By some quirk of mathematics, a one-degree 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 five degrees will result in five miles (5.25 to be strictly accurate). The system keeps going more or less up to fifteen degrees, at which point a 16-mile sideways error is reported. Thereafter, you need a calculator to work out the result. It’s easy to steer three or four degrees consistently off course. An undiscovered compass deviation can do it at a stroke. If it lands you four miles down-tide of a cross-Channel destination, you’ll be disappointed, so don’t forget to swing that compass, and remind the person on the helm to make more effort!