There is a very good description of the process in BOATBUILDING by Howard
Chapelle. Wood as fuel to melt lead is entirely satisfactory, it has been
used for years. Beware of using concrete blocks to support the tub;
firebrick is much better as suggested above. If you really want to cast a
keel larger than a bathtub can hold, finding a foundry may be a good (safe)
alternative.