This an extremely dangerous thing to do. It is a field of mines to the
inexperienced and most of them can kill you or worse, make you an invalid.
Your plan will not work. There is no shortcut. You must do this correctly,
using the correct materials and tried and true processes. Lets forget about
the mold for just a moment. Just consider the task of melting the
appropriate amount lead and pouring it into the mold and the required
machinery. Pure lead is 11.7 metric tonnes per cubic meter. Do the math.
Think how much heat is required to raise your melt to 600 F. Then think how
long this will take to cool to room temperature. A 6 ton plug takes about a
week to cool. There are no shortcuts. There is a legitimate reason why you
contract this out. How do you justify the one time expense to build the
tools and machinery? Think about the volume of poisonous gases you will
generate. There is absolutely no way this can be done in a residential area.
Just for your information, creating your ballast will one of most expensive
parts of a boat project and for very good reasons.
Steve

wrote in message
...
I am to the stage of casting the keel for my project. Please comment
on my proposed method of making the mold:

--make a male plug out of styrofoam with a thin coat of bondo for
fairing and smoothing.

--coat the plug with half a dozen thin coats of no texture drywall
mud.

--suspend the coated plug in the mold box and surround with cement.

--burn out the styrofoam and bondo, leaving a smooth female mold for
pouring the lead into.

note: I have never cast with sand and have no desire to learn
another new skill.

TIA mm