Wayne.B wrote in
news
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.


Hmm. I hadn't thought of the weight distribution aspect. Lots of
tumblehome would necessarily limit the amount of weight that could be
carried on deck, or in an aft cockpit while maintaining waterline beam.
Looking at the photo of _Posh_ in the link in the OP, it seems that she
carries very little deadrise at the transom and not all that much draft
which would not make a lot of sense in a full-displacement boat like a
sailboat or Whitehall skiff (i.e. one limited by hull speed). This
suggests to me that she has the ability to plane, or at least go faster
than her wake. Thinking about planing boats, it seems that most carry
their full beam all the way aft, probably for stability. So if you need
the beam for stability at speed, but want to discourage half a dozen portly
gentlemen with beer from hanging out on the fantail for weight distribution
reasons, then a lot of tumblehome at the transom makes sense, at least in
something as big as _Posh_ (51 feet). Very clever, our ancestors...