How To Calculate Course To Steer (Illustrated Guide)
Your GPS has a handy "Course to Steer" field on it. Just point the compass at that and go, and you'll get there, right? Well, not exactly. That's the compass heading to the next mark you've plotted, not the most efficient route to get there if you have current.
The GPS can not accommodate currents into that number, so you may take a circuitous route if you've not accounted for these forces yourself.
How do you calculate course to steer? You plot a course on a paper chart, then account for current set and drift to create an accurate course to steer. You will need to adjust your sailing on the water or autopilot use to handle this, and there are a few steps to take to make sure it works as you've plotted it.
Sound complex? It is...a little. Following your GPS may get you there, but you may sail miles out of the way to do it. So grab your straight edge and dividers and let's take a closer look.
The Shortest Distance May Look Like an "S"
I learned this lesson years ago on my first Gulf Stream crossing during an offshore sailing class. The Gulf Stream is a broad band of warm current running north along the U.S. East coast. It's position moves, but we can estimate its breadth and current, which runs up to six knots but averages around three between Florida and the Bahamas.
Traveling from the coast of Florida, if you point your boat at Grand Bahama on the rhumb line, you will miss the island as the current sweeps you north. However, if you point your boat enough south of it, the current will push you north enough to end where you want to if you adjust your course to suit on the end. It's easy to overshoot and end up too far south and have to sail more distance.
If you calculate it at just the right course south and hold it, the current will push you north, and maybe even north of the rhumb line. But your calculated course will take you to the mark you aimed for while sailing the same compass course. The GPS track of this will look like an "S" as the strong current in the middle of the stream takes you further north, and your angle changes less and less as the current drops on each end.
But you have sailed in a straight line course - the shortest course - while you traveled in an S-curve because of the current.
How to Calculate the CTS With Current
All we are doing to calculate the correct CTS is plotting a course without external forces, then adding the effect of those courses. Here the correction is for the set of the current - the direction it's flowing, and the drift - the speed of the flow.
For a simplified example, let’s plot a course from Point A to Point B eight miles across a body of water with a known current. The average current in is 2.5 knots running at 255 true, though it may be higher in the middle and lower at the edges, the average is what we need.
- Draw a line from A to be, and measure and note it’s length (8 miles) and direction.
- If your boat speed is eight knots, it will take you an hour to sail from A to B normally. This means you will be exposed to one hour of current - so the current will sweep you 2.5nm in that time.
- Staring at Point A, draw a line bearing 255 True with a length of 2.5nm. This is the current.
- Next, draw a line from the end of the current line to Point B.
- Measure the compass reading of this new line - this is your new Course to Steer.
If your data and assumptions are good (boat speed, current set and drift) then you will end up at Point B when you sail this course. Your Course Over Ground may look like an S-shaped curve, but you will have sailed the most optimal course.
What about when the numbers aren’t so neat?
It's a rare case when the numbers add up so nicely that your units are in easy multiples. So let’s assume the distance from A to B is 12 miles and keep the same 8 knot boat speed. Now it will take you an hour and a half to get there instead of an hour. How is this different?
There’s only one difference - in step 2, you will be in the current for 1.5 hours instead of one hour. To account for this, your current line now must be 2.5 knots x 1.5 hours = 3.75nm long instead of 2.5.
The rest of the process is just the same.
What Other Factors Could Add Errors?
Set and drift in areas of variable current - like bays or coastal waters - can introduce errors since the time and date you transit them will affect your CTS. Fixed currents like the Gulf Stream are easier to calculate; when you plot your CTS make sure you’re planning close to the right dates for tidal currents.
The other major source of error is leeway.
Leeway is the amount that a boat sideslips while under sail. Every boat, no matter how well designed, has some leeway, but it will vary somewhat with the boat and with the point of sail.
Because of the physics behind how a boat moves, the track of a boat through the water is always a little down wind from the compass heading you're sailing. The closer you sail to the wind, the more your boat will sideslip. This may be as much as a ten or fifteen degrees in some boats, though it's often less.
Low boat speed and high winds also increase leeway, as does excessive heeling. So in high winds, reefing your sails will reduce side slipping. Even the skill of the helmsman can contribute to leeway.
Leeway is difficult to estimate, and even harder to plot proactively on a chart since you don't know the wind direction you'll be sailing in until you're on the water. The best you can do is try to learn your boat's typical behavior and adjust your CTS on the fly for the conditions you are in.
Most leeway calculations are figured out once a course is under day and plotted. From the actual positions of the boat compared to the plotted course, you can figure the leeway.
Using Calculated CTS
With paper charting, using a CTS calculation adjustment was an automatic part of the plotting process. If you're expecting current, you'd put a way point at the edge of the expected current, then do the CTS calculation to the other edge of the current. After that you'd just figure the rest of the course around hazards and marks as usual.
You can do the same thing with your GPS. Plan your course right up to the edge of the current. Then do a CTS calculation to get a heading to steer through the current, and drop a mark on the other side of the current on that range and bearing. When you get to the current zone out on the water you will point to the correct amount up current to sail the CTS.
It is helpful to write your CTS course down in case where you arrive at the current edge isn't at the exact waypoint you plotted.
Working With an Autopilot
If your autopilot isn't integrated with your GPS, you will adjust it step by step to the course to steer as your GPS changes waypoints. You're all set if you put in the CTS calculation in the course, just follow the numbers.
If your instruments are like mine and your autopilot integrates with your chart plotter, you can just push a button and the autopilot will follow the course you plotted. And this does not work so well when you've plotted a current crossing.
If your autopilot is in "course" mode, it's trying to sail to the next waypoint. And as you know, as we start sailing that heading, we will be carving out that big S-curve. But your autopilot will keep pointing your boat at that mark no matter how much the current side slips you. It will not keep the same heading as it adjusts to keep the boat pointed to the mark.
Instead, for this specific leg, switch your autopilot to "compass" or "auto" mode, where you choose the explicit compass heading to steer. You need to dumb the autopilot down and manually set it to stay on a proper CTS to get that shortest crossing instead of letting it flounder trying to deal with current it doesn't know about.
The XTE Mode Error
Many autopilots have a Cross Track Error (or "CTE") mode of navigation. In normal course mode, most autopilots point at the next programmed mark, which works well enough without big currents.
XTE mode drives the boat to stay within a certain distance of the plotted course line - it keeps the cross track error within set parameters. It will adjust the helm so your boat does not exceed a maximum distance from the course you plotted, no matter what. It's very convenient in light current and leeway situations, and autopilot navigation where tight tolerance to the plot is critical, like near reefs, land and obstructions.
While this sounds good, when sailing across a current your boat will do odd things. As it sweeps you off course, your boat will turn up current to come back to the route. Your heading will change back and forth as you sideslip and come back, but your up current turns will become more and more severe until you are back in the tolerance zone.
While this isn't a big deal when motoring, it's a nuisance sailing because your sail trim will be changing as your autopilot turns up-current to sail back to the course.
While this will get you straight across a patch of current the way you plotted it, it will make for aggravating and inefficient sailing if the current is high. It's better to keep your boat on a fixed heading, so your sailing wind stays settled and you aren't steering all over trying to stay on a plotted course.
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