Set and drift

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The term “set and drift” is used to describe external forces that affect a boat and keep it from following an intended course. To understand and calculate set and drift, one needs to first understand currents. Ocean currents are the horizontal movements of water from one location to another. The movement of water is impacted by: meteorological effects, wind, temperature differences, gravity, and on occasion earthquakes. Set is referred to as the current's direction, expressed in true degrees. Drift is referred to as the current's speed, which is usually measured in knots.^[1] “Leeway” refers to the amount of sidewards translation of a vessel drifting off of or away from the intended course of travel (with no correction or compensation by altering the heading of the vessel such as pointing her into the wind.)

Ignoring set and drift can cause a mariner to get off their desired course, sometimes by hundreds of miles. A mariner needs to be able to steer the ship and compensate for the effects of set and drift upon their vessel while underway. The actual course a vessel travels is referred to as the course over the ground. The current of the ocean alters this course whether pushing it away from its desired course or in the same direction. The vessel's speed through the water is referred to as the boatspeed^[2] and the current can affect how fast or slow the vessel moves through the water.