The Nasmith Director was a British torpedo attack calculator developed by Captain Nasmith for use in helping submarine commanders envision the proper intercept course to steer to achieve a possible shot. It was supplanted by his later device called the "Is-Was".
Despite its name, it was not a Torpedo Director.
The Director consisted of a compass card mounted in a fixed azimuth ring, a bearing pointer, and an enemy bar carrying a model ship. The bearing pointer and enemy bar are mounted atop the compass card and so can rotate around with it while still being set independently. The enemy bar also carried a card marked in degrees from 180 degrees from ahead.
In use, the operator would obtain a representation of the attack by setting:
- the submarine's course on the compass card
- the enemy bar to enemy's course on the compass card
- the bearing pointer to the periscope angle against the azimuth ring
He could read the angle on the enemy bow by noting the difference between the enemy bar and the bearing pointer. A graduated card on the enemy bar likewise gave the course for any given track-angle.
- The Technical History and Index, Vol. 3, Part 21. p. 20.
- Admiralty, Technical History Section (1921). The Technical History and Index: Submarine Administration, Training and Construction. Vol. 3, Part 21. C.B. 1515 (21) now O.U. 6171/13. At The National Archives, Kew, United Kingdom.