The measurement of a vessel’s ’net tons’ or ’gross tons’ actually has nothing to do with the weight of the boat. The measurements are concerned with the volume of the vessel, and the use of these terms developed as a yardstick to levy taxes (such as registration fees and port taxes) on merchant vessels.
The ’gross tonnage’ of a vessel is an approximation of the total cubic volume of a vessel, where one gross ton equals 100 cu. ft. The exact number is the product of a formula used by marine architects, and there are a few ’tricks of the trade’ that they use to allow the boat to fall within certain regulatory schemes.
’Net tonnage’ is a measurement of a merchant vessel’s ’earning capacity,’ and it is calculated by subtracting certain unproductive areas of the boat, such as the engine room, from the gross tonnage calculation. These terms are defined in the Code of Federal Regulations, Title 46, section 69.9. The terminology has carried over to recreational vessels, even though there is no real ’earning capacity.’
The weight of a vessel is usually expressed by its displacement. This number will actually vary, depending upon factors such as the temperature or salinity of the water, so we have yet another measurement, ’deadweight tonnage,’ to contend with. Deadweight tonnage is the actual weight of a vessel.
But, getting back to the tax question (this is, after all, a legal column and not an engineering discussion) …
As noted above, the ’50-ton’ reference in the reader’s question is probably a reference to a provision of the California State Constitution which exempts certain commercial vessels from the annual assessment of personal property tax. Specifically, Article XIII, Section 3(l) exempts vessels that are ’more than 50 tons burden in this State and engaged in the transportation of freight or passengers’ (’tons burden’ is another way of saying ’net tons’).
The recreational sportfishing boat described in the reader’s question will, unfortunately, not fall within this exemption.