Had one on my 40ft yacht, Perkins 4.108, driving a 1.25" shaft
through a flexible drive to a Maxprop look alike.

It was fitted in 1988, and I sold the boat in 2003. Shaft ran for
about 6,000 hours in that time. Never a leak, no sign of wear. I
inspected the bellows regularly looking for any signs of cracking;
none seen.
--
JimB
http://www.jimbaerselman.f2s.com/
Describing some Greek and Spanish cruising areas