Nav ToolboxJan 1, 2003
The problem I have with calculating set and drift when crossing a current is not in the layout, nor in finding the correct heading. The mechanics of the problem are simple enough if it is laid out graphically. However, when I do come upon a situation that requires a new heading due to a crosscurrent, I find that I am unable to leave the wheel long enough to go below and work out the new heading in the classic, graphic way on a piece of graph paper or plotting sheet.
On a number of occasions, I have found that when sailing some distance offshore and crossing a bay, the incoming tide has taken me off course. The change is not perceptible, for all is moving in the same direction, and the first sign that something is amiss is when my boat is a lot closer to shore than I would like it to be. The last time I came upon this situation, I was single-handing, bringing my boat back from winter storage in Casco Bay to Sheepscot Bay in Maine.
Somewhere between the mouth of the Kennebec River and Seguin Island, a dense fog bank rolled in. I have the buoys in this area entered as waypoints in my GPS, and this late afternoon I was hitting the nuns and bouys right on the nose. All was fine until I came to the narrow part of the bay, where the bay turns to river. At this point the markers were not in the storage bank of my GPS, for I never imagined that fog could be this thick.
The problem was this: I had to go from one side of the river at nun No. 10 to the other side and find the green marker No. 1. This green marker is at the entrance to a channel that takes me to another bay and my mooring. Due to the short distance, I did not believe that the current would take me off course enough to require a correction in my heading. I found the green marker. It was fastened to a pole, on top of which a couple of ospreys nested comfortably. From this landmark, I was able to start my move into the channel. Something did not feel right, however. When the top of an island came into view, I knew wasn't where I was supposed to be.
The Maine coast is famous for two things: rocks and fog, neither of which does my beloved sloop care for. For a solution to this vexing problem of staying at the wheel and mentally correcting for set and drift, I have come up with the following rule of thumb:
Let's assume that you have a crosscurrent of 0.5 knots.
· If you're moving at 4 knots, head up into the current 7°
· If you're moving at 6 knots, head up into the current 4.6°
· If you're moving at 8 knots, head up into the current 3.5°
Generally speaking, these figures will get you to your mark, but a number of other items always enter into play. Wind, waves, boat displacement, full keel vs. fin keel, the amount of sail presented, and so forth. The variables are too numerous to take into consideration. So your best guess, based on knowledge of your boat and the distance you must travel, will get you in the neighborhood of your mark. On the other hand, if you have a long distance to travel and you are not close in, by all means go below and plot your course. Don't guess at the course to steer if you can calculate it.
Let's consider another set of figures. Let's say the crosscurrent is 1 full knot.
· If you're moving at 4 knots, head up into the current 14°
· If you're moving at 6 knots, head up into the current 9.3°
· If you're moving at 8 knots, head up into the current 7°
If you have a calculator in your pocket, plug in the current speed and divide it by your boat speed, multiply this number by 56°. The resulting number will give the amount of degrees you should head up into the current (see figure 1). If you have the time to make a layout but have forgotten how, here is a reminder. Lay out a triangle with 6,000 feet (one nautical mile rounded off) on side A of the triangle. Here I use an engineer's scale marked off in tenths. Let's assume you are moving at 4 knots, so it will take you and your boat 15 minutes to cover one nautical mile. The current is pushing you a half-knot at right angles to your course or 3,000 feet in one hour. Divide the 3,000 feet by one-fourth (the amount of time to cover the nautical mile) and you have 750 feet. Lay 750 feet on side B of your triangle and connect the C side. Measure the angle with a protractor and you have the angle of 7° (figure 2).
The advantage to laying out your course is that you are able to draw in the direction of the current, which is not always at right angles to your course. Using the calculator is quick but only gives you the answer if the current is at right angles to your course, so if you have a long course to cover and the set is not at right angles to your course then a graphical layout is preferable. To double check side B of your triangle, use the ratio formula in figure 3. With the above set of numbers posted near your wheel, you can adjust your best guess depending upon the direction of the set, since these figures are based on the set being at right angles to your bearing. Bearing is the course you want to make good, and heading is the course you are steering. When the set is not at right angles to your heading, then decrease these numbers slightly. The greater the angle of the set off 90° to your course, the less the adjustment to your heading. Information on the current may be taken from current charts or tables, or it may be determined by observing the current as you pass a lobster buoy. Observing the water as it passes a buoy will give you an idea of the speed of the current, which is the drift, and it's direction or set. Have a look at the wave created by the buoy you are passing and check it against the tide or current tables. This will help you estimate current speed when you are unable to leave the wheel.
Test your current skills in the light of day. Find a crosscurrent, judge it's speed and take a new heading mentally. Try using these numbers to see how close you come to your mark. You can also use your pocket calculator. Remember that these numbers are based on the current being at right angles to your heading. In your boat's logbook (every sailor should keep as detailed a log as possible), make a note of your findings when you are not hindered by fog so thick that you can cut it with a knife.
There are obviously many occasions when you are going to want as precise a solution as possible. At those times, it makes sense to do a careful graphical solution.
In my case, I did find the entrance into the channel for, as luck would have it, the fog lifted as soon as I came off the river. I have plugged these "close together" marks into my GPS, for I have learned the hard way that the fog around these parts can be as thick as pea soup.
Theodore L. Davis sails a 1976 Pearson 35 and lives in Bath, Maine.