(Jokes) Old Sailor

An old retired sailor, puts on his old uniform and heads for the docks once more, for old times sake.
He engages a prostitute and takes her up to a room.
He's soon going at it as well as he can for a guy his age, but needing some reassurance, he asks, 'How am I doing??
The prostitute replies, 'Well, Pops, 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.