The "Course to Steer" boating app has particular value for sailors whose relatively slower speed through the water is easily affected by tidal currents and the leeway of the boat in the wind. When these factors throw the boat off its ideal course straight to a waypoint or destination, experienced sailors have always learned to steer more to one side or the other to compensate (and to avoid obstacles toward which the wind or current may move them). But while common sense can tell you which side to steer to, you need trigonometry to calculate the actual course to steer in order to stay on the direct shortest path to the destination.
Most of us can't do trig that quickly in our head, and most chartplotters don't calculate that. But the free Course to Steer app does the work for us.
Version reviewed: 1.5c
Available on Android Google Play Store - free (at time of testing)
Tested on Lenovo A1 Tablet running Android 2.3
What It Does
"Course to Steer" is a relatively simple app that essentially does one thing only: when you input the appropriate variables about your boat, your course, and other factors, it calculates for you the exact optimal course to steer to stay on the direct line to your destination or waypoint - the best course over the ground (COG). With it you can avoid what is often called cross-track error (the deviation from your intended course), which results from a current or wind taking you off course.
It also calculates your true speed over the ground (SOG) regardless of what your boat's knotmeter may tell you about your speed through the water.
Once you go through the initial learning curve, the app is very fast and easy to use. Here are the steps in which the user typically enters data:
- Ground track - the direction you want to go (in true, not magnetic, degrees)
- Tide direction - the direction in which a tidal or other current is moving (in presumably true degrees)
- Tide rate - current speed in knots
- Boat speed - through the water in knots
- Wind from - direction from which the wind is coming (choose among 8 directions, or none)
- Leeway - entered in degrees (may be confusing at first - see comment below)
- Magnetic variation - the variation of local magnetic direction from true direction (enter this in degrees W or E - from your local chart)
- Magnetic deviation - presumably a deviation factor related to your boat's compass
Then tap the "calculate" button to receive the following information:
- CTS true - course to steer (or intended course over ground, COG) in degrees true, not magnetic
- CTS magnetic - course to steer in degrees magnetic
- CTS compass - course to steer in degrees magnetic using your compass (if you entered a deviation number previously for your boat's compass)
- Speed over ground - your actual SOG along the COG
Why This Matters
Even just playing around with this app almost immediately helps you improve your understanding of wind and current on your actual course over ground. It's very easy to be way off in your own common sense calculations. For example, imagine you are sailing due north in a very light wind, going only 2.5 knots. From due west is a 2-knot tidal current sweeping you east. You know you need to turn westerly in order to reach your destination - but how far to the west? If you like math puzzles, take a guess before reading the next paragraph.
I might have guessed that you'd have to turn maybe 40 degrees toward the wind to compensate for the current, to a heading of around 320. In reality, because this is a trigonometric function, you have to turn 53 degrees to a heading of 307 - just to be able to make a COG of 360. But with the wind from 270, you can't sail at the point of sail of 307, so you'd have to do some tacking. That means you better be aware of possible underwater hazards well off to both sides of your ideal course or run some significant risk. That's a lot to have to pay attention to, but at least this app helps keep you from making blind assumptions about how to sail to maintain the clear straight course to your destination.
As far as I have been able to test it, the app does seem to make its calculations correctly. On the water, of course, every boater should be constantly aware of boat position at all times anyway, such as with a navigation app that shows your boat's position against a chart - so it would be dangerous to trust "Course to Steer" or any other app to keep you free of underwater or other hazards.
My only complaint is that the app in this version has no help screens at all and saves no settings. While for the most part you could view this as just a fancy calculator, to use it well you need to understand the concept of leeway to be able to enter that data into the app. Leeway is the degree of movement of the boat off course due to wind. If the wind blows hard from the west as you travel north, for example, then your boat's track will vary from a line straight ahead off to easterly. How much leeway you experience depends on both how hard the wind is blowing and how much wind your boat "catches." A boat with high freeboard generally has more leeway than one with low, and leeway increases with wind speed. In the app, however, you enter only a number for your boat's degree of leeway along with the wind direction (but not speed). So to do this well, you have to estimate, for different wind speeds, how much leeway your own boat experiences. For an app that otherwise seems so precisely mathematical and accurate, this estimate could throw your results off considerably if you guess wrong. I would urge the app developer to think about this and perhaps offer some advice for how one should do this estimating. Perhaps you could calculate your usual leeway from using other GPS data commonly available in other navigation apps?
Overall, this is a nice little app that boaters should consider using. As a free app, it's nicely not cluttered with ads or requests to update to a paid version. (Hopefully it will stay that way.) Just remember that because of the leeway function, your own results could vary from the predicted results. And even if your results are correct, you may have to make other changes in navigation, such as in the tacking example earlier.
If you use a regular navigational app or chartplotter instead of this app, you can still easily learn to compensate for currents.
Related navigation apps useful for boaters: