It is easiest to plan a route when currents are at 90º to the course, but the same basic rules apply when the currents are other than 90º. Two examples are given: one for a current with us, and another for a current against us.
Using the example below, our intended track (rum line) from the starting point to the finish point on the island is a heading of 180º. If there were no current, we could follow the rum line by maintaining a heading of 180º. But we know that a current will be flowing. Assuming an average current of 1.5 knots at 225º TN (True North) during our crossing, we first draw a vector 1.5 units long from A to B at a heading of 225º. Now we need a vector that represents our paddling speed. Since we estimate that we can paddle at an average speed of 3.0 knots, we swing an arc (with a ruler or compass) from point B with a radius of 3.0 units - representing our paddling speed - until it intersects with our rum line. This vector, B-C, gives us our "Adjusted Heading," which turns out to be 160º. This is the heading we must maintain throughout our crossing in order to ferry glide from the starting point to the finishing point along our intended track.
There's more to consider here than there would be for a current at 90º. We are being moved diagonally, to the side and forward, as we paddle. The current actually speeds our journey. If we combine our paddling at 3.0 knots at 160º with the current of 1.5 knots at 225º, the resulting vector A-C gives our speed and direction "over the ground." Measuring this vector A-C shows that our speed will actually be 3.8 knots. If the distance from start to finish is 6 miles, then we would arrive at our destination in an hour and 35 minutes. Had there been no current, the trip would have taken 2 hours.
The solution works the same way if the current is working against us. Say the current will average the same 1.5 knots at 300º. This would require a ferry angle of about 152º, and will reduce our effective speed to 2 knots. The six-mile crossing would take three hours.
Note that this is a simple example. If the current is projected to increase or decrease during the journey, we might want divide the trip into segments, and plan a separate ferry angle for each segment. For relatively short crossings, planning for a single average current works well enough, but for longer exposures, more careful planning is required. In the first example, a two-hour exposure over six miles might get by with a simple calculation. But the difference in current that extends the exposure by an hour should prompt a more thorough approach.
How close will this get us to the island? With all the inaccuracies built in, we are lucky to get within 5º to 10º. As stated on the previous page, plan your routes with short distances to known points and use transits to check your drift.
Did you notice that the headings in this example were labeled (TN) for True North? While it's usually easier working with True North on a chart, the process is the same regardless of whether you work things out using True North or Magnetic headings. Just be aware of which referent you're using, and don't mix them up!
Start learning by doing short crossings (under 3 miles) with slow currents (under 1.5 knots), where a mistake doesn't matter. Remember that current predictions are not highly accurate. As you approach the shore, currents will be slower or even ebbing. Also, currents will vary depending on the lunar cycle: spring and neap tides are different. Understand that the current speed changes continuously during the tide cycle, not just on the hour as the current charts show. The last part of the equation is the wind. You can't plan for the wind. While underway, you'll need to adjust your course to compensate for the wind. And as the wind changes, you'll need to readjust your course.