How to judge distance at sea

Practical sailing tips on how to estimate the distance of a boat to objects at sea and on land

I was taking over the night watch on a sailboat and the captain's orders were clear: "There's a ship ahead, wake me up when it's close." I nodded and fixed my gaze on a small light in the distance. After about five minutes of intense observation, in desperation, I finally risked the captain’s annoyance and woke him up, asking, "How do I know   when it’s close?". Gauging distance at sea is not as easy as it might seem. Our eyes are not precise instruments and, especially at night, all visual perceptions are distorted. Yet estimating the distance of various objects is one of the basic requirements for trouble-free navigation. So let's take a look at a few methods of gauging distance at sea with the naked eye.

Men on a yacht estimating the distance during a race.

Of course, everyone has GPS on their boat these days, either as part of the boat's equipment or on their smartphone. This will provide most of the information we need to sail and then we can just set our destination. But there is some data we can't get from it, such as the position of non-stationary objects like other boats, fishing nets etc. And even though GPS is highly effective, it is always a good idea to verify what it tells us with our own observations.

According to yachting guru Tom Cunliffe, “nothing can ever be as good close-in as your own eyes. They remain the best instrument you have by far, and can define distance down to a fraction of a millimetre. If you are within 100 yds (100 m or so) of a charted buoy, you know where you are. In all probability, it will be more than accurate enough to say, for example, ‘The buoy bears dues north and I am about the length of a football pitch away from it’.”

Depending on the distance from an object, our eyes are able to discern the following: 

  • Up to 400 m: details of clothing, familiar people, etc.
  • Up to 500 m: clear human figures, window frames
  • Up to 600 m: clear outlines of people
  • Up to 800 m: arm and leg movements
  • Up to 1,000 m: tree trunks
  • Up to 1,500 m: car movements
  • Up to 2,000 m: medium-sized solitary trees
  • Up to 4,000–5,000 m: roads and house


However, this is only applicable when we can see land from the boat — for example, when sailing between islands. But on the open sea, gauging distance is more complicated, and there’ll always be a margin of error. The accuracy of the estimate is directly proportional to the experience of the navigator. For example, experience tells us that whilst an object one nautical mile away from the boat appears to the observer to be within reach in good visibility, in poor visibility it appears much further away.

Sailboat at sea towards the mainland.

If your eyes are around 3 metres above sea level, they are able to discern the following (at different distances):

  • 1 nautical mile: crew members can be recognised on board another boat, sail rigging can be seen. 
  • 2 nautical miles: large buoys can be identified with difficulty, at night the navigation lights of boats are becoming visible.
  • 4 nautical miles: sailboats are just white dots on the sea, individual floors of large ships can be recognized.
A woman on a sailboat looks into the distance.

Knowledge you can use at sea today and every day

Constantly be checking whether the object ahead (most often a boat) is on a collision course and whether or not you are in danger of collision. A trained eye and good judgement are vitally important, so you can know you’re in the clear with just a quick glance. Depending on the speed and distance away, a boat ahead on a steady course and speed that’s heading on a course to cross your path is potentially on a collision course. Any decision to act should be made when the object is about 3 miles away, as a boat travelling at 18 knots can cover that distance in 10 minutes. If you are at the helm of a boat maintaining a steady course, choose a fixed object, such as a windmill or post in alignment with the other boat. If the other boat's position does not change during your observation, i.e., it remains in alignment, it is on a collision course and action is required. Remember that regardless of the COLREG rules, it is your responsibility to do everything possible to avoid a collision, even if you have the right of way and the other boat doesn't. Action must be timely and clear, and understandable to the other boat. If you change course by just a few degrees, the other boat may not notice, so ideally alter course by at least 30 degrees. It is safest to head astern of the other boat rather than ahead of its bow, even at the cost of taking a detour.

Sailboat sailing towards another, imminent collision.

Eye height and estimating the distance to the horizon

Looking out to sea, we see an imaginary line: the horizon. In the past, people believed the Earth was flat and that this was the end of the world. Nowadays we know that the horizon is due to light rays spreading in a straight line across a curved Earth. The line from our eye to the horizon is, in mathematical terms, the tangent to the globe’s radius. How far you can see across the ocean to the horizon depends on two factors — the radius of the globe and the height of your eyes from the surface of the sea.

So how far is it to the horizon? To calculate it accurately, you need to know the mean radius of the earth, which is 6,371 kilometres, and also the Pythagorean theorem, which we were all tormented with at school. If you want to reminisce about your school days and calculate the exact distance to the horizon for your eye height, assume a right-angled triangle with one side being the distance from the surface to the centre of the earth, the second being that distance + our eye height (+ boat decks) and the third being the distance to the horizon - this is want we have to calculate. However, since the distance to the centre of the earth does not change, and your eye height remains the same (1.7 m for an average adult) it is enough to remember that at this height the distance to the horizon is about 4.7 kilometres. A child with an eye height of one metre, on the other hand, can only see about 3.6 kilometres away. These figures are valid if we are standing on land (or at sea level). On a boat, however, you have to add the difference between sea level and the deck. So if you are at the top of a 30-metre mast, you will be able to see up to 20 kilometres away. That's why there used to be a crow's nest so high up on the mast, for a lookout to announce that land had finally been sighted.

View from the ship in the distance to the horizon.

Right hand rule of thumb at sea and on land

The simplest and most commonly used method for gauging the distance of various objects on land and at sea is the thumb method. This method uses the observation that the average distance between the left and right eye of an adult is about 6.5 cm and the length of the forearm is about 65 cm. Their ratio is therefore 1:10. Based on this fact, we can estimate the distance of different objects in the following way. Extend the right hand forward, thumb pointing upwards in a perpendicular position. Close the left eye and look with the right eye at the thumb, which we raise so that it lines up with the object whose distance we are estimating. Then close the right eye while opening the left eye. We must remain as motionless as possible (which, admittedly, can be a problem on a boat), and the outstretched arm must not change position. The thumb seemingly jumps to the right. Instead of looking at the object we are now looking a little further away. The only thing left to do is to estimate as accurately as possible how far this spot is from the object being tracked and multiply this distance by ten.

The thumb method is used to determine the distance to an object.

Distance estimation when observing with binoculars

As well as the naked eye, we often use binoculars at sea. But how do we estimate the distance of objects we see through binoculars? Each fixed-magnification set of binoculars has a two-number designation (e.g.: 6x15; 8x20; 8x40; etc.) indicating the magnification and the diameter of the front lens in millimetres. For us, the first number, indicating the magnification, is of most interest. For example, a magnification of 10x means that we can see an object observed 1 km away as if we were looking at a distance of 100 m with the naked eye. Referring back to the table above, we can assume that if we have binoculars with 10x magnification and looking through them we can clearly distinguish the details of a lighthouse on the shore, this lighthouse will be about 5 km (500 m x 10) away. When choosing binoculars, however, it’s important to remember that a higher magnification isn’t necessarily better. There are many other factors that affect the quality, an important one being the diameter of the lenses (the bigger the better — greater angle of view, colour rendition and contrast, and above all, luminosity). For yachting, a 50 mm diameter is ideal. 

The captain of the ship looks through binoculars.

Whether you’re choosing one of these methods or you’re just making a rough estimation, remember that it is always best to trust your own eyes than what the GPS says: it may be out of date, experience lag or not contain some data at all.

So what ship will you look out for on the horizon?

How about estimating your next sailing trip? I can recommend both boats and destinations.