Tuesday, March 11, 2008

Factors Affecting DR Positions

A DR track is based on an assumption of making good an exact course and speed. There are many factors prevailing against the ship to prevent this. Some of these factors are poor steering and the inability to make good the plotted speed, current, and leeway.

Terms that must be understood in regards to dead reckoning include:

This is the horizontal motion of water. The direction in which the water is moving is called the set and the velocity of the flow is called the drift.

This is 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
This is 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.

This is the direction toward which the current is flowing. If the broader definition of "current" is used, the resultant direction is of all offsetting influences. Note that the description of the set of a current is directly opposite from the naming of a wind, a westerly current sets toward the west, a westerly wind blows from the west.

This is the speed of a current (or the speed of the resultant of all offsetting influences), usually stated in knots. However, some publications, notably pilot charts and atlases, express drift as nautical miles per day.

Course Made Good
CMG is the resultant direction from a given point of departure to a subsequent position. It is the direction of the net movement from one point to another, disregarding any intermediate course changes en route. This will differ from the track if the correct allowance for current was not made.

Speed Made Good
SMG is the net speed based on distance and time of passage directly from one point to another, disregarding any intermediate speed change. SMG is speed along the CMG.

Course Over The Ground
COG is the actual path of the vessel with respect to the earth. This may differ from CMG if there are intermediate course changes, steering inaccuracies, varying offsetting influences, and so forth. In current sailing triangles, CMG (not COG) is used.

Speed Over The Ground
SOG is the ship’s actual speed with respect to the earth along the COG. In current sailing, SMG (not SOG) is used.