August 22, 2007
-
Water-Ballast vs. Lead-Ballast
I overheard a discussion today about water-ballast versus lead-ballast, and to be honest, I had no fudging clue what they were talking about, probably because I’ve never sailed a boat with a keel! (Oh, wait. I did sail a ketch with a keel of *some* sort 3 years ago.) Anyway, I found it an interesting topic, and I learned something about what I would prefer in the boat I plan to build.
The following is an excerpt from a discussion about the water-ballast of a MacGregor 26. It’s from http://www.faqs.org/faqs/boats-faq/part1/
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)
