Thames Measurement, also known as Thames Tonnage, is a system for measuring ships and boats. It was created by the Thames Yacht Club, and was designed for small vessels, such as yacht

Yacht

A yacht is a recreational boat or ship. The term originated from the Dutch Jacht meaning "hunt". It was originally defined as a light fast sailing vessel used by the Dutch navy to pursue pirates and other transgressors around and into the shallow waters of the Low Countries...

s. It was originally used for calculating the port dues for yachts; the formula was also used in some early handicapping

Handicapping

Handicapping, in sport and games, is the practice of assigning advantage through scoring compensation or other advantage given to different contestants to equalize the chances of winning. The word also applies to the various methods by which the advantage is calculated...

 systems for yacht racing

Yacht racing

Yacht racing is the sport of competitive yachting.While sailing groups organize the most active and popular competitive yachting, other boating events are also held world-wide: speed motorboat racing; competitive canoeing, kayaking, and rowing; model yachting; and navigational contests Yacht racing...

. It is a variation of Builder's Old Measurement

Builder's Old Measurement

Builder's Old Measurement is the method of calculating the size or cargo capacity of a ship used in England from approximately 1720 to 1849. It estimated the tonnage of a ship based on length and maximum beam...

.

The calculation of Thames Tonnage uses the following formula:



where:

• length is the length, in feet, from the stempost to sternpost;
• beam is the maximum beam
  Beam (nautical)
  The beam of a ship is its width at the widest point. Generally speaking, the wider the beam of a ship , the more initial stability it has, at expense of reserve stability in the event of a capsize, where more energy is required to right the vessel from its inverted position...
  
  , in feet.

This can be simplified as: