When Size Matters

“Hatteras’ specifications list the boat length as 50’ 3” and that is what I would like on my survey”. This was a recent request from a client who had just purchased a 2003 Hatteras 50 convertible. “And I would like to be referred to as Sir Kells, bring peace to the world and arouse women when I walk into the room”, I thought, though I responded “well let’s see if I can help”.
“I measured your boat and usually I am close. I measured 53’ 4”. I would be happy to measure it again”. I continued, “Boats are rarely the length that the manufacturer specifies or the broker lists”. Of this I am certain. But why? How are boats measured?
Per the 2009 version of the U.S. Coast Guard’s Simplified Measurement Tonnage Guide (uscg.mil/hq/msc/tonnage/docs/TG-1_Current.pdf), the overall length is the horizontal distance between the outboard side of the foremost part of the hull and the outboard side of the after most part of the hull. It does not include bow sprits, rudders, outboard motor mount brackets, swim platforms that do not contain buoyant volume and other similar fittings and attachments that are not part of the buoyant hull envelope.
We measure vessels length overall (LOA) and we do include swim platforms and bow sprits. This is the most useful length, as it is the length used by most marinas and yacht clubs. For our purposes we do not include the anchor hanging over the bow or the tender hanging on the transom, these can always be removed. While permanent fixtures such as swim platforms can be removed, it is not simple.
We rarely refer to length on the waterline (LWL) or length on deck (LOD) as these lengths are generally only useful when discussing designs and theoretic hull speed.
The Coast Guard defines overall breadth as “the horizontal distance taken at the widest part of the hull, excluding rub rails. This measurement is rarely disputed. Another important measurement of a boat’s width is the breadth at the waterline, as most boats are much wider at their rub rail (flair) than they are in the water and can thus fit into a slip which is not as wide as the boat.
The Coast Guard defines overall depth as the vertical distance at or near amidships from a line drawn horizontally through the uppermost edge of the skin (excluding the superstructure) to the bottom skin of the hull, excluding the keel. This length is not the vessel’s draft. Thus the depth on your Coast Guard documentation should not be confused with your vessel’s draft. We measure draft as the distance between the waterline of the vessel and the deepest submerged part of the vessel, be it keel, propellers or rudders. This is the minimum depth of water your boat needs to remain happy.
The Coast Guard allows simplified measurement for the calculation of tonnage. A discussion of the meaning of tonnage was addressed in a prior article and can be found on our website (http://themarinesurveyors.com/a-primer-on-displacement) it is not a measure of weight.
The boat’s length thus can vary, but it only varies based on the specific definition of the length that is sought. It is neither shorter when you are paying for a slip (or marine survey) nor longer after a couple scotches.