Three Knots

A retired old sailor decides that just one last time he will put on his captains uniform and head to the docks.

After a couple of drinks, he decides that just one last time he will hire a prostitute for the night.

While engaged in the act, the old sailor says to the prostitute "How am I doing? Not bad for an old guy huh".

The prostitute replied "Well sailor, you are doing three knots. You not hard, your not in, and you are not getting your money back".

Stolen joke from a boating forum.