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 was going at it as best he could for a guy his age, but needing some reassurance, he asked.  

"How am I doing?"  

The prostitute replies, "Well 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".