Calculating Vessel Weight
By Don Perrine
There are three key weight definitions and specifications used with vessels. It is important to understand the differences between these terms for various reasons, including hauling out.
Displacement tons:This is the actual weight of the vessel, and it is the number you need to know prior to hauling out. It is sometimes expressed in long tons or metric tons, which equal 1,000 kg or 2,240 lb. Long tons are 10 percent heavier than commonly used “short” tons of 2,000 lb. Sometimes, displacement weight is specified as being measured at no load, half load, or full load.
The vessel always will follow Archimedes’ principle, which states that “any body partially or completely submerged in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the body.” In other words, a boat sinks in water until it has displaced its own weight.
For our purposes, displacement weight in tons is the actual weight that will be lifted by a mobile boat hoist. Generally, a vessel’s manufacturer can provide design displacement weight specifications. Also, most mobile boat hoists have scales to read the displacement tons fore and aft, although they may be somewhat inaccurate. After several haulouts, and by asking the lift operators for the scale readings, one can get an estimate of the measured weight.
When hauling out, I suggest using displacement tons at full load if known; if this value is not known, to ensure safety, I suggest communicating a weight that is considerably higher than anyone’s best estimate.
Gross tons:This measurement is used by vessels that are in commercial service or are U.S. Coast Guard–licensed. The metric is an estimate of the cargo capacity of the vessel based on calculating units of 100 cubic feet of interior volume for all areas inside the vessel. Each unit of 100 cubic feet is deemed to correspond to 1 long ton or 1 metric ton. Therefore, a vessel could be described as being 100 gross tons but could have an entirely different displacement weight.
Net tons:This is gross tons minus spaces in the vessel that cannot under any condition be used for cargo, such as engine and machinery spaces, the pilothouse, and similar areas. The non–cargo areas are subtracted from the gross tons, resulting in net tons of theoretically usable cargo space. Thus, net tons will always be lower than gross tons. Neither one bears any direct relationship to displacement tons.