John,
The displacement of any vessel is easy to calculate in the water simply by
measuring the freeboard. However, you would need the lines plan to
calculate its volume. You have given very little data to the reader on the
hull shape, so only a very crude guesstimate can be made, but 25% of the
displacement as ballast is a good starting point. As it is a swing keel,
other ballast may exist elsewhere within the hull itself and if so, that
mass would have to be subtracted from my guess. Yes, it is common that
ballast is contained within a fabricated fin on metal boats. That practice
reduces the cost of ballast substantially, as the ballast is usually in
ingot form. The only downside of that practice is that it is impossible to
acheive the load density of a cast lump, but with dilligence, you can get
close. The Specific Gravity of pure lead is 11.7 metric tons per cubic meter
and it would be less in alloy form. This is given because you can measure
the fin and make a guess. It is also normal that the lead will not be
distributed uniformly within the fin, because the architect will also
deliver a load schedule to the builder in order to exactly place the CG to
the plan. Assuming the builder used due-dilligence in packing the ballast,
you can easily determine the lead/space boundary by lightly tapping the fin
with a hammer when making your measurement, but without access to the
ballast yourself, it is very possible the ballast is not lead because of
cost. Your concern for water inside the fin is real. Especially with
aluminum, as it is very reactive. Occasionally, a builder will fill the
voids around the ballast with oil for corrosion protection, but if it is a
production boat, don't expect it.
Steve

"EASI54" wrote in message
...

Lot of interesting keel threads going on, and I thought it would be good
time to enter mine.

The boat: 50ft custom built aluminum sloop with modified swing keel.
(no data on vessel)
draft: 4 1/2 ft. With swing extended: Approximately 9ft.


The vessel is on the hard and have no data to it's displacement other
than a scale ticket during it's move to my back yard, indicates the
boat weighs approximately 38,000 lbs.
Anybody have any idea on a ball park approximate on how much the swing
blade weighs (keel)?
I also would like to know if it is customary to wrap a lead keel in
aluminum to blend in with the hull?
In additon to all of this, I have noticed that the forward portion of
the keel indicates that it is hollow (forward section) which was made
to streamline it's shape to the hull. Since this is true and appears
to have been patched since being built, should I be concerned with
holding sea water and will this damage the metal over time?
John

John




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EASI54