[2] 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.
[3] These issues were eventually solved, and the Forbes Log remained a standard solution well into World War II, by which time they were found on almost all British and allied ships, including submarines.
[5] 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.
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.