“Net tonnage” measures a vessel’s volume. It does not measure a vessel’s weight (which is often also expressed in tons).
If a vessel is 26 feet or longer, then it most likely measures five net tons or more.
According to the United States Coast Guard, “net tonnage is a volumetric measure of a vessel’s useful capacity.” Parts of the vessel that do not hold cargo (such as the crew’s spaces and the like) do not count as part of a vessel’s net tonnage.
Volume measurements are from a vessel’s bow to its stern and funnel to the keel.
The “Simplified Measurement System” can be used to find the net tonnage of any vessel under 79 feet in overall length, as according to GovInfo.
Net tonnage is calculated, in part, from gross tonnage. To find the gross tonnage of a monohull vessel (a vessel with one hull), add the vessel’s hull volume to the deckhouse volume and divide by 100.
The formula for “Deck Structure Volume” is “length” X “breadth” X “depth.” For hull volume, calculate “length” X “breadth” X “depth” and multiply by “S” and “K.” The dimensions of deck structure are defined as the length, breadth, and depth of the principal deck structure.
“S,” or “the shape factor,” is .84 for barges and other vessels with boxed-shaped hulls, .67 for powerboats and other vessels with circular hulls, and .5 for sailing vessels.
“K,” or “the keel factor,” is .75 for sailing vessels where the keel is included in the vessel’s Overall Depth and 1.0 for all other hull configurations.
Twin-hull vessels and tri-hull vessels have other formulas for gross tonnage.
The Simplified formula for net tonnage (or “net register tonnage”) is NRT = GRT X M.
“GRT” can be found through the formulas above.
“M,” or “the machinery factor” is .8 for vessels that have propulsion machinery inside the hull that aren’t designed for sailing, .9 for sailing vessels that have propulsion machinery inside the hull, and 1.0 for vessels that have propulsion machinery outside the hull as well as non-self-propelled vessels.
For more information about how tonnage is determined, email firstname.lastname@example.org