How to work out your course to steer (CTS)

When you’re sailing or navigating a boat, pointing the bow straight at your destination doesn’t necessarily mean you’ll get there. Wind can push you sideways, tide or current can carry you off course, and your boat could make a little leeway as it moves through the water.

This is why navigators use course to steer (CTS), which is the course you need to steer through the water so that your boat follows the track you want over the ground. In this guide, we explain what CTS means, how to calculate it, and show some simple course to steer examples.

What is course to steer (CTS)?

Course to steer (CTS) is the direction you steer to achieve the desired track over the ground, accounting for current and, where relevant, leeway.

Your CTS depends on many things, including:

• Your starting position
• Your destination
• The distance between them
• The direction and speed of the tide or current
• Wind and the resulting leeway
• Your boat's speed through the water

Course to steer and course made good

CTS is the course you choose to steer through the water, while course made good (CMG) is the average direction your boat has travelled over the ground. Essentially, CTS is what you steer, and CMG is where you go.

Course to steer and course over ground (COG)

Course over ground (COG) is the direction your boat is moving across the Earth's surface and is affected by your boat's movement through the water and the movement of the water itself.

GPS normally shows your movement over the ground, so its course COG can look different from the course you’re steering.

What’s the difference between course to steer and heading?

Heading is the direction the bow of your boat is pointing, while course to steer is the direction you intend to steer through the water. These can differ due to leeway.

If a strong wind is coming from your right-hand side and pushes your boat towards the left as you sail, you may need to point the bow slightly towards the wind to continue to move in your intended direction.

Why is course to steer important?

Working out CTS helps you arrive at your destination efficiently and safely. Without allowing for tide, current, and leeway, you can end up:

• Off your planned track
• Further from your destination
• Using more fuel
• Sailing a longer route
• Arriving later than planned
• Closer to hazards than expected

This is especially important when navigating near rocks, shoals, traffic, narrow channels, or a busy harbour.

How do you calculate course to steer?

To calculate course to steer, you need to work out the direction from your starting point to your destination, then adjust it for the forces that will move you away from that track. A simple CTS calculation normally considers:

• The course between your start and destination
• Your boat speed through the water
• The tide or current direction and speed
• Any leeway caused by wind
• The amount of time you will be exposed to the current

The calculation is easiest when you think of the boat's movement and the current as vectors.

1. Work out the direct course by finding the bearing from your starting position to your destination.
2. Add the current. For example, if the current is flowing towards the north at 2 knots and you steer 090°T, the current will push you north of your destination.
3. You can calculate the required steering angle using vector or triangle calculations. The exact calculation depends on how the current direction is expressed, so care is needed when using a course to steer formula from a navigation book or calculator.

For practical navigation, a vector diagram, navigation calculator, or plotting method can be used rather than trying to remember a formula.

Adding leeway to course to steer

Tide isn’t the only thing that can move a boat away from its intended track. Wind can push a sailing boat sideways. This sideways movement is called leeway. When adding leeway to course to steer, you need to consider which way the wind is pushing the boat.

For example, if your calculated CTS is: 030°T and the wind is pushing your boat to the south-east, you may need to steer further into the wind to compensate. Your final steering direction could therefore be different from the CTS calculated from tide alone.

You can think about it as: Final steering direction = tide-adjusted CTS + leeway correction

However, you can’t simply add the numbers every time as the right correction depends on which side the wind is coming from and which way the boat is being pushed.

Plotting a course to steer

Plotting a course to steer on a chart can make the calculation much easier to understand. Using a vector triangle is a traditional method.

Start by plotting:

1. Your starting position
2. A line towards your destination
3. The tidal stream vector, showing its direction and speed
4. Your boat's speed through the water
5. The resulting course to steer

For example, if your boat can travel at 6 knots and the tide is carrying you at 2 knots, you can draw a 2-knot tidal vector. Then, use your 6-knot boat-speed vector to find the direction that produces the required ground track. This is often called a tidal vector triangle, and it will give you a visual picture of what’s happening.

Using a course to steer calculator

A course to steer calculator can save time, especially when you’re working with several variables. A typical calculator may ask for:

• Start point
• Destination
• Boat speed
• Tide direction
• Tide speed
• Leeway
• Time or distance

It will then work out the course you should steer. Before relying on the calculator, check that:

• You entered true or magnetic directions correctly
• Tide direction is entered in the correct format
• Speed is in knots
• Leeway is entered on the correct side
• The result makes sense on the chart

A quick chart check can highlight any input errors.

Can course to steer change during a passage?

Yes, CTS isn’t necessarily a single number that stays the same for an entire passage, as tide changes direction and speed and wind can also change. Your boat's speed may vary, too.

For example, you might calculate a CTS of 075°T at the start of a passage. Two hours later, the tidal stream may have changed enough that your best CTS is now 090°T. This is why good navigation involves checking your position and progress during the passage. If your actual course over ground (COG) is different from your expected track, investigate why rather than simply continuing to steer the original course.

Is course to steer a true or magnetic course?

CTS can be expressed as either a true course or a magnetic course, and you need to know which one you’re using before applying it to a compass. Charts commonly use true bearings and courses, but a magnetic compass points towards magnetic north. This means you may need to convert a true course into a magnetic course before steering it with a magnetic compass.

How do you convert true course to magnetic course?

The difference between true north and magnetic north is called magnetic variation or declination. The variation changes depending on your location and also changes over time. Essentially, your magnetic course is your true course adjusted for variation. Whether you add or subtract the variation depends on whether the local variation is easterly or westerly.

For example, if your CTS is 100°T and the local variation is 5°W, you would make the appropriate west-variation correction to obtain the magnetic course. Always use the current variation shown on your chart or navigation data rather than relying on an old figure.

Bearing, heading and course to steer: What's the difference?

The key differences are:

• Bearing - Bearing describes the direction of an object or position from you
• Heading - Your heading is the direction your boat's bow is pointing
• Course to steer - Your CTS is the course you need to steer to achieve your planned track, after allowing for things such as current and leeway

Why doesn't my GPS match my calculated course to steer?

Your GPS normally displays COG, not CTS. If there is tide, your boat can be pointing in one direction while travelling over the ground in another.

For example:

• Compass heading: 350°
• CTS: 350°
• COG: 000°

This can be perfectly normal if a current is pushing the boat sideways. There can also be a delay in GPS data, especially when you change course or travel slowly.

Other possible causes include:

• Wind and leeway
• Incorrect tidal information
• A change in the current
• Incorrect compass calibration
• Incorrect variation
• Steering errors
• GPS position or COG smoothing

Don’t expect CTS and COG to be identical when you’re navigating in moving water.

A practical CTS workflow

When you need to work out a course to steer, use this simple process:

1. Mark your start point
2. Mark your destination
3. Find the direct course
4. Check the tide
5. Allow for boat speed
6. Allow for leeway
7. Calculate CTS using a vector triangle, navigation software, or a reliable course to steer calculator
8. Convert to magnetic if needed
9. Check your progress
10. Recalculate when conditions change

The easiest way to calculate course to steer

For a beginner, the easiest way to calculate course to steer is usually a navigation app, plotting tool, or course to steer calculator that lets you enter your boat speed and tidal stream. For learning and traditional navigation, it’s worth understanding the vector method.

Essentially, your boat's movement through the water plus the water's movement over the ground gives your actual movement over the ground. To reach your destination, you need to choose a boat direction that makes those two movements combine into the ground track you want.

Keen to improve your sailing? Learn more about tacking in sailing and how to use a tripping line to protect your anchor.

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