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Maximum spotting distance for sailing vessels
Posted: Sat Jan 18, 2020 6:33 pm
by Erling
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?
Re: Maximum spotting distance for sailing vessels
Posted: Tue Jan 21, 2020 1:57 am
by Byron Angel
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
Re: Maximum spotting distance for sailing vessels
Posted: Thu Jan 18, 2024 3:55 pm
by chuckfan3@gmail.com
Other considerations include the sails being silhouetted against dawn or evening sky and How the sails are set,
In sailing days a man of war sailing independently would often shorten sail just before dawn, so the ship with almost bare masts would be harder to see once there is light for observation. This gives the ship a better chance of seeing without being seen at the beginning of the day, so as to gain a head start on any maneuvers needed to counteract any other ships revealed by the dawn.
Re: Maximum spotting distance for sailing vessels
Posted: Thu Jan 18, 2024 11:17 pm
by OpanaPointer
On a Windjammer cruise in the Caribbean "a few years ago" I got to the top of the main mast and spotted the ship we were going to meet for a beach party. I saw a man waving at me, apparently standing on the ocean. As we got closer I saw he was in the same I was, albeit on a smaller ship.
Re: Maximum spotting distance for sailing vessels
Posted: Fri Jan 19, 2024 3:42 pm
by Thorsten Wahl
Graf Spee
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