Why is Net Tonnage carved into my boat hull?

"Net" followed by a number and carved into a beam of a vessel (or otherwise permanently affixed) refers to the vessel's Net Tonnage, which is an attempt to measure the useful volume of a hull, excluding machinery and other spaces that are not available for cargo.  On a Certificate of Documentation, the Gross Tons and Nets Tons are listed.  These were determined by a "Tonnage Admeasurer" when the boat was documented. Gross Tons would be for the entire hull volume, whereas Net Tons is less since it doesn't include machinery spaces.

The use of the word TON here is confusing because both Gross and Net Tonnage are measures of VOLUME, NOT WEIGHT.  There is no comparison to short tons, long tons, or metric tons (2000 lbs, 2240 lbs, and 2204 lbs respectively.)  In fact, tonnage as used in measuring vessels precludes the word ton used in weight measurements.  Originally ships were measured by how many casks of wine could fit in her hold, and the casks were known as "tuns".  The long ton was set at 2240 pounds to correspond to the weight of a tun of wine. (They must've been large casks!)  At some point, vessel admeasurement was standardized to use 100 cubic feet per ton, so admeasurers would measure the interior volume of the hull in cubic feet (excluding framing and certain other deductions) and then divide by 100 to convert to Gross Tons.  Both the Gross and Net Tons are listed on your certificate of documentation.

But remember -- Gross Tons or Net Tons have no relation to the weight of your vessel.  A boat's weight is often referred to as "displacement" since a floating body displaces it's weight in water (Archimedes' Principle.)
FYI, you can look up your vessel's documentation info online:


Note, there are also specific rules about what constitutes the Length, Breadth and Depth of the hull for use in determining tonnage.  Length has several possible methods such as stem to rudder post, etc.  So it is usually less than the overall length.  Breadth is pretty standard, but does not include guards and rub rails.  Depth is the vertical dimension from the rabbit line (intersection of keel and bottom) to the main deck at the side.  (So don't confuse Depth with draft.)

Oh, and why is the Net Tonnage carved into a beam?  It is because many port fees are, or were, based on tonnage. So a vessel's Master can't lie about the size of his ship in order to pay a lesser fee.


-R.E., Nov. 2011