An old retired sailor puts on his old uniform and goes down to the docks for old times sake.

He hires a prostitute, takes her up to a room and goes at it as best as he can for a guy his age.

After a couple of minutes he asks, "How am I doing?"

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

"Three knots?" He asks. "What's that supposed to mean?"

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