Thursday, October 2, 2008

Lesson 20: Set and Drift

Interestingly, the mere mention of the words Set and Drift strikes paralyzing fear in the hearts of real and armchair mariners. The reality is that set and drift problems are relatively easy to solve if one understands and correctly practices the basic concepts learned so far.

In simple terms, current, when running, usually acts on a vessel moving it off course. Therefore, as a vessel advances through the water, the vessel will be set off in a certain direction by a current moving in a particular direction. This is termed Set. The velocity in which the vessel is carried on the current is just called Drift (drifting along with the current). Hence, current is the horizontal movement of water and can be defined in terms of both direction and speed or just Set and Drift.

Remember !

When calculating Set and Drift problems in here (or out there), make dog-gone sure to determine SET in TRUE degrees. Always go to the compass rose with the angle that you get starting at your “missed” DR position and going TOWARDS the point or position your vessel actually ended up at, (ie, the 2nd fix). This is SET.

DRIFT speed is determined by measuring the distance in nautical miles between the same “missed” DR position and, again, the same position where you actually ended up at, (ie, the 2nd fix). Work this distance from nautical miles up to speed in knots by using the S = (D X 60) ÷ T formula.

Need to Know

1.The difference between the dead reckoning position and an accurate fix is the result of the action of various forces on a boat, plus any errors introduced when laying out course and speed.

2.Remember, a vessel can be moved from its intended course by the combination of set and drift, as well as steering error. The results are often disastrous of not corrected.

3.The direction in which the current is moving, or the direction a vessel is moved by current is called set. Direction of set is expressed in true degrees.

4.A vessel set off course by wind and current. The speed of the current and the speed in which a vessel is being set is called drift, and is expressed in knots.

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