Three Knots

Jerry told a pretty good joke the other night:

There was an older man who lost alot of weight during rehabilitation from open heart surgery.
He thought he looked slim enough to try on his old Navy uniform. It fit.
To celebrate the achievement, he decided to do what he had done the last time he wore the uniform - go down to the bar at the dock and find himself a hooker. He did.
After they were busy for awhile he asked her, "So, how am I doing?"
She answered, "Sailor, you are doing about three knots. You're not hard, you're not in and you're not getting your money back."