stu, you'll have to experiment, 20-25 knots it plenty.... at differnet speeds the tube will do different things... I have a 3 man one... which if it catches the air can stay airbourne for a little while.. also, for differnet people different speeds is better... the faster you go the harder it is to hold on... take it easy at first, and try people at different speeds
And slow down on turns... or make them wide... the force of a turn gets a lot harder when you're at the tube end