It is from the departure that the voyage is reckoned out.
In determining the course and position of a vessel at sea, allowance must be made for leeway and for the set of the tide. The leeway is greater when the sea is rough and when the sails are reefed. The amount of leeway can be roughly estimated by looking over a vessel’s stern at her wake, which will not be in the same line as her keel, but at an angle to it.
Having measured this angle, apply it to the left when the vessel is on the starboard tack, to the right when she is on the port tack.
If the strength and direction of a current are known, its effect upon the vessel’s course and distance made must be allowed for.
If the set of the current is in the same direction as the ship’s direction—either with her or opposed to her—her course is unaffected, but her rate of motion over the ground is increased or lessened by as many knots an hour as the current is flowing. The rate of current must therefore be added to or deducted from the distance logged. The log, of course, only indicates the vessel’s speed through the water, and does not register the current.
If the current is across a vessel’s direction, it will influence both her course and rate of sailing.
In order to find the course that should be steered so as to make good a required course in a cross current we proceed as follows. In either of the two Figs. 51 and 52, let A be the position of the vessel, B the port we desire to make, and let the arrow represent the direction of the current. With the dividers take from the scale at the side of the chart the number of miles the current runs per hour, and lay down this distance A C in the direction of the arrow. Then take from the scale the number of miles the vessel is going per hour, and with this distance as radius, and C as the centre, describe a circle.
Fig. 51.