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
- Looking towards the shore: Possible to recognize the shape of houses and trees. A beach is not distinguishable.
- Looking only over the water: One can recognize the superstructures of a large ship. Sailboats are white dots.
2 Nautical Miles
- Looking towards the shore: One recognizes doors and windows but not human beings.
- Looking only over the water: One barely starts to identify large buoys. At night, boats navigation lights start to be visible.
1 Nautical Mile
- Looking towards the shore: One can see details of houses and of traffic. Persons are small dots.
- Looking only over the water: One can make out crewmembers on deck. Sailboats’ rigging are visible.
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)
- When the object (Point C) you want to measure the distance to from your boat is at a precise bearing of 45°of the boat's heading, that is point A.
- Start measuring the distance covered by the boat with the speedometer.
- When the object (Point C) is on 90° bearing, that is the boat’s point B.
- Clock the distance run by the boat from point A to point B.
- That distance is your boat’s distance from the object at that moment.
The Double Bearing Angle Method (see illustration)
- The boat starts at point A.
- Take a bearing of the object (point C) at any point over 20° of the boat's heading.
- Start measuring the distance run by the boat with the speedometer.
- When the object is on a bearing angle double of what it was at point A, that's the boat’s point B.
- Clock the distance run by the boat from point A to point B.
- That distance is your boat’s distance from the object at that moment.
Measuring Angles with a Hand (see illustration)
- Arm extended, fist tightly closed, thumb extended out of the hand:
From one knuckle extremity to the other, it is about 10°.
From one knuckle extremity to the outside of the thumb, it is about 15°.