3.3.1 Does water ballast work?
Yes, but not nearly as well as a more dense ballast like lead. We are talking here about a fixed tank of water placed as low in the boat as possible and completely filled. An air bubble in the tank means that the some of the water is free to move to the low side and in this case stability can actually be worse than if the tank were left empty. If it is kept empty, the entire boat will float too high, reducing stability. So if your boat has a ballast tank, keep it *completely* filled while you are afloat. To answer the question in more detail, it needs to be broken down into two questions, one comparing water with lead ballast and another comparing water with no ballast.
How does a water-ballasted boat compare with a lead-ballasted boat of the same length, beam, draft, freeboard and interior headroom, and the same weight of ballast?
Water ballast is much lighter for trailering, as it can be drained. A water tank is cheaper than the same weight of solid lead. These benefits are purchased at a cost however.
The water-ballasted boat will have less static stability, This is because the less dense ballast cannot be concentrated as low in the boat. The water-ballasted boat therefore cannot carry as much sail as the lead-ballasted boat, but will have similar resistance to motion. This means decreased speed. Also, this ballast occupying relatively high areas of the boat will require a deeper shaped hull for the same interior headroom which leads to a shorter (vertically) fin or centerboard for the same total draft. This adds up to worse windward performance. These are the costs of the more convenient trailering and lower expense.
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.
3.3.2 Summary
Could you make a SHORT summary of all this?
Yes. Just consider a water-ballasted boat to be an unballasted boat but with improved capsize angle and all the plusses and minuses of added weight while afloat but not while trailering. There is a cost in performance. (gf)
from http://www.faqs.org/faqs/boats-faq/part1/
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