As for lead keels... they're denser than iron keels, so provide more ballast for the same volume. A keel of equal mass and righting moment made of lead will be smaller and have less underwater surface area—resulting in a faster boat. The metal is more malleable, giving in the case of an impact. It is less likely to cause damaging corrosion. Iron expands when it rusts...leading to the breaking of encapsulating fiberglass... etc.
Overall, if you have to have a heavy keel... lead is really a much better material for it.
Osmium is denser than either tungsten or deplete uranium... in fact it is about the only metal that won't float on mercury. Tungsten is about 19.25 gm/cc and uranium is about 19.1 gm/cc—Osmium is 22.61 gm/cc, and not very radioactive.. but it is considerably harder than lead... so not as forgiving. Iridium is a close second to Osmium in density, and both are in the platinum metal family.
It is very expensive... with the one stable isotope of osmium going for as much as $25,000 per gram.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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Still—DON'T READ THAT POST AGAIN.