When applying for an FCC SHIP STATION LICENSE, applicants use FCC Form 603 Schedule B. At line 15 the applicant can specify the length of the vessel and the tonnage. If this information is provided, then the FCC will share that information in the ITU SHIP STATION list.
The length of the vessel is straightforward. The tonnage is less obvious. Here the term tonnage does not refer to weight, but to the interior capacity of the vessel.
CALCULATING TONNAGEThe federal guidelines for tonnage are given by the USCG in their document
Simplified Measurement
Tonnage Guidehttps://www.dco.uscg.mil/Portals/9/DCO%20Documents/Marine%20Safety%20Center/Tonnage/Tonnage%20Guide%201%20-%20Simplified%20Measurement.pdf?ver=2017-06-09-123757-6805. MEASUREMENT SYSTEM APPLICABILITY
The USCG provides a form to be used: Form CG-5397 “Application for Simplified Measurement”
A vessel is eligible to be measured under the Simplified measurement system if it is..a...recreational vessel.
Refer to section 12. GROSS TONNAGE FORMULATION (MONOHULLS) for the procedure. Generally on small mostly-open boats the volume of any superstructure is ignored. In that case, the formula for tonnage simplifies to
Hull Volume = S x K x L x B x D
where
S = 0.67 (the shape factor for power boat hulls)
K = 1.0 (because the boat is not a sailing boat)
L = overall length
B = overall beam
D = overall depth (measured from the deck edge to the keel)
A machinery factor must be applied. If the propulsion is outside the hull (for example, outboard engines) the machinery factor is 1.0.
The lengths and depths are to be measured in feet and inches or to decimal feet to the nearest tenth.
Using my own boat as an example, I estimate as follows
S = 0.67
L = 24-feet 3-inches or 24.25-feet
B = 7-feet 5-inches or 7.6-feet
D = 3-feet (estimated for the purpose of this example)
This gives a hull volume of
0.67 × 24.25-feet × 7.6-feet × 3-feet = 370 cubic-feet
To determine tonnage divide the hull volume by 100, giving 3.7-tonnes.
You can also use an interactive webpage to compute the tonnage. Visit
https://www.dco.uscg.mil/Portals/9/DCO%20Documents/Marine%20Safety%20Center/Tonnage/CG-5397_10-17.pdf?ver=2017-11-15-070703-783When I entered my information in the form linked above it computed the vessel at 3 for both gross and net tonnage. Note that in order to be a documented vessel under USCG rules the tonnage must be greater than 5, so the interactive form alerts you to this problem.
V
ESSEL OFFICIAL NUMBERApplicants should also be sure to complete line 8, Official Number of the Ship. For most recreational vessels this information will be the state registration number of the vessel. If the vessel is a Coast Guard documented vessel, then that number is used instead.