The Gyroscopic Compass: A Non-Mathematical Treatment

by T. W. Chalmers

Fig. 29. Effect of Rolling on due North Course.

The gyro-compass on board ship is usually, although in no way necessarily, mounted above the vessel’s metacentres—transverse and longitudinal—about which rolling and pitching take place. In [Fig. 29] we show a section, looking aft, of a vessel steaming due north with a gyro-compass mounted on its deck. It is clear that, so far as the weight S is concerned, it may be regarded when the ship rolls as the bob of an inverted pendulum vibrating in an east and west plane, through an angle equal to the angle of the ship’s roll. From what we have already said, it will be seen that at the end of a roll to port the hand holding the string attached to the flying stone, hitherto following it, begins its withdrawal to the east. The weight checked in its movement to the west communicates a kick to the sensitive element. This kick, a westward force, is applied at the centre of gravity of the weight, and clearly does nothing more than throw a stress on to the journals at E F and H J, and through the square frame on to the deck. It does not therefore affect the direction in which the gyro-axle is pointing. If, as is actually the case in practice, the equivalent of the square frame is not fixed directly to the deck, but is mounted inside the binnacle on athwartship and longitudinal gimbals, the square frame will swing on the longitudinal gimbal axis. The weight S will, however, still act during the ship’s roll as an inverted pendulum, as at Z Z, in which the bob remains more or less parallel with its original direction. Under these conditions the kick of the weight at the end of a roll to port will rotate the frame in the direction of the curved arrow about the longitudinal gimbal axis. As this axis is coincident or parallel with the gyro-axle, the gyro-axle is not moved otherwise than parallel with itself either by the actual roll of the ship or by the kick of the weight at either out position. Thus a ship steaming north may roll with impunity without affecting the accuracy of the direction in which the gyro-axle is pointing.

Fig. 30. Effect of Rolling on due West Course.

It is clear that rolling on a due south course or pitching on a due east or west course is similarly without effect on the gyro-compass.

Now consider the case of the ship rolling when on a due west course ([Fig. 30]). It is clear that here again the weight S may by itself be considered as the bob of an inverted pendulum vibrating through the angle of the ship’s roll, this time in a north and south plane. The weight S has, in addition, its own pendulum action on the horizontal axis E F, and tends to keep the gyro-axle horizontal throughout the roll.

At the end of a roll to port the kick of the weight S, a southerly force acting at the centre of gravity of the weight, tends to turn the spinning wheel in the direction R about the horizontal east and west axis E F. This kick is clearly equivalent to the application of an upward force at the end C of the axle or a downward force at the end B, and therefore, as we know, will cause the axle not to turn about E F, but to precess the north end B towards the west about the axis H J.

The roll to starboard now takes place, and at its end the kick of the weight occurs in the northerly direction. This kick will tend to turn the spinning wheel about the east and west axis E F in the direction T, and is clearly equivalent to the application of an upward force at the end B of the axle. Such a force, as we know, will cause the end B of the axle to precess towards the east.

It will thus be seen that the western deflection of the axle produced by the precession which occurs at the end of the roll to port is counterbalanced and automatically eliminated by the eastern deflection produced by the precession occurring at the end of the roll to starboard. The only effect on the compass caused by the rolling of the ship is therefore a vibration of the axle about the north and south direction. We should really say that the only effect is the application to the sensitive element of a vibratory influence in tune with the rolling of the ship. As the ship’s period is very small compared with that of the compass about the axis H J—about 5 to 12 seconds as compared with about 85 minutes—this vibratory influence practically fails to disturb the steadiness with which the axle points to the north.