On Wed, 03 Jun 2009 09:15:38 -0500, Jim Willemin
wrote:

I've never understood why old powerboats often have such pronounced
tumblehome at the transom. Was it style, or was there a performance
reason?

Maybe both. Older powerboats had less horsepower available to them
so speed depended on building boats that moved easily through the
water, similar in some respects to sailboats. The most efficient
shape is long and narrow like a torpedo or fish, however on a boat it
is water line length that is important, not length on deck. Extending
the water line aft but disguising it with reverse sheer can give a
somewhat sleeker appearance with improved speed potential. On
sailboats a reverse transom sheer also reduces weight in the stern
which decreases pitching when going through waves.