An old retired sailor puts on his old uniform and goes down to the
docks once more for old times sake.  He hires a prostitute and takes
her up to a room.  He's going at it as best as he can for a guy his
age.  

He asks, "How am I doing?"  

The prostitute replies, "Well sailor, you're doing about three
knots."  

"Three knots?" he replies, "What's that supposed to mean?" 

She says, "you're knot hard, you're knot in, and you're knot getting
your money back!"