Kayak Navigation Part 3: Crossing Current; How to Calculate a Ferry Angle

by Tsunami Ranger John Lull

Introduction
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.

Resting in an eddy behind the bridge abutment, prior to crossing the Golden Gate Channel

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.

Discussion
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.

Share on Facebook

Comments

Fat Paddler says
December 10, 2012 at 4:40 am
For a quick and dirty way to see the effects of currents, get out in some river whitewater and pick your ferry angles on the fly, and quickly! Of course, the threat of big bad holes makes for a more exciting ferry glide, as does firing yourself out of an eddy and hoping you’ve got the angle right to get past that hole/strainer/rock/waterfall – haha!
Reply
PeterD says
December 10, 2012 at 9:58 am
Here’s a video I took showing a ferry angle, as part of a blog post I did specifically for people doing safety boating for the large Alcatraz swims:
http://youtu.be/vf0k7RfJJYI
In this case, the current is ebbing, so going from right to left. We need to get to Alcatraz before the boat with swimmers get there. Though we aren’t pointed to Alcatraz, we are going there.
I guess if John was looking for more tow write on, a follow up post on this could be using eddies to work your way against currents. There is currently a conversation on the BASK email list how to best go against a current in Raccoon Straights.
Reply
Scott Becklund says
December 10, 2012 at 6:45 pm
Ouch! John I always think of you as a geologist not a mathematician or….
Ok, now I not only have a head ache but known I need to work this out Don’t worry though I get even .
To Fat Paddler; I agree but this is also a great tool for changing our thought process when dealing with big water. I can’t tell you all the times I wish I had tide with current charts in places like Alaska or even SF bay with small motor or row boats.
Great article John. Cheers to all and happy holiday paddling.
Reply
Nancy Soares says
December 11, 2012 at 9:32 am
Great vid, Peter. That’s a really good example of heading vs destination. @ FP, I have never ferried on a river in a kayak, but I have done it many times swimming, caveman style, just observing the swiftness of the current, looking at where I want to end up, and then figuring how hard I have to swim to get there. It’s always interesting to see how accurate those calculations are. With practice, you can get pretty good.
Great article, John. I think Peter has a point. Maybe in the future you could do a post on the eddy thing. And hey, Scott, good to hear from you! Hope all is well with you and your little Tsunamis 🙂
Reply
John Lull says
December 11, 2012 at 10:33 am
Hey thanks for the quick comments. There’s a lot more to say about this subject and I’m glad to continue both with comments and another article.
Fat Paddler, yes indeed in a river you’ll use ferry gliding all the time. It’s pretty much the same thing as in tidal currents, but also points up one significant difference. In the river you get immediate feedback on what the current is doing because you can see the banks of the river for reference. No need to calculate anything.
In tidal currents, it’s the same if you’re close to shore, but on longer crossings or in fog, you won’t be as aware of the current. In fact, without some relatively close stationary point of reference or a handy range, you won’t ‘feel’ any current and can easily be fooled into thinking its having no effect on your progress. It’s a lot like the fact we are going a gazillion miles an hour on this earth in orbit of the sun, while also spinning around on the earth’s axis. But we feel none of that because we have no ready point of reference. Luckily tidal currents are predictable (at least on a general basis) and ranges, even distant ones, are extremely useful in determining what’s happening.
Reply
John Lull says
December 11, 2012 at 10:48 am
Peter, that’s an excellent video that illustrates the ferry angle very well. Thanks for posting that! I can say something in general about Racoon Strait (been there, done that, many times). It’s actually pretty easy to go against the current there. You simply hug the coast of Angel Is, using the eddies behind the points and paddling your ass off for the short distance around any points. By ‘hug the shore’, I mean so close you can reach out and touch the rocks (at least with your paddle). Obviously it’s better to go with the current, in which case you move right out into the channel and get a free ride.
Hey Scott, I’m not mathematician, lol. This vector thing is really easy to see and use, much simpler and more powerful than I can describe in an article. Get some sticks of different lengths to use as vectors (for both current and paddling speed), then play around with them to see what happens when you vary current speed, paddling speed, and the angle at which you are crossing the current. You’ll discover a lot doing that, without needing any math. Well, you might have to review what you learned in grammer school about using a protractor…
I’ve used this ferry angle calculation to cross the Bay between Crissy Field and Angel Is (3 mile crossing), not far in from the Golden Gate, and even though the current changes across that channel, using an average current speed, the ferry angle worked really well, on several ocassions. Sometimes you have to factor the wind in too, but I do that by ‘feel.’
Nancy, yeah we can talk about some future articles on current and whatever else.
Reply
Tony Moore says
December 13, 2012 at 4:30 pm
John, as always, GREAT treatment of a critical area of knowledge for sea kayakers. I myself am partial to the vector solution to determine ferry angle because I was familiar with vectors (through physics) before I started kayaking, and also, I am very visually oriented, and much prefer a “picture” over a formula. Vectors are perfect for this, having both magnitude and direction.
When actually out on the water, it really is a mix of science and art, (“art” I am defined as a sense of the situation through experience, a “sea sense” so to speak). This could be things like a feeling for the wind and how land masses might alter it, the effects of chop or wave fronts coming from whatever direction, etc…all these things can affect your path on the water to your destination. Nothing but time on the water in many different situations can develop this sense.
Happy Holidays to all, and get out there and paddle!
Tony M.
Reply
John Lull says
December 13, 2012 at 4:58 pm
Thanks Tony, and a very good point about ‘sea sense.’ That’s a really important part of the whole process. Once you figure out what the ferry angle SHOULD be, you still don’t totally ignore whatever happens on the water. If you’re in thick pea soup fog, you have nothing much else to go on, but even then you can look for ‘way points’, like a buoy or an island or something, that might lie along your course. But if you can see something and pick up a temporary range, or other indicator, you can check and refine your calculation if necessary.
Regarding the effects of chop or wave fronts, here’s a little tip that I discovered on a crossing from Angel Is to Alcatraz on an ebb tide. On that crossing, there is a point where the current changes from a side current to a ‘head’ current (right in your face) and without a range or other indicator you’ll end up on a treadmill going nowhere without realizing it. The first time this happened to me, the first red flag was the fact I was surfing small waves, but there was no wind to account for those waves. Since the steep face of a wave caught in the current faces upstream, if you find yourself surfing, you are probably headed directly into the current. That’s fine if you want to play in the tide rip (another good topic for an article), but if you don’t want to fight against the current, it’s not a good sign.
The next thing I did was look for a range. I found several and detected no movement in any direction! Now it was clear what was happening, so I changed course until one of the ranges showed me I was moving. Better to go somewhere than nowhere!
Reply
Nancy Soares says
December 14, 2012 at 8:23 pm
Hey this is great! Thanks to this post, we now have subjects for 2 new articles. Yay! I’ve been scanning the archives for other requests as well, and I think we’ll have some good offerings for 2013.
Thanks to everyone who is reading and/or commenting. It means a lot to all of us. Happy Holidays to everyone! We’ll be taking a short break and will return in January with more Tsunamilosophy (we can thank the Ocean Dragons for coining that term!) Love to all!
Reply
Nancy Soares says
December 14, 2012 at 8:38 pm
Hmmm – not being notified of comments – messing around with WP – don’t mind me!
Reply