Water Ballast

Discussion in 'Boat Design' started by Guest, Aug 8, 2003.

  1. tspeer
    Joined: Feb 2002
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    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    I think that sketch is very revealing as to what underfloor water ballast is all about. What you really have is a catamaran with a bottom skin connecting the two hulls. The water could be captive or it could be flowing through - either would be the same with respect to the initial stability. If the ballast tanks are not filled, the boat will sit higher in the water, and the waterline will be narrower, perhaps at about the location of the stringers, reducing the form stability.

    The same stability could be obtained with less wetted area and wave drag by omitting the center bottom skin and making it a true catamaran. Or by moving the bilge out to provide a wider waterline. So evidently the water ballast in this case is not for the purpose of adding stability. There must be some other motivation.

    Probably the biggest contribution the ballast would make is to enlarge the range of positive stability, and make the boat self-righting after a knockdown. The ballast would also add to the inertia of the boat, making it less lively in a seaway.
  2. Guest

    Guest Guest

    TSpear, your catamaran analogy is really good. It shows the inefficiency of this type of design. I think there is some benefit, however. If we imagine the boat heeled at say 30 degrees, the CB of the hull does shift to leeward of the CG of all that contained water, which starts to lift above the waterline. So it does provide some stability besides increasing the BWL.

    I suspect that some of the impetus for having water ballast is the marketing angle of having a water-ballasted boat, just like Volvo 60's, Open 60's, etc.

    Another note on the water ballasted keel idea. If you had a keel which was already hollow, then the air-filled keel would tend to decrease the righting moment of the boat (imagine heeling the boat a bit and having the bouyant force of the keel pushing upward). Filling it with water would neutralize that effect. But to purposely make a high volume keel to fill it with water, will just add wetted surface. After all, you are only replacing the weight of the water that you just displaced. You will gain some small benefits. As someone pointed out earlier, you will add inertia, which would help stabilize motion of the craft (it won't feel so tippy). The lateral surface of the keel will also resist rolling. And finally, if your craft does find itself upside down, that water filled keel sticking up in the air will tend to right it. But these small benefits will come at the expense of speed, manouverability and acceleration. It would be better to take your keel bulb and fill it with air and stick it out to the side, which is really just an outrigger.

  3. shu
    Joined: May 2003
    Posts: 45
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    Location: SoCal

    shu Junior Member

    Sorry, did it again....that last post was me.
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