Marine Glossary

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layline

Usually applied to sailing upwind, these are the two imaginary lines on a chart emanating from your destination or the next mark of course, one on each side of the true wind direction, that mark the optimum final track line to that point. Optimum here meaning fastest way to get there in the present conditions, which includes everything, wind, current, sails, helm etc. Normally we learn this by recording our track on both tacks.

When sailing up wind we ideally have these in mind at all times, or even plotted on the chart. Ideally we make our last tack right onto one of these lines. If we tack too early, then we will have to tack again to get there; if we tack too late ("over stand the mark") then we have to fall off the wind to get there which will typically be slower.

Choosing the best time to reach the layline is a crucial navigation decision. If we get there too early, ie right on the layline at the moment, but rather far from the mark, then we risk that the layline will change, either due to a wind shift or change in current. But if we take extra tacks before the final tack onto the layline, then we lose a bit of time at each tack.

Tactical sailing navigation programs compute and draw in dynamic laylines at the position of the vessel and at the position of the mark to assist the navigator. These are based on the boat's polar diagram and the forecasted wind loaded into the program in grib format.