Thursday, March 26, 2009

Piloting and Currents

One of the problems in small boat piloting has to do with currents, and how they effect your boats speed and the courses that you steer make good a desired track, and the time required to reach a destination. Sometimes this is called current sailing. As your boat is moved and steered through the water, it moves with respect to it. At the same time the water might be moving with respect to the bottom and the shore beause of the current. The direction and speed of your boat is the effect of these two motions combined. The actual course you make good over the bottom will not be the same as your DR track, in terms of course or speed.

Tidal currents are important and should not be underestimated. Unexpected current is always a threat to a vessel because it can carry your vessel off course and into dangerous water. The risk is even more with slower boat speeds and conditions of low visibility.

Leeway is the leeward (away from the wind) motion of a vessel due to the wind. While sailboats are most affected by it, larger vessel's are not immune to its action. The wind's effect need not be considered separately from current, but the two may be lumped together, with such factors as wave action on the boat, and the total off setting influence termed "current."

Definitions of Current Sailing Terms
The terms "Course" and "Speed" are used in DR plots for the motion of the boat through the water without regard to current. The intended track is the expected path a vessel, as plotted on a chart, after consideration has been given to the effect of current.

Track is the direction (True) of the intended track line.

Speed of Advance (SOA), is the intended rate of travel along the intended track line. The intended will not always be your actual track.

Course over Ground (COG), is the direction of the actual path of your boat, the track made good is sometimes called "Course made good."

Speed over Ground (SOG), is your actual speed of travel along this track, this is sometimes called "Speed made good."

Currents have two basic situations:
1. When the set of the current is in the same direction as the boats motion, or if it's in exactly the opposite direction.
2. When the direction of the current is at an angle to the boats course, either right or an oblique angle. The first is the simplest and is easy to solve. The speed of the current (Drift) is added or subtracted from the speed through the water to get the speed over ground. When the boat's motion and the set of the current form an angle with each other, the solution is not difficult. Their are several methods that you can use, one of which is a current diagram.

Basically, a current diagram represents the two component motions separately, as if they occurred independently and which, of course, they do not. These diagrams can be drawn in terms of velocities or distances. Distances are easier and usually used. If distances are plotted, be sure to use the same period of time for each component motion, one hour is commonly used since the units of distance will then be the same numerically as the units of speed.

The accuracy which the course and speed can be found depends on the accuracy with which the current has been determined. Values of the current usually must be taken from tidal current tables or charts, or estimated by the operator from visual observations.

Current diagrams are also called "vector triangles of velocity". The term "vector" in mathematics means quantity that has both magnitude and direction. In current sailing, the directed quantities are the motions of your boat and the water (the current).

A vector can be represented graphically by an arrow or a straight line with an indicating the direction, and the length of the line scaled to the speed of your boat. When two motions are not in line with each other they form two sides of a triangle. Completing the triangle will give you the third side which will be the course or speed of your boat.

If your set and drift can be estimated, a better position is found by applying the correction to the DR position. This is called an estimated position. If a current is setting in the same direction as your course 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 boats crossing a current, three current vector diagrams can be made giving the information needed to determine speed and courses to be steered. These diagrams can be made on scrap paper or on a plotting.

Example 1: Find your course and speed made good through a current with your boats speed at 10 knots, course 080°, current set 140°, and drift 2 knots.
Step 1: From point A draw the line AB. This is your boats course and speed (080° at 10 knots) in length.
Step 2: 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).

Example 2: Find the course to steer at a given speed to make good a desired course, your boats speed is 12 knots, the desired course 095°, the current is 170°, and the drift 2.5 knots.
Step 1: From point A draw in your course line AB in the direction of 095° (indefinite length).
Step 2: From point A draw in the current line AC for the set 170° and drift 2.5 knots. Using C as a center, take your dividers, swing an arc of radius (boats speed 12 knots) CD, intersecting the line AB at D. Measure the direction of line CD (083.5°). This is your course to steer. Measure the length of the line AD, 12.4 knots is your speed made good.

Example 3: Determine what course and speed you must do in order to make a desired course and a desired speed good. Desired course 265°, desired speed to be made good 15 knots, current set of 185° , and a drift of 3 knots.
Step 1: From A draw line AB in the direction to be made good (265° ) and for a length equal to the speed to be made good (15 knots).
Step 2: From A draw AC, the set and drift of the current 185° and 3 knots.
Step 3: Draw a line from C to B. The direction of this line is 276°, this is your course to be steered. The length of the line equals the speed required (14.8 knots).

These current vectors can be made to any convenient scale and at any convenient place such as the center of the compass rose, unused area of the plotting sheet, a separate sheet of paper, or directly on the plot. Leeway is the leeward motion of a vessel due to wind. It can be expressed as distance, speed, or angular difference between the course steered and the course made good through the water. The amount of leeway depends on the speed and relative direction of the wind, type of vessel, exposed freeboard, trim, state of the sea, and depth of water. Leeway is applied by adding its effect to that of the current and other elements introducing geographical error in the dead reckoning.