SOG is the ship’s actual speed with respect to the earth along the COG. In current sailing, SMG (not SOG) is used.
In navigation, "current" is to include all factors that introduce geographical error in dead reckoning. When a fix is obtained, one assumes that the current has set from the DR position at the same time as the fix and the drift equals the distance in miles between these two positions divided by the hours since the last fix. This is true, regardless of the number of changes of course and speed since the last fix.
If set and drift can be estimated, a better position is obtained by applying the correction to the DR position. This is referred to as an estimated position. If a current is setting in the same direction as the course of the ship or its reciprocal, the course made good is the same, only the speed changes. If course and set are in the same direction, the speeds are added. If in opposite directions, the smaller speed is subtracted from the larger.
For ships crossing a current, three current vector diagrams can be made to give the information needed to determine speed and courses to be steered. These diagrams can be made on scrap paper or an area on the plotting sheet.
Find course and speed made good through a current with ship’s speed 10 knots, course 080° , current set 140° , and drift 2 knots.
Solution: From point A draw the line AB. This represents the course and speed (080° at 10 knots) in length.
From B draw in BC, the set and drift of the current, 140° at 2 knots. The direction and length of AC are the estimated course made good (089° ) and speed made good (11.2 knots ).
For this types of problems I like to use a Radar Transfer Plotting Sheet # 5089 or you can use a Maneuvering Board Sheet # 5090.
