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Programmed Traverse Light

SPOTTISWOOD and Richardson, in the January issue of the Journal (34, £3), described an optical aid to the speed control of ships approaching their berths. There is repeated reference to the device measuring the speed of the ship. In fact, it measures bearing from the light source, which, if the ship's track is well defined, can be converted into position. It is position error that their ingenious display system gives to the master or pilot. If, however, he applies an acceleration to the vessel in an attempt to keep it at the planned position, the effect will be a pendulum-like oscillation in speed. (To correct a position error the vessel must travel at the wrong speed and after the error is corrected the wrong speed will produce a new position error with the opposite sign.) To hold the desired speed, the vessel should be controlled to hold the position error constant, not to correct it. There is now the difficulty that the accumulation of speed errors may cause the vessel to drift out of the useful cover of the traverse light, and it would then be necessary to select a new and more appropriate traverse.