer to 10%. If it is full ended (or just has a really wide transom) and shallow section, the number is probably closer to 15%.

    Guestimate where your boat lies within these characteristics and you can then further guestimate the actual amount you have increased your sail carrying power.

    There are a few things to look out for.

    1.) The water ballast should be a small fraction of the boat's fixed ballast. Say, no more than 25%.

    2.) The height of the Center of Gravity of the water ballast from the bottom of your hull (not the bottom of the keel) should be no more than half the distance it is from the centerline of your boat. (half as high as it is far).

    3.)There should be a means to pump the water ballast over board other than relying on gravity. This is incase it ends up on the low side and your boat is knocked down from the wind. A gravity dump system should be in place as well.

    Best of luck.

    Bob
     
  8. PAR
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    PAR Yacht Designer/Builder

    You may also want to look at the loading on the bits and pieces of the rig and the rig as a whole, which will increase, possibly substantially.
     
  9. razor
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    razor Junior Member

    Thank you for your responses. Sharpii2 would your 25% rule apply to total capacity or per side? Does any one have access to information on the systems used on the old Volvo open 60s?
     
  10. RHough
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    RHough Retro Dude

    IIRC the limit was 10 degrees of heel from the ballast.

    For most boats the RM curve from 0 to about 30 degrees is linear.

    Measure the righting moment at 2 degrees by adding weight to the boat where the ballast will be, multiply that weight by 5 and you should be very close to being able to heel the boat 10 degrees with the water ballast.

    Take the 2 degree number and multiply by 15 to get the RM at 30 degrees, add the RM from the ballast (10 deg) and use that number to check the rigging sizes to make sure the rig will not be overstressed.
     
  11. sharpii2
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    sharpii2 Senior Member

    I meant 25% of fixed ballast total.

    Having water ballast on both sides makes no sence at all. Because of the sectional shape of most boats, the water ballast, to be out far enough to do any good, will end up being relatively high in the boat. This will raise the vertical center of gravity and thereby make the boat more tender.

    Also, water ballast high up, even on the windward side can contribute to a knock down once the boat heels beyond a certain point. This is the reason that I and other posters on this thread say that the water ballast should be able to be gotton rid of easily.

    25% of fixed ballast does not sound like a lot. for a 2000 lb boat with about an 800 lb keel, that would give you only 200 lbs of water ballast. But think of it this way. 200 lb is about the weight of a healthy crewman out on the weather rail, rain and shine, who eats not one morsel of food, and who drinks not one can of beer.

    I have thought of using water ballast myself. Not for leaning the boat toward the wind, but to lean it away from it. That would be to make the sails set better in light wind conditions because, even becalmed, the boat would have a list. It would be interesing to see if using it for that purpose actually improves over all performance more than using it for its original purpose.

    Bob
     

  12. asathor
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    asathor Senior Member

    waterballast

    Water ballast can be made self leveling by drilling a series of small holes at the waterline:idea:
    If the waterline is raised it will run out :)
    If it is low it will run in :(

    However some people prefers to have the water on the outside of the boat :D

    Whats so bad about lead anyway. It would be easy to build a track for lead ballast to move to the desired location and that way you could have the keel cant in the direction where it would do some good. Sure the ballst would not get as far out but it could make a canting keel inexpensive.
     
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