In response to a few questions on the Nor'Sea Yahoo group site a friend of mine posted a YouTube video of his Yanmar 2GM20 prop shaft at different RPMs.
You can take a look at it at;
Nor'Sea Prop Shaft With Ed and Ellen Zacko - http://youtu.be/hMGN4OnmH0s
Greg
Below, our Yanmar after we arrived in Mazatlan Mexico, and I was servicing the raw water pump.