Leeway

Leeway is the amount of drift motion to leeward of an object floating in the water caused by the component of the wind vector that is perpendicular to the object’s forward motion. The National Search and Rescue Supplement to the International Aeronautical and Maritime Search and Rescue Manual defines leeway as "the movement of a search object through water caused by winds blowing against exposed surfaces". However, the resultant total motion of an object is made up of the leeway drift and the movement of the upper layer of the ocean caused by the surface currents, tidal currents and ocean currents. Objects with a greater exposure to each element will experience more leeway drift and overall movement through the water than ones with less exposure.

A navigator or pilot on a vessel must adjust the ordered course to compensate for the leeway drift and more important set and drift, an all encompassing term for drift that includes the steering error of the vessel. Failure to make these adjustments during a voyage will yield poor navigational results.Bowditch's American Practical Navigator (1995) offers a comprehensive free guide to navigation principles.

An object can be classified as either an active object like a ship navigating through a waterway or a passive object like a liferaft, drifting debris, or a person in the water (PIW) (Figure 3). A passive object will experience the greatest leeway drift and it is this drift that is of utmost importance to those involved in search and rescue (SAR) upon inland waterways and open oceans.

Figure 1. Relationship between Relative Wind Direction (RWD) and Leeway Angle.

Figure 2. Relationship between the Leeway Speed and Angle and the Downwind and Crosswind Components of Leeway.

Figure 3. The 5.5-meter V-hull, open skiff, Tulmar 4-person life raft, Beaufort 20-person life raft, and 1-cubic meter wharf box.

The most important elements of search and rescue are accurately assessing the last known position of a search object and accurately predicting its future position given hindcast, current and forecasted environmental conditions. Because the search object is located within two dynamic boundary layers with high vertical shear in the velocity profiles of wind and current, Fitzgerald et al. (1993) proposed an operational definition of leeway that helped standardize atmospheric and oceanic reference levels:

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