Leeway

A Nicholson 32 , a Snapdragon and a Centaur. The Nicholson can sail closer to the wind than either the Centaur or the Snapdragon. When close hauled, the Snapdragon will make more leeway than the others. These characteristics are due to the profile of the boat below the waterline.. Given your choice of  keel, the leeway will affect your course over the ground:

Five degrees is a reasonable estimate of the leeway which my boat makes. At least I went out onto Cardiff Bay which is non tidal and compared my compass heading against my GPS heading. When running before the wind I found that these two headings were the same, no leeway,so far as I could tell. I then sailed onto port and starboard tacks:

I concede that I have not got around to making a deviation card for my compass - although it does benefit from an external mount well away from magnetic fields. It is interesting to see that on the port tack, compass and gps headings are similar whilst on the starboard tack they differ by about ten degrees. This happens because the leeway and magnetic variation are about the same size, by coincidence (Cardiff, 1994). Whilst Cardiff Bay is not tidal anyone sailing at slack water at sea should be able to reproduce these results. ( UK!)

There are two distinct techniques by which the navigator deals with leeway:

ESTIMATED POSITION : in the first case, leeway is allowed to happen; the leeway angle is used to adjust the true boat heading and an estimated position is plotted onto the chart.

COURSE TO STEER: leeway can be prevented by turning the boat into the wind by an amount equal to the leeway angle. You will need to measure the true bearing of the ground track which is required; construct (or calculate) a heading such that the tide will not take you away from this ground track; and finally adjust the true CTS by pointing upwind by an amount equal to the leeway angle.

CTS is preparation for a future event, a particular ground track for the future. Estimated position is a summary of past events, leeway has happened so you accept it in plotting the EP onto the chart.

Sheila Brown made the following diagram which demonstrates how leeway affects an estimate of position and a calculation of a course to steer:

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