A vessel’s safety may depend upon her compass. Cruising under conditions of poor visibility, the small craft pilot may have no other means of keeping to his desired track and, crossing a body of open water, no other means of making a good landfall. Running out of time on given course in thick weather and neither seeing nor hearing the expected aid to navigation is not conductive to peace of mind. There is little comfort in a chain of soundings that does not match what the chart shows in the expected vicinity. Stranding because of unexpected contact with rock, shoal or any bottom is unnerving.
The are ways of ascertaining and using variation and deviation, compensating the reduction of deviation to a minimum.
SELECTING A COMPASS
Any new compass looks good in the store, or aboard in the quite motion of the mooring or the marina slip. Its behavior under way, when the sea makes up and the little ship rolls, pitches and yaws, is of supreme importance. Will the card stick at some angle of heel? Will its apparent motion be jerky or smooth and easy? Are the card graduation legible and different heading easily distinguished? Is the instrument to be subject to large temperature changes? What is under the glass appears a bubble which may distract the helmsman? Answer depend on the quality of the compass.
No aviator will knowingly accept a cheap parachute, no sailor a bargain in life-jackets or fire extinguishers. No boatman should settle for cheap compass. Select one adequate for your expected needs. Look at number of them before buying. Pick them up, tilt and turn them, simulating motions to which they would be subject afloat. The card should have a smooth and stable reaction, come to rest without oscillations about the lubber’s line. Reasonable tilting, comparable to the rolling and pitching of your boat, should not materially affect the reading. In fairness to the
compass, if it has internal compensators, they must be zeroed-in before making test.
Pay particular attention to the card. Its graduation should be suited to the intended use. That a large craft may be held more easily on the course than a small one axiomatic.
Except under ideal conditions a small boat cannot long be held on the course with single degree accuracy. The varying effects of wind, sea and indeed of trim brought about by the movement of a person aboard, swing her off the desired heading.