I have been studying rigging dimensions, and that has been an adventure. First, here is a drawing showing various dimensions used for determining mast, spar and rigging diameters.     The biggest problem is that many authors use the term "length" ambiguously and just assume you know what they are thinking. This is known as "functional illiteracy" and it is very common.   The Hull   The length of a sailing ship's hull is often the length on deck - for the