Ray, 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 Ray, old 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."