Three knots.
Norman, an old retired sailor, puts on his old uniform and heads for the docks once more, for old times sake and some hot sex. He engages a lovely 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?"
She replies "Well, Norman, ya old sailor; I'd say that 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!"