Navigation: Estimating Distances at Sea


During a navigation, it is a precious safety skill to be able to evaluate distances on the fly: How far from shore is your boat? Or how far is your boat from that vessel which seems to be on collision course with yours? Here are some simple ways to evaluate this. Note: The methods described below are approximations. Always try to confirm your findings with another method.

Visual Indications

Standing on a boat deck, eyes about 12/14 feet above water level, with clear weather, here is what one can see.

4 Nautical Miles

2 Nautical Miles

1 Nautical Mile

As darkness falls at the end of the day, it becomes more and more difficult to appreciate distances. Therefore keep an additional safety margin at night.

Methods described above are ESTIMATES only. So always try to confirm or double-check this information with additional tools, like a GPS, if available.

Bearing Methods

With a hand compass and the boat’s distance speedometer, it is pretty simple to evaluate precisely the distance of a fixed object. It is a simple geometric method based on the isosceles triangle principle.

Right Angle Method (see illustration)

The Double Bearing Angle Method (see illustration)

Measuring Angles with a Hand (see illustration)

Hand open and vertical (see illustration) close to the face, at the same height as the eyes, it is possible to evaluate angles according to this illustration.