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3.3.1 Does water ballast work?


This article is from the Boats FAQ, by John F. Hughes with numerous contributions by others.

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.


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