Applying leeway

Applying leeway

In the worked example above, the heading of the DR position of our first leg was 243°M (237°T) for 8.2 miles without any application of leeway. If we had a leeway of 15° caused by a moderate southerly wind, it will drastically affect the DR position of the vessel (Fig 7.6)

Applying leeway
Fig 7.6 – The leeway that a vessel experiences can drastically alter its course of the ground.

The southerly wind would have pushed the vessel more northward by 15° and while the heading of the vessel would remain the same, the ground track would have changed to 237°T + 15° = 252°. If you plot the 252°T position line on the chart you will see that we have passed on the northern side of the dangerous rocks which would have been passed to port of the vessel Fig 7.7.

Effects of leeway
Fig 7.7 – Due to the effects of leeway the vessel has travelled to the north of the submerged rocks.

A good tip when plotting up positions on the chart and figuring out what effect leeway is having on the vessel is to draw in a wind arrow on the chart so that it is clear what way the wind has pushed the vessel and how you need to adjust the water track bearing to account for leeway.

For instance if we had a northerly wind we would need to subtract the angle of leeway from our water track bearing to get a better representation of our real ground track. If we mixed the leeway calculation up we could be up to 30° off course, which is unacceptable especially when sailing near dangerous rocks.