The Is-Was 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 an improved version of his earlier Nasmith Director.
The Is-Was improved upon the original Nasmith Director by having the compass card connected to a Sperry repeater so that it would be automatically updated. The enemy bar was also here graduated in knots. A circular deflection ring, graduated radially in tracks and circumferentially in speed was mounted around the repeater the Is-Was was fitted to and was kept set in the direction the torpedo was to be fired.
Setting the enemy speed on the enemy bar caused the pointer to indicate on the deflection ring the proper deflection in degrees. This method eliminated the need to mentally calculate bearings, and the gyro repeater permitted it to be easily used while turning. The periscope need only be kept trained on the indicated deflection angle and the torpedo could be fired when the periscope came onto the target.
A bearing ring would continually indicate where the target could be found if the periscope had to be dipped, no matter how the submarine may have manoeuvred in the interim.
The device also showed the courses the submarine need steer to go perpendicular to the bearing and on the course of the enemy, as well as indicating the Track Angle on which the approach was being made. This allowed quick adjustments to be determined when a target was zig-zagging.
- The Technical History and Index, Vol. 3, Part 21. pp. 20-21.
- 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.