Last time we distinguished three sorts of tonnage applicable to boats: displacement, deadweight tonnage, and gross and net register tonnage. The displacement of your sailboat will be given in the owner’s manual. The deadweight tonnage is not normally a big issue, as we do not use our boats as cargo vessels. Cruisers, of course, must consider how much weight to carry before it will adversely affect the sailing characteristics of their yacht.
The third measurement, gross register tons or GRT, is a measurement of volume, not weight. On cargo ships this measurement is done professionally as it relates to tax revenue. But on recreational boats, you’re on your own. Emails to Catalina Yachts on this issue were not answered, and at Beneteau I was informed that the manufacturer does not provide the register tonnage of their boats. The only reason this would matter to you is if you desired to document your boat with the Coast Guard instead of getting a CF number from the state of California. The federal documentation includes boxes for gross and net register tons, but the state registration does not require it.
To calculate these values, we may rely on the simplified measurement form provided by the Coast Guard at https://www.dco.uscg.mil/Portals/9/DCO%20Documents/Marine%20Safety%20Center/Tonnage/Tonnage%20Guide%201%20-%20Simplified%20Measurement.pdf?ver=2017-06-09-123757-680
Basically, you calculate length x beam x depth, multiply by two factors they give you to account for hull shape and keel type, and divide by 100. Depth is not to be confused with draft. Depth is the vertical measurement from the deck (where it meets the hull, not counting the height of the cabin trunk) to the bilge. The result is a pretty rough measurement of volume, but precise enough to satisfy Coast Guard regulations.
Using the formula, I measured Tradewinds’ Megalina and Lionheart, two boats that are nominally the same size at 31 feet. Megalina (displacement 8933 lbs.) comes out to 6.5 GRT, while Lionheart (displacement 9170 lbs.) is 6.1 GRT. Since a register ton is 100 cubic feet, this means Megalina’s internal volume is 650 cubic feet, Lionheart’s 610. (Actually, Megalina’s design with the beam carried aft and an after cabin, yields a lot more usable interior volume but this is not represented in the given procedure.) Next, to obtain net register tons or NRT, we multiply the GRT by .9, yielding 5.85 for Megalina and 5.49 for Lionheart. Voila! We are now admeasurers!