The Three Knots

Arthur, an old retired sailor, puts on his old uniform and heads for
the docks once more, for old times sake.
He engages a typical docklands working girl and takes her up to a room.
He's soon going at it as well as he can for a bloke his age, but
needing some reassurance he asks, 'How am I doing?'
The girl 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.