The rule is to allow westerly deviation to the left to get the correct magnetic course, and easterly deviation to the right to get the correct magnetic course.
To find out the error of the compass in order to steer a true course, the _sum_ of the deviation and the variation when both are of the same name, and their _difference_ when they have different names, must be ascertained. For instance, deviation 20° West and variation 25° West, would give an error of compass 45° West, which should be applied to the left.
If the deviation was 20° East and the variation 10° West, the difference between them would be 10° East, which compass error should be applied to the right to steer a true course.
In order to find the compass course or course to steer, proceed as follows, the true course being North 40° East, the variation being 38° West and the deviation 18° East:
Variation, 38° W., being of contrary names, take their difference. Deviation, 18° E. ------ Correction, 20°, apply to the right, being westerly.
True course N.40° E. ------ Compass course N.60° E.
Another example is given where the variation and deviation are both easterly and the true course is S., 75° West.
Variation, 24° W., being of same name. Deviation, 16° W., add together. ------ Correction, 40°, apply to the left, being easterly.
True course, S. 75° W. ------ Compass course, S. 35° W.
A volume might be written on the mariner's compass. It is a fascinating study, but unfortunately my space is limited.
There is another correction to the compass that the amateur should have cognizance of. It is called leeway, and is, in untechnical language, the drift that the ship makes sideways through the water because of the force of the wind or the impulsive heave of the sea. Some craft, because of deficiency in the element of lateral resistance, such as in the case of a shallow, "skimming-dish" sort of a boat, with the centerboard hoisted up, will go to leeward like a crab. Others of a different type, such as the "plank-on-edge" variety, with a lead line attached, will hang on to windward in a wonderful manner. It requires, therefore, a certain amount of judgment as well as of knowledge in this particular section of nautical lore to be able to estimate with any degree of approximate certainty the leeway a vessel may happen to make. It should not be forgotten that build has much to do with this, and that trim and draught of water are also two powerful elements in this connection. For instance, a boat with outside lead and a centerboard in a strong breeze and a lumpy sea, so long as the wind permitted her to carry a commanding spread of sail, might make no appreciable leeway, but, on the contrary, might "eat up" into the wind. But given the same boat without the lead and without the adventitious aid that the centerboard affords, she would be compelled to dowse her muslin at the first puff, and as a purely physical consequence she would retain no hold on the water and would drift off to leeward like an irresponsible she-crab.