On Mon, 28 Aug 2006 11:56:35 -0400, Walt
wrote:

How about this for a working definition:

"A monohull keelboat that's capable of planing."

Doesn't work for me, there are lots of monohull keelboats that are
capable of planing under certain conditions. I used to race on a B-29
that was right on the cusp of almost being a sport boat but it really
wasn't. Downwind with 20+ kts it was one exciting ride however, and
we once planed at 17 to 18 kts for over 2 miles. The B-29 carried a
conventional spinnaker pole which enabled it to go lower on a
spinnaker run than a true sport boat, but also at lower speed.