The challenges of course-keeping

There are several challenges in keeping your course, while you are sailing.

It is almost impossible for you to keep your course as you have planned it: you have to take into account some important factors that affect it, such as fuel shortage, drift or sideslipping. It is also important, in addition to the theoretical knowledge needed to plan the voyage, fuel and course, to know what can hinder navigation in everyday sailing life. 

This will be the topic of this guide.

The difference between course and heading

During navigation, it may happen that one points the heading in the correct direction as indicated by the compass, only to find oneself at a different point than one had imagined. 

The effect of wind and current must also be considered during navigation.

Wind can shift the orientation of the heading and may even take you on a different course. 

So while ideally the course is a line drawn on the seabed, it is not so simple to orient the boat’s heading correctly. 

It takes more than a compass to tell if you are on the right course; you also need a GPS to tell where you are and to possibly adjust your course.

Do you know what a true course is?

A true course is the path of the ship relative to the seafloor. It goes from 0 to 360 degrees clockwise from true north.

The true course angle is thus the azimuth that the course forms at each point with the direction of true North as indicated by the geographic meridian.

True heading

True heading, on the other hand, is the direction of the keel line, longitudinal axis or direction of the ship’s bow.

True heading angle, like true heading, is read on wind roses on nautical charts clockwise from true north as indicated by the geographic meridian.

How to find the correct heading of the boat?

Let us now introduce two fundamental concepts to understand why true heading and true course do not always coincide.

Drift and sideslipping

Current and wind can propel a vessel to drift or sideslip. 

Drift is when the current horizontally displaces water masses, affecting the hull. 

Pay attention to drift!

You won’t notice it if you don’t see fixed references. The current has the peculiarity of being the same for all vessels different in shape and size that drift, that is, move in a certain direction with a certain speed. 

All vessels even with different volume and weight drift in the same direction and at the same speed.

Instead, sideslipping is the direct consequence of the effect of the wind is to drift. The wind acts on the dead work and moves the boat horizontally. 

Those with some sailing experience can distinguish the sideslipping by looking at the wake from the boat. 

Sideslipping acts differently on different boats, depending on the size of the dead work.

Calculating drift and sideslipping angle

It is important to learn how to calculate the drift angle and the sideslipping angle, thus evaluating the deviation obtained by current and wind from the true course.

The drift and gybing angles can have a positive or negative sign depending on whether the deviation is to starboard (positive) or port (negative) from the held heading.

The motion of the ship undergoes a change in speed but not in direction. 

On the other hand, when the wind and the current push in opposite directions to each other, the heading and the course coincide. When the current and wind push in opposite directions to each other, a steep wave is generated.

Proper motion, surface motion, actual motion

When we talk about course-keeping, we must also consider the concepts of proper motion, surface motion and actual motion. 

1) Proper motion

Proper motion is generated by the propulsion of propellers and thrusters in a situation without current and wind.

In this case, the produced speed (also called the proper speed) is easily checked using a simple log. 

The orientation of the sailed course corresponds in the proper motion to the orientation of the bow.

Key Concepts 

• true heading
• proper speed

2) Surface motion

Surface motion, on the other hand, is generated by the combined actions of propellers, thrusters and wind. The actual speed of the boat is obviously different from the produced speed, and varies depending on the say.

Surface motion, on the other hand, is generated by the combined actions of propellers, thrusters, and wind. The actual speed of the boat is obviously different from the produced speed, and varies depending on the direction of the wind, which slows or accelerates the boat.

Key Concepts 

• surface course
• surface speed (or effective speed)

3) Effective motion

Finally, we have effective motion, which is generated by the propellers, thrusters, wind and current (as opposed to surface motion, which only considers wind).

Key Concepts 

• true course 
• actual speed