In navigation, in current sailing, the total of all the factors which cause a ship to depart from its intended course and DR are termed current. Among the factors included in the term are:
Ocean current
Tidal current
Wind current
Windage on the ship
Heavy seas
Inaccurate steering
Undetermined compass error Inaccurate determination of speed
Error in engine calibration Error in log calibration Excessively fouled bottom Unusual conditions of trim
It can be seen that current sailing has two meanings as used in marine navigation. First, it refers to the horizontal movement of water due to ocean currents, tidal currents, or wind currents. Second, in common use it refers to the combined effect of all the factors listed.
Some terms to be talk about are:
Track (TR): The intended horizontal direction of travel with respect to the earth, taking into consideration known or predicted offsetting effects such as current, wind, and seas.
Speed of Advance (SOA): The intended speed with respect to the earth, taking into consideration the effect of known or predicted current. SOA is also used to designate the average speed that must be made good to arrive at a destination at a specified time.
Set: The direction toward which the current is flowing.
Drift: The speed of a current usually stated in knots.
Course Made Good (CMG): The resultant direction from a given point of departure to a subsequent position.
Speed Made Good (SMG): The net speed based on distance and time of passage directly from one point to another.
Course Over the Ground (COG): The actual path of the vessel with respect to the earth.
Speed Over the Ground (SOG): The actual ship's speed with respect to the earth along the COG.
Three problems come up with currents of estimated set and drift:
1.To find the anticipated track and speed of advance of a vessel steaming an ordered course at a specified speed through an estimated current.
2.To find what course a ship steaming at a given speed through an estimated current should take to make good an intended track.
3.To find what course and speed must be ordered to steam through an estimated current to arrive at the destination on time.
If a current is setting the same direction as the course or its reciprocal the track is the same as the course through the water.Speed can be found by adding or subtracting depending on if your going in the same direction or opposite direction.
Another example is given the estimated set and drift of the current and the speed of your ship,what course should you steer to make good the intended track and what the SOA will be.
The last thing is given the set and drift of the current and the direction of the desired track and the SOA,what will your course and speed be. I like to use a maneuvering board when working these types of problems.
