The Forbes Log is an instrument for measuring the speed and distance traveled by

ship A ship is a large watercraft, vessel that travels the world's oceans and other Waterway, navigable waterways, carrying cargo or passengers, or in support of specialized missions, such as defense, research and fishing. Ships are generally disti ...

s, notably those of the

Royal Navy The Royal Navy (RN) is the naval warfare force of the United Kingdom. It is a component of His Majesty's Naval Service, and its officers hold their commissions from the King of the United Kingdom, King. Although warships were used by Kingdom ...

. They were introduced commercially by Elliott Brothers in 1909 and were fitted to all

capital ship The capital ships of a navy are its most important warships; they are generally the larger ships when compared to other warships in their respective fleet. A capital ship is generally a leading or a primary ship in a naval fleet. Strategic i ...

s by 1912, generally to aid the use of the

dumaresq The Dumaresq is a mechanical calculating device invented around 1902 by Lieutenant John Dumaresq of the Royal Navy. It is an analog computer that relates vital variables of the fire control problem to the movement of one's own ship and that o ...

as well as battle reporting.

History

Elliott Brothers licensed the concept and began production in 1909. It was a standard fit to all

s by 1912. The company also licensed the device to Anschütz in 1911, in exchange for a license of Anschütz's

gyrocompass A gyrocompass is a type of non-magnetic compass which is based on a fast-spinning disc and the rotation of the Earth (or another planetary body if used elsewhere in the universe) to find geographical Direction (geometry), direction automaticall ...

technology. The Anschütz compass proved less accurate and less reliable than models introduced in 1913 by

Sperry Gyroscope Sperry may refer to: Places In the United States: * Sperry, Iowa, community in Des Moines County * Sperry, Missouri * Sperry, Oklahoma, town in Tulsa County * Sperry Chalet, historic backcountry chalet, Glacier National Park, Montana * Sperry Glac ...

, and the Anschütz models produced by Elliot were replaced with Sperrys. Initial tests of the Forbes Log in 1912 demonstrated it was not entirely accurate, which added to a larger set of problems being encountered by the Navy's new gunlaying instruments. These issues were eventually solved, and the Forbes Log remained a standard solution well into

World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

, by which time they were found on almost all British and allied ships, including

submarine A submarine (often shortened to sub) is a watercraft capable of independent operation underwater. (It differs from a submersible, which has more limited underwater capability.) The term "submarine" is also sometimes used historically or infor ...

s. One of the remote outputs of the Forbes Log was placed in the gunnery control rooms where they were initially used to set the

and range clocks by their respective operators; in the post

World War I World War I or the First World War (28 July 1914 – 11 November 1918), also known as the Great War, was a World war, global conflict between two coalitions: the Allies of World War I, Allies (or Entente) and the Central Powers. Fighting to ...

-era, the system was often automated, eliminating the need for an operator to do this.

Description

The measuring system of the Forbes Log consists of an L-shaped tube that projects through the hull with the bottom portion of the L facing forward. The ship's forward motion forces water into the tube where it spins a propeller and then exits through a second pipe facing aft. The propeller is connected to a small generator, whose electrical signal is then sent to the Log itself. This consists of a

voltmeter A voltmeter is an instrument used for measuring electric potential difference between two points in an electric circuit. It is connected in parallel. It usually has a high resistance so that it takes negligible current from the circuit. A ...

pointer indicating the instantaneous speed, and an

odometer An odometer or odograph is an instrument used for measuring the distance traveled by a vehicle, such as a bicycle or car. The device may be electronic, mechanical, or a combination of the two (electromechanical). The noun derives from ancient Gr ...

-like system recording elapsed distance. The advantage to the Forbes Log is that the signal can be amplified and sent anywhere in the ship. This made it valuable on large Navy ships where the information was being used by many different parts of the crew. In typical use, the main log would be repeated on the bridge and combat center, with additional speed-only indicators sent to the fire control office and turrets to aid gunlaying.

References

Citations

Bibliography

* * {{cite tech report , first=John , last=Brooks , title=Fire control for British Dreadnoughts : choices of technology and supply , publisher= Department Of War Studies, King's College, London , date=2001 , url=https://kclpure.kcl.ac.uk/portal/files/2931913/274933.pdf Military computers Mechanical computers Analog computers Naval artillery