If it wasn't for currents, kayak navigation would be easy. Figure A shows the planned route, the current direction and speed, and the paddling distance. Figure B shows what would happen if we didn't plan for the current. In this scenario, the current carried us 2.6 miles to one side of the light house on a route that is just six miles long. This could be more than a slight inconvenience. To learn about dealing with the currents we will start with an easy example and then move on to more complex examples. Note that none of the following methods is exact. If the current were at exactly 90º and didn't change speed, figure C would be exact. In reality, the current is constantly changing speed, is almost never at exactly 90º, and we can't keep a kayak traveling on a perfect heading. Kayak navigation is all approximate. If you can keep within 5º your accuracy is excellent, but precision within 10º is more realistic. The longer your crossings, the larger the error will be. This is why we don't paddle the rum line (a straight line between starting and ending points). Instead, our routes consist of sets of shorter distances to fixed points i.e. buoys, islands, light houses. Each time we reach one of these fixed points, we can re-establish our known position and start on our next leg of the trip. The shortest route is not necessarily the best route. Plotting your course is an important part of accurate navigation. You can further improve your precision by using transits and taking fixes along the way.
Be aware of the effects of tide and current on immovable objects such as retaining walls. Large reflecting waves form when waves hit solid vertical objects like cliffs and retaining walls. Currents can push your kayak into piers or other objects. Give a wide berth when rounding objects.
Click for Current Thirds Rule.
"A" shows the planned route, the current direction and speed, and the paddling distance. "B" shows where we would end up if we followed the plotted course without adjusting for the current. "C" has the course adjusted for the current.
Currents At 90º
Figures B and C show how to plan for the currents. The first leg of our planned route ends at the Middle Ground Lighthouse, which is a 6 mile paddle at 165º True. With an average paddling speed of 3 knots it should take 2 hours to paddle. In the example we are using, there is only one set of current speed numbers on the chart. In a real situation we would have to look up the current speed for both hours of the trip since the current would change over each hour of the tide cycle. In figure B, with only two hours of paddling, the current moved us 2.6 miles off course to the west (Y). We adjust for this by taking into account the current speed and direction, and an estimated paddling speed. Since this computation tells that we would drift 2.6 miles west of the lighthouse, we measure 2.6 miles to the east (parallel to the current) and plot a course to point Z (138º). While we are paddling our kayaks we will be pointed to a heading of 138º. At the same time, the current will be pushing us to the west, and the combined forces (the current and our heading) keep us on our rum line of 165º. This technique -- pointing the bow "upstream" to compensate for the force of a current, is called ferrying. In river kayaking we ferry from one side of the river to the other side by pointing upstream diagonally into the current. Sea kayakers use this same technique, but usually in a larger, slower moving body of water.
One of the easiest ways to cross a 90º current is to time the crossing so that the tide changes at the midway point (Y). Keeping the kayak pointed 180º we drift 1.5 mile to the east by the end of the first hour to point Y. The next hour of paddling brings us 1.5 miles to the west, and we arrive at point Z.