HB's Home Page

I like having guests on our boat. Most people enjoy the experience of steering, so very often there are novice sailors at GYPSY's helm.

When people take the wheel for the first time there is usually a lot of weaving back and forth until they get the feel of how the boat responds to the helm and how to compensate for the delay in the compass reading. One can always tell an experienced helmsperson because he will only make sparing adjustments to keep the boat on course.

This raises the question of how close to the course one ought to steer. How far off the prescribed course do I let the boat drift before making corrections? If I average 5 or 10 degrees off course, how will this affect my arrival time?

Let us suppose I zig-zag back and forth between 5 degrees port and starboard off the straight line between the startpoint and the destination on a 10 nm traverse:

Referring to the diagram, the question becomes "How much longer does it take to cover the distance A - B - C - D - E -F than the distance AF?

It turns out that the longer distance is 10.038 nm. At GYPSY's cruising speed of of 5 kt the direct course would take 2 hours, while the zig-zag course would take 2 minutes and 20 seconds, or (0.4%) longer. If the angle is increased to 10 degrees, the increased time taken would be 9 minutes 15 seconds, or 1.5%. These increased times really aren't anything to lose sleep about. Increasing the number of zigzags does not affect the results.

When one crosses Georgia Strait there is always either an outgoing or an incoming tide that causes the boat to drift sideways off ones heading. Choosing the optimal heading to steer the boat now becomes a challenge which I'll discuss next.

But first let's review some terminology.

Heading is what the compass says; the direction in which the boat is pointing. Track is provided by the GPS. It is the direction in which the boat is actually moving over ground. When there is no sideways current these numbers are the same. Changing the heading (intentionally or for lack of vigilance) will normally lead to a change in track. While crossing Georgia Strait you will likely make several changes in heading.

Course is the direction from my startpoint to the destination. The course value is traditionally found by using parallel rulers on my chart. The course does not change during a crossing. By contrast, bearing is the direction to the destination from where you are now. This number is very conveniently supplied by the GPS, and under normal circumstances bearing will vary during a crossing because of uncompensated-for drift. Conversely, if you compensate for drift perfectly, the bearing remains constant at the course value.

So, we have four different terms to keep straight. Here is a diagram to illustrate:

Before continuing it would probably be worthwhile to have a quick look at something called vector diagrams. In this illustration we assume that my course is due east (90 degrees), my boat travels at 5 knots through the water and there is a 2 knot current moving towards NNE (30 degrees).

The vector diagram consists of arrows the length and direction of which are proportional to the speeds and directions of the current and the boat through still water. Two parallel lines are drawn at the ends of the arrows and the arrow from the origin to the point where these lines meet is the vector representing the resultant track.

Now back to the original problem. I want to go to a point 10 nm due east of my present location. There is a 2 kt cross current from the south at right angles to my course. My boat does 5 kt it still water. Should I keep the compass heading at the course, the bearing or the track? Let's examine these one at a time.

Each dot represents the boat's position at 10 minute intervals. An illustrative vector diagram is shown at the third position. In this scenario we end up 4 nm north of where we want to be after 2 hours. To get from where we end up to where we want to go we will have to travel 4 nm against the current at (5kt - 2 kt = 3 kn) over ground requiring another hour and 20 minutes.

Next we examine what happens if the boat's heading is continually adjusted so as to correspond to the changing bearing.

Again w have shown where the boat will be at 10 minute intervals. An illustrative vector diagram is shown for the 9th position. It would take 2 hours and 25 minutes to follow this route.

In the final illustration we look at what happens if the heading is adjusted so that the track is the same as the bearing:

Notice that in order to keep the track the same as the bearing or course line, we have to steer somewhat into the current. This is quite intuitive, but difficult to do over long distances unless you have a GPS. In this case we arrive shortly after completing the 13th 10-minute period, or after 2 hours and 13 minutes.

In these illustrations we have explored only one set of values for boat speed, current speed and current direction. Obviously as these values change the results will be different.

If you are interested in exploring this subject further I'll be glad to let you have the Excel templates on which the calculations are based.