An old retired sailor

An old retired sailor puts on his old uniform and goes down to the docks once more for old time's 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. The old sailor 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."