3 Knots

A sailor returns to shore after 6 months at sea and needless to say he is longing for some female companionship. So, he finds himself a lady of the night and they agree to terms and find a hotel room to carry on their business arrangement. After having made love for quite a while the sailor asked his companion "how am I doing?". She replied, "three knots". Puzzled, he asked her, " what do you mean by that?". She says, "you're not hard, you're not in, and you're not getting your money back."