How to Calculate Course to Steer (CTS) with a Known Current

Calculating course to steer (CTS) with a known current is one of the most practical navigation skills a recreational sailor can learn. It answers a simple question: if the water is moving, what compass heading should you hold so that your boat actually travels along the intended track over the ground? The method uses a vector triangle—combining your boat’s speed through the water with the current’s set and drift—to find the heading that offsets the current. This article explains the procedure, the tools, common mistakes, and when to rely on more than a plotted line.

Main Explanation

What it is

Course to steer (CTS) is the compass course you steer through the water to make good a desired track over the ground when a known current is present. It is not the same as the bearing to your destination, and it is not the same as course over ground (COG) or course made good (CMG). In the context of independent sailing reference and tools platforms, CTS is presented as a universal navigation procedure—not tied to a specific boat brand, sailing school, or equipment manufacturer. It is a standard calculation taught in coastal and offshore navigation courses worldwide.

The core idea is simple: the current pushes your boat sideways or ahead/astern while you move through the water. To offset that push, you must aim the bow upcurrent of the intended track. As CaptainsGround puts it, the steered course always falls upcurrent of the track, and the closing leg of the vector triangle is always your speed through the water.

Why it matters

Without correcting for current, a boat will crab sideways and miss its intended destination. Over a long passage, even a modest current can add miles, delay arrival, or push a vessel toward a hazard. The stakes are high: grounding, collision, or an unplanned night arrival are all possible consequences of ignoring set and drift.

Recreational sailors face real challenges when learning this skill. Information overload is common—forums, videos, and apps offer conflicting advice. Outdated resources may not reflect modern electronic charting, while manufacturer manuals rarely explain current correction in a practical way. A reliable, independent reference platform helps by breaking the procedure into clear steps, using consistent terminology, and providing worked examples. That reduces the risk of misunderstanding a concept that must be executed correctly under pressure.

How it works

The CTS calculation is a vector triangle. You need three pieces of information: your intended track over the ground, the current’s set (direction toward which it flows) and drift (speed), and your boat’s speed through the water. The triangle is drawn on a chart using a consistent time interval—often one hour, but any interval works as long as all distances are scaled to that same interval.

Typical tools include a paper chart or training chart, parallel rules or a course plotter, dividers, a pencil, and a basic calculator for time-speed-distance math. A compass rose is used to read true directions. Independent sailing reference platforms often supplement these physical tools with step-by-step guides, practice problems, and chart plotting simulators. The structure usually covers boat knowledge and reference material, seamanship education, passage planning tools, and decision-support features such as worked examples and common error warnings.

In practice, a sailor draws the intended track from a known fix to the destination. From that fix, they lay off the current vector—pointing in the direction of set, with a length equal to the drift distance for the chosen time interval. From the head of that current vector, they swing an arc equal to the boat’s speed through the water for the same interval. Where that arc intersects the intended track is the point the boat will actually reach. The line from the head of the current vector to that intersection is the course to steer.

How to do it

Here is a practical walkthrough for a typical coastal passage. Suppose your boat makes 5 knots through the water, and the tidal current is forecast at 1.8 knots setting 017° true. You choose a half-hour time interval. In half an hour, the current will move you 0.9 nautical miles in direction 017° true. Your boat will travel 2.5 nautical miles through the water in the same half hour.

1. Plot a fix and draw the intended track from that fix to your destination. Measure its true direction—this is your course over ground.
2. From the fix, draw the current vector: direction 017° true, length 0.9 nm.
3. Set your dividers to 2.5 nm, representing your boat’s speed through the water for the half-hour interval.
4. Place one divider point at the head of the current vector and swing an arc to intersect the intended track.
5. Draw a line from the head of the current vector to that intersection. The direction of this line is your course to steer.
6. Measure that direction on the compass rose, then apply variation and deviation to get the compass heading to give the helm.
7. To find speed over ground, measure the distance along the intended track from the fix to the intersection, then divide by the time interval. Use that speed to calculate your estimated time of arrival.

This same procedure works for any current and boat speed. The key is keeping the time interval consistent for both the current vector and the boat speed arc.

When not to do it

CTS calculation assumes the current is known and constant over the time interval. In reality, tidal currents change strength and direction, often every hour. A single CTS may be valid for only a short leg. If the current varies significantly along the route, you may need to recalculate at intervals or split the passage into multiple legs.

Do not rely solely on a plotted CTS when entering confined waters, crossing busy traffic lanes, or navigating in reduced visibility. In those situations, real-time observation, radar, AIS, and professional pilotage take priority. A human weather router or professional navigator is more appropriate for ocean passages with complex current systems such as the Gulf Stream. Likewise, a certified marine surveyor—not a reference platform—should evaluate your boat’s structural and systems capabilities. CTS is a planning tool, not a substitute for situational awareness or professional judgment.

Visual

The following step sequence summarizes the CTS vector triangle method at a glance:

1. Fix your position on the chart and mark it clearly.
2. Draw the intended track from the fix to the destination; measure its true bearing.
3. Plot the current vector from the fix, using set for direction and drift distance for the chosen time interval.
4. Set dividers to boat speed distance for the same time interval.
5. Swing an arc from the head of the current vector to intersect the intended track.
6. Draw the CTS line from the head of the current vector to the intersection; measure its direction.
7. Convert to compass heading by applying variation and deviation.
8. Calculate speed over ground and ETA using the distance along the intended track divided by the time interval.

Common Mistakes

Confusing CTS with course over ground

Many sailors think the bearing to the waypoint is the heading to steer. The consequence is that the boat crabs downcurrent and misses the destination. The correct approach is to treat CTS as the heading through the water, while the intended track is the path over the ground.

Using GPS course-to-steer without current correction

A GPS or chartplotter often displays a course-to-steer field, but that is usually just the bearing to the next waypoint. It does not account for current. Relying on it can lead to a curved, inefficient ground track. Always calculate CTS manually or verify that your device applies current correction.

Mixing time intervals in the vector triangle

If the current vector is drawn for one hour but the boat speed arc is drawn for half an hour, the triangle is invalid. The consequence is an incorrect CTS. Always use the same time interval for both the current drift distance and the boat speed distance.

Using outdated or generic current data

Tidal currents vary by location, date, and time. Using a generic average or yesterday’s data can produce a CTS that is dangerously wrong. Always consult current tables, tidal stream atlases, or official electronic current data for the specific area and time.

Ignoring leeway and speed changes

Wind-driven leeway adds another sideways component that the current vector does not capture. Similarly, if your boat speed through the water changes due to sail trim or engine RPM, the CTS becomes invalid. Account for leeway separately and recalculate if speed changes significantly.

Failing to update CTS as conditions change

A CTS calculated at departure may be wrong an hour later if the current has changed. The consequence can be a significant off-track error. Recalculate at regular intervals or when the current forecast changes, and plot estimated positions to monitor progress.

Safety Note

CTS calculation is a planning aid, not a safety system. Always cross-check critical navigation data—weather, tides, and currents—with official sources such as national hydrographic offices, coast guard broadcasts, or recognized tide tables. Never use a tablet or phone as your only navigation tool; carry paper charts and know how to plot a fix manually. Take a certified navigation and safety course, and practice CTS plotting on a training chart before relying on it at sea. Follow the International Regulations for Preventing Collisions at Sea (COLREGS) and local maritime regulations at all times. If conditions deteriorate or you are unsure of your position, slow down, reassess, and do not hesitate to call for assistance. Digital tools and reference platforms complement—never replace—proper training, physical checks, and human judgment.