The initials are for Titcomb and Walsh who designed a USCA racer with high sides for downriver racing. The legend had Jim buying the plug, gluing a rope down the keel line, them fairing it in with Bondo to yield the famous Mad River semi-V bottom.
Only the goddess knows if that's true, and she ain't saying. I had two TW's over the years, the first if Glass, the second in Kevlar for obvious reasons. That V bottom was a great tracking aid.