by Tsunami Ranger John Lull
Tidal currents form in inland marine waterways in response to the rise and fall of the tide; the water from the rising/falling ocean flows in and out of bays and estuaries. Stronger currents will form around the times of new and full moon when the gravitational pull of the sun and moon is working in tandem, creating higher and lower tides. The current is strongest in deeper channels between landmasses and at the mouths of estuaries. These currents are often strong enough to be a major factor for sea kayakers. Typical tidal current strength runs from 1 to 3 knots, but in some areas can run 5 to 7 knots or more, faster than most kayakers can paddle. Ideally, you want to go with the flow and plan your trip accordingly. But that’s not always possible. When paddling against the current the best strategy is to hug the shore very closely, where the current is usually weakest. If your route takes you across a channel, you’ll need to set a ferry angle if you don’t want to get set downstream. A ferry angle is the angle between your actual course across the current and your heading (where the boat is pointed). This article will cover how to calculate a ferry angle.
The most accurate and easiest way to determine a ferry angle is to use a range (see Navigation Part 1), but when a range is not available (in fog, a long crossing, etc.) you’ll need to calculate the ferry angle, based on your paddling speed vs current speed. Most paddlers in a sea kayak can maintain a paddling speed of 3 knots, so for now we’ll assume a 3 knot paddling speed. To determine current speed, you’ll need a tide table or tidal current chart (readily available for most areas with significant tidal current). The current chart will give you current speeds at given locations throughout the tidal cycle and the table will give you the currents on any given date. Take an average current for the channel (it will usually vary across the channel, as the current chart will show) during the time period you’ll be crossing.
Calculating the Ferry Angle
Once you know the current speed you can estimate your ferry angle using the following formula:
Ferry angle = current speed ÷ paddling speed x 60
Examples, using a paddling speed of 3 knots:
Current speed 1 knot: 1 ÷ 3 x 60 = 20⁰ ferry angle
Current speed 2 knots: 2 ÷ 3 x 60 = 40⁰ ferry angle
With current speed close to paddling speed, the formula is no longer accurate. So to maintain a course without being set downstream across a 3 knot current, you have to paddle faster than 3 knots.
Current speed 3 knots; paddling speed 4 knots:
3 ÷ 4 x 60 = 45⁰ ferry angle
These are estimates, assuming a beam current (current perpendicular to your desired course). A more accurate, and visual, way to calculate a ferry angle is to use a vector solution. Looking at the diagrams below, keep in mind a vector is a measure of force acting in a certain direction; it’s NOT a distance measurement. Pick any convenient scale (say 1 inch = 1 knot) and draw up a vector diagram as follows:
Step 1: Draw a line representing your course made good (CMG) across the current.
Step 2: Draw a current vector showing the current direction relative to the course, with vector arrowhead meeting the course line. Scale the vector in knots.
Step 3: Construct a paddling speed vector by setting dividers (or use a ruler) to your paddling speed in knots, using the same scale units as the current vector. Then place one end of the dividers at the base of the current vector and swing the other end to intersect the course line. Connect these two points, giving you your ferry angle and heading. If you don’t have dividers a ruler can be used.
Once you know the ferry angle (whether you estimate it with the formula or vector solution) you can adjust your heading on the water accordingly. In the vector example above, if your course is 045 (magnetic north) your heading will be 085 (add the 40⁰ ferry angle because the current is coming from your right; if the current was from your left, you’d subtract). Follow a compass heading of 085 and maintain a steady paddling speed of 3 knots and you will stay on course. Your actual course across the current will be 045.
The ferry angle only works if you maintain a relatively steady paddling speed. If you slow down or stop paddling, you’ll be set downstream. If you need to stop and hold your position, turn directly into the current and paddle at roughly the same speed as the current. Also, on a sidenote, if the current varies across the channel, you’ll be using an average current speed to determine your ferry angle. So your course will be slightly zigzag; the ferry angle will be too large in the area of weaker current, and not large enough in the stronger current. But that’s all right because it will average out to the same destination as the straight-line course.
So far we’ve assumed the current is directly on your beam (at 90⁰ to your course). This is often the case when going directly across a channel, but if your destination is at an angle across the current, simply construct the vector diagram with the current vector at the appropriate angle. Ideally you’ll have the current on your stern quarter (coming at you at an angle from behind) because then it will be helping you along and your speed made good will be greater than your paddling speed. If the current is on your bow quarter, it will be impeding your progress and your speed made good will be much slower than your paddling speed. Vector solutions will show this clearly, if you plot them with the current vector at different angles to your course line.
Obviously if you can maintain a faster paddling speed (say 4 knots), you can cross more quickly and efficiently. Paddling at 4 knots, you can also hold a ferry angle easily when crossing a 3 knot current, where a 3 knot paddling speed wouldn’t be sufficient to maintain your course. Try plotting the vector diagram using a 4 knot paddling speed vs 3 knot paddling speed, measure ‘speed made good’ along the course line, and this will be apparent.
The above might sound like a lot of theoretical mathematics, but it’s really pretty simple in practice. You can even draw a rough vector diagram in the sand, using a paddle or a stick for scale and your compass to determine the angles (or just estimate the angles). That will be well within the margin of error. And once you know you need a ferry angle of 40⁰ to cross a 2 knot current when paddling 3 knots, you never have to calculate it again (until you forget and want to check to be sure).
Finally remember that current is your friend (usually) because you can use it to your advantage. Not to mention the fun of playing in tide rips! But you definitely want to be aware of current because it is possible to paddle on a ‘treadmill’ if you’re fighting the current and not aware of it. For a lot more information on paddling in current (warning: here comes shameless self-promotion), you can check out my book, Sea Kayaking Safety & Rescue, in which I wrote an entire chapter on dealing with tidal currents.
Please feel free to comment or ask questions.