A Tangent Bar is a small metal accessory that could be added to a simple torpedo director which would permit the director to be situated at a distance from the firing tube, and would factor out the parallax errors thus introduced. The remote placement of the director was often necessary on capital ships with submerged broadside tubes, or desirable on ships where the aiming and control of torpedo fire was deemed better kept near the command centre.
Tangent bars proved to be a pain in the ass. They greatly increased the operational complexity of the torpedo director and compromised its rigidity. In exchange, they solved a problem that was tiny, in the big picture.
A tangent bar looked like a small metal ruler of 3-5 inches and would be fitted to the rear post of the director. The rear sighting arch of the director would be removed from its former position on the rear post and placed on a slider that rode along the tangent bar. The proper use of the device, then, was to orient the tangent bar to point to the firing tube and position the slider to the present range to the target against a irregularly graduated scale. The purpose was to factor out the offset between tube and sight to cause the sighting line to converge on the torpedo's track.
Intrinsic in this function is that the markings on the tangent bar are related to a given fixed distance from director to firing tube. As such, each ship's director positions would require their own tangent bars. The range set upon the tangent bar need not be extremely precise, but the convergence will be imperfect if the range is not correct.
Use in Gyro Angle Firing
In 1912, Vernon studied the geometry of how the radius of turning of a torpedo seeking to take up a given Gyro Angle impacted the use of tangent bars. The solution hit upon was to affix a disc at the base of the tangent bar and to not point the bar directly at the mouth of the tube. Its markings were arrived at through considerable graphical analysis and exemplify the further complexity the tangent bars were imposing.
Impressions on Use
For too long, the Royal Navy regarded torpedoes as magical weapons which, when fired, should perform exactly and strike their target every time. They willfully ignored the likelihood that an enemy would turn while the torpedo was swimming toward him, even in the incredibly unlikely case where his speed and heading had been judged exactly. For instance, the fore conning tower of H.M.S. Jupiter was deemed a firing position of "last resource", owing to the errors introduced in the pre-tangent-bar period by the lateral offset that would result between sight and torpedo. They seemed to fail entirely to consider the many greater and almost certain sources of error in the firing solution.
Even when in place, the tangent bar was regarded as a distasteful accoutrement which was often the source of director user error. In 1911, the Royal Navy was opting to retrofit armored sighting positions at the gunwale immediately over the broadside tubes of some battlecruisers. While this change may have in part been inspired by a wish to free up space within the conning tower, it also would eliminate the need for a tangent bar.
Those sighting hoods never appeared, however. Rather, the bars were finally being seen with a clearer vision as correcting an error that was comparatively small.
In 1916, the decision of five years earlier to do away with Fore Bridge Firing Gear in deference to placing an offset between torpedo and sight was reversed under the rationale that "close-range attacks [previously] in vogue" had warranted the change but that now the error is likely small at any range when compared to errors in estimating the other parameters of the firing solution.
- Annual Report of the Torpedo School, 1912. pp. xii-xiv.
- Annual Report of the Torpedo School, 1898. p. 58.
- See the Lion class.
- Annual Report of the Torpedo School, 1914. p. 38.
- Annual Report of the Torpedo School, 1916. p. 31.
- Admiralty, Gunnery Branch (1917). Handbook of Torpedo Control, 1916. C.B. 302. Copy No. 141 at The National Archives. ADM 186/381.