Maximum spotting distance for sailing vessels

General naval discussions that don't fit within any specific time period or cover several issues.
Erling
Junior Member
Posts: 1
Joined: Sat Jan 18, 2020 6:16 pm

Maximum spotting distance for sailing vessels

Post by Erling » Sat Jan 18, 2020 6:33 pm

Excuse me if my question doesn't quite fit the topic of these forums, but I failed to find the answer at other web resources.

What's the approximate maximum distance at which an observer sitting in a sailing vessel's crow's nest can spot another sailing vessel?
Let's assume following input:
1) Observer's crow's nest height – approx. 35 m (115 ft);
2) Other vessel masts height – approx. 35m (115 ft);
3) Fair weather;
4) Open sea.

Numerous manuals and calculators (like this one: http://www.ringbell.co.uk/info/hdist.htm) easily allow one to find distance to the horizon, which is about 20 km (12.4 Imperial miles, 10.8 nautical miles) in this case. But it doesn't take into account the fact that the other vessel is not a mere dot – it has masts sticking up 35 m high! I'm sure it got to affect spotting distance, since I've heard that in reality spotting a vessel at 20 nautical miles is far from impossible.

And by the way, how will (un)abailibility of a spyglass affect the spotting distance?

Byron Angel
Senior Member
Posts: 1095
Joined: Sun Mar 06, 2011 1:06 am

Re: Maximum spotting distance for sailing vessels

Post by Byron Angel » Tue Jan 21, 2020 1:57 am

Erling wrote:
Sat Jan 18, 2020 6:33 pm
Excuse me if my question doesn't quite fit the topic of these forums, but I failed to find the answer at other web resources.

What's the approximate maximum distance at which an observer sitting in a sailing vessel's crow's nest can spot another sailing vessel?
Let's assume following input:
1) Observer's crow's nest height – approx. 35 m (115 ft);
2) Other vessel masts height – approx. 35m (115 ft);
3) Fair weather;
4) Open sea.

Numerous manuals and calculators (like this one: http://www.ringbell.co.uk/info/hdist.htm) easily allow one to find distance to the horizon, which is about 20 km (12.4 Imperial miles, 10.8 nautical miles) in this case. But it doesn't take into account the fact that the other vessel is not a mere dot – it has masts sticking up 35 m high! I'm sure it got to affect spotting distance, since I've heard that in reality spotting a vessel at 20 nautical miles is far from impossible.

And by the way, how will (un)abailibility of a spyglass affect the spotting distance?

Hi Erling,
Double the computed distance for an observer at 35m height sighting a masthead of 35m height - so a maximum location distance approaching about twenty sea miles or 40,000 yards would not be amiss. Do keep in mind, though, that this would require extremely good conditions of visibility, lighting, as well as good visual acuity on the part of the observer. The likelihood of discovery versus observing time would tend to improve with -
(a) use of a glass/telescope by the observer.
(b) the quarry carrying lofty sail -topgallants/royals - as opposed to naked yards.
(c) a/m sails showing a sunlit side against a more distant dark/cloudy horizon.
(d) confining one's search upon a limited arc.

FWIW.

B

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