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How does a water-ballasted boat compare with an unballasted boat of the same length, beam, draft, freeboard, and interior headroom?

If designed to do so, water ballast could make a boat uncapsizable. At least, it will increase the capsize angle. Water ballast also adds mass and therefore easier motion in a sea and better way-carrying in a lull or a tack. It will do this for little increased expense and trailering weight.

Basically, the advantages are bought at the cost of performance. A water-ballasted boat can carry little if any more sail than an unballasted boat. This is because it has little if any more stability at small angles of heel. However, for the same length, headroom, freeboard, etc. it must displace a greater amount of water equal to the tank of ballast. The same length, combined with greater displacement and no greater sail-carrying ability means less speed. Compared with an unballasted boat even more than compared with the lead-ballasted boat, the hull must be deeper, which again means less of the draft constraint can be allowed for the centerboard. This means poorer windward performance. Also the draft with centerboard up must be greater than the unballasted case. The better carrying of way and easier motion are at the cost of slower acceleration in puffs or after tacks. The increased mass is a double-edged sword.

Why does it add little if any more stability at small angles of heel? Remember we are comparing a water-ballasted with an unballasted boat of the same length, freeboard, cabin headroom, etc. The increased weight of water must be put in an increased underwater volume of the hull located as low as possible. This added volume of water underneath what could have been the bottom of the unballasted boat has no net gravitational force under static conditions as long as it is completely submerged. That is, neglecting the additional weight of the tank and added hull material, the increased weight is exactly balanced by the buoyancy of the increased volume to hold it. It therefore can have no effect on either heeling or righting moment if the tank is full of water of the same density as that in which it is submerged. Another way to think of it is that the center of buoyancy is lowered by exactly the same amount as the center of gravity.

Then how does it increase the capsize angle? At large angles of heel more or less of the water tank rises above the waterline. Now the relationship between the center of gravity and the inclined center of buoyancy becomes more favorable than the unballasted case. All of the weight of the water is no longer balanced by its buoyancy.


next up previous
Next: Summary Up: Does water ballast work? Previous: .
John F. Hughes
11/6/1997