Judging distance at sea by eye – a how-to

Distance judging at sea 5


How far is the mudbank opposite? Half a mile? A mile and a half? On board sailing barge Lady of the Lea in the Swale

Each year I tell myself I will now memorise the ways old fashioned used to judge distances by eye to improve my ability to navigate by eye. Perhaps this will be the year I manage it.

Anyway, it would be a great skill for all of us to have and so when I found this little chapter in The Yachtsman’s Weekend Book  I thought should share it…

Distance judging at sea 1 Distance judging at sea 2 Distance judging at sea 3

Distance judging at sea 4


Some things I mean to try to remember are the following:

  • ‘For the man with his height of eye at 8ft, the sea level horizon will be about 3 1/4 miles.’
  • ‘Anything with its base just touching the horizon line, or a power vessel under way and showing her bow-wave just above the horizon – all these objects would be approximately 3 1/4 miles away.’
  • ‘Had his eye been between 5 and 6 feet above sea level, this distance would be about 2 1/2 miles.’
  • ‘At two miles distance (from the 8-foot observer) a large navigation buoy should be visible, in smooth water, but its shape and colour will as yet be indistinguishable to the naked eye’
  • ‘At a distance of 1 1/2 miles small-sized navigational buoys can be made out in smooth water.
  • ‘At a distance of 1 mile to 1 1/4 miles the shape of the smaller buoy can be made out… as also can the colour and markings of the large type of buoy.’
  • [At a distance 1 to 1 1/4 mile]’A man moving to and fro on board of a ship or on shore shows up… as a black mark but his limbs and features are, of course undistinguishable.’
  • ‘At 600 to 800 yards a moving man… resolves himself into a featurless vertical mark. At 400 to 500 yards distant the movement of a walking man’s legs are noticeable, and the rower’s arms in a dinghy can be seen working… ‘





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