Graduated Sight Bar
A Graduated Sight Bar was a sight bar for a Torpedo Director with markings on it which permitted the user to answer the question, "Is the enemy presently within range of my torpedo?"
Owing to the principle of scale that underlay their design, the British torpedo directors were largely indifferent in their use to the range of the enemy. However, it is naturally important to know whether the torpedo will have enough endurance to meet the enemy before firing if one is to avoid wasting torpedoes.
The first British solution to this problem, suggested in 1903 by Lieutenants (T) Frank Brandt and Arthur E. Silvertop, was to add markings to the sight bar of the director on the same scale as the speed markings on the torpedo bar and enemy bar. Its geometry was as follows:
Torpedo set for 1,000 yards having a speed of 30 knots; enemy's course and speed as shown on (plate to right).
According to the principle of the director: —
- AD/AE = AB/AC
But AC is 30 divisions and AB as shown is 40:—
- AD = 40/30 * 1,000 = 1,333 yards.
Thus, the torpedo could be fired if the enemy was within 1,300 yards and the torpedo running 1,000 yards would still reach her.
The nice thing about this approach is that the sight bar markings are entirely independent of the specifications of the torpedo being considered, but the downside is that the operation requires mathematical calculation to perform... a definite drawback.
By 1909, this realization prompted the Royal Navy to decide that it would be better to create custom-marked sight bar scales that would absolve the user to do any calculation, although the switch would require the markings to be tied to the torpedo settings in use. These custom scalings were dubbed Possible Shot Scales.
- Annual Report of the Torpedo School, 1903. Plate 5.
- Annual Report of the Torpedo School, 1903. pp. 47-48.
- Annual Report of the Torpedo School, 1908. pp. 412-413.
- Annual Report of the Torpedo School, 1909. p. 23.