JP 3-50 National Search and Rescue Manual Vol I - US Navy

5. Other on scene observations can improve the accuracy of drift
estimates. Ships and stations near the incident can be asked
for recent wind and local current observations, but these
should also be used with caution.
515 Enclosed and Coastal Waters
A. Datums in enclosed and coastal waters are derived by adding
force vectors for the incident area. Typically, the applicable forces
are leeway and water current.
B. Leeway direction is considered as directly downwind. The
reciprocal of the average wind direction between the incident and datum
determines the direction of the leeway vector. Leeway speed is
calculated using the leeway graph or formulas in paragraph 514.A.
C. Two currents normally encountered in coastal environments are
tidal and wind-driven currents. However, other currents should be
included in calculations if their effect is significant.
1. After leeway, tidal currents cause the greatest drift for most
objects. Procedures for determining tidal current vectors are
discussed in Vol. II, Chapter 2, and the appropriate tidal
current manual.
2. Wind current, normally present where wind and a long enough
fetch generate the force, is difficult to quantify. The tidal
current manual for the East Coast of the United States has a
wind current table based on historical data. Most other areas
have no data. Whether to compute a wind current depends on
SMC local knowledge and the environmental parameters. Wind
current for enclosed and coastal areas, including water depths
less than 100 feet and distances closer than 20 miles from
shore, is not normally calculated because of variability and
short fetch distances. On some larger or deeper lakes, such
as the Great Lakes, wind current can be determined
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