I weigh in at around 17 1/2 st.
My 150 Opti just about gets me up on a mono IF my technique is good, otherwise I can win the tug of war.
When I was around 15 st I could get up behind a 50hp, so the extra weight seems to make a huge difference to required grunt ...
"If everything seems under control, you're just not going fast enough."
- Mario Andretti