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helm bias

An error in DR that can sometimes be identified. It means the helmsman did not steer the intended course at all times and the off course errors tended to be in one direction by some average amount.

This potential error has been know to navigators since the very earliest days of any writing on the subject of DR, though not often addressed in modern textbooks. It can come about for various reasons, and will differ from one helmsman to the next, and for the same helmsman in different conditions.

Wind and waves are often aligned, but not always. When meeting waves headed into the wind there is a tendency to alter course to avoid pounding into the seas, or maybe under sail downwind one might alter course trying to "catch a wave" for better speed. Or it could be that some sailors can sail closer or farther from the wind than others can when the course is on the edge of good trim.

The only way the navigator can detect this in a reliable way for dead reckoning is to compare the present helm's track with the desired course on a plot of the track, and then noticing a deviation, stand for a while (quietly!) and watch the helm and heading long enough to spot a pattern that confirms the charted observations.

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