60-mile rule

By some quirk of mathematics, a 1° course error delivers a vessel a mile to one side of a destination that is 60 miles distant. Two degrees sets her two miles off and 5° will result in five miles (5.25 to be strictly accurate). The system more or less hangs on all the way up to 15°, at which point a 16-mile sideways error is reported. Thereafter, you need a calculator to work out the result. It’s all too easy to steer three or four degrees consistently off course. An undiscovered magnetic deviation can do it at a stroke, so swing that compass and develop some technique other than a winch handle to the cranium for persuading the helmsman not to allow the boat to nudge up to windward on a reach. They all do it, don’t they!