Defining the Length
There are three lengths commonly referred to:
• Length overall, LOA.
• Length on the waterline, LWL. This will vary with the draught at which a ship is floating but, unless otherwise specified, should be taken as the length on the design waterline, usually the summer load waterline.
• Length between perpendiculars, LBP. The fore perpendicular is a vertical line at the intersection of the bow with the summer load waterline. The after perpendicular is a line through some convenient point aft, usually through the rudder pintle. The point midway between the perpendiculars is known as amidships.
The upper deck often rises towards the ends of the ship to make the ship drier in waves. This rise is known as sheer.