Wikipedia:Reference desk/Archives/Miscellaneous/2023 February 5

= February 5 =

Did anyone ever put distance finders on ships?
If they painted a distance finder on the hull then when you looked at the ship through telescope or binoculars you could have a vertical line for viewers who's telescope is 200 ft above water, one for 180 ft and so on, find your height above water then look down from there to the horizon, the ship is closer than the line of the other set of lines that's marked x nautical miles and further than the one that's hidden by Earth's curvature at that vertical line and marked y nautical miles. Or the highest thing with enough space could be painted with a ruler and/or text saying something like my masts are 267' apart (which needs a way for you to estimate angular size like a ruler superimposed over the telescope view which exists). Unless you have a circular funnel or something this would be affected by orientation though. I've never heard of anyone doing something like this. Sagittarian Milky Way (talk) 15:41, 5 February 2023 (UTC)


 * If you know the identity of the vessel, you can find its length $$L$$ in various registries. Unless it is headed straight at you or away from you, you can combining the difference $$\delta$$ between its bearing and the angle at which it is spotted with its apparent length (an angle) $$\omega$$ to estimate the distance to a far-away ship as being $$(L\cos \delta)/\omega.$$ --Lambiam 16:52, 5 February 2023 (UTC)
 * How to find instead of estimate its orientation? I guess if the registry had the number of feet between the red and green light you could figure it out with some math or you could ask its bearing by radio or optical telegraph (signal flag(s)). If the registry also has the ship's tip height above water and refraction is the same as your lookup table then you wouldn't need to know which way it's pointing or anything else (though which way it's pointing could be useful for other things i.e. if nothing changes will we collide yes/no/maybe). Sagittarian Milky Way (talk) 17:31, 5 February 2023 (UTC)
 * The waterline of a ship can change considerably, depending on its load. It is not a constant datum that can be meaningfully registered. A ship having a radar installation can see the course of another ship that is in range. --Lambiam 23:17, 5 February 2023 (UTC)
 * Yeah oil tankers show so much red when empty. The heights of the various Plimsoll lines could be registered but you'd still have to guess if it's fully loaded, empty or in between. So little of large cruise ships are underwater compared to their heights that the tip altitudes don't vary much percentage-wise but everyone would have radar by then. Sagittarian Milky Way (talk) 00:20, 6 February 2023 (UTC)
 * Wanted to mention the Plimsoll line and that the information can be found in registers as Lambiam indicated, and I don't know what age are you referring to, but for a good few dozen years now most ships will have a marine radar on board to help you get the ballpark figure for a ship that could be in your vicinity. --Ouro (blah blah) 03:43, 7 February 2023 (UTC)

A coincidence- or stereoscopic- rangefinder (telemeter) measures the distance to a ship regardless how it is painted. Such optical means are limited at night and ineffective in fog. A ship owner can assist possible search and rescue detection by installing a low-cost radar Corner reflector. Conversely, warships that prefer not to be targeted have employed Dazzle camouflage to confuse rangefinders. One hopes that an on-board ship navigator remembers the ancient skills of Celestial navigation but today (s)he probably relies for accurate location information on a Global Navigation Satellite System (GNSS) receiver, using the GPS, GLONASS, Galileo or BeiDou satellite systems. Philvoids (talk) 22:53, 5 February 2023 (UTC)


 * So good telescopic rangefinders were invented pretty early (1778, the 1899 stereoscopic rangefinder not really being more accurate according to the articles). Yeah all but the most amateur vessels probably have radios and satnavs these days, they could just ask the other vessel what their navigation computer, chart app or Apple Maps says if they really needed to know and don't have radar or lidar. As an amateur astronomer give me a sextant, 2023 nautical almanac with anything that says how to use the data blacked out and a Mickey Mouse watch set to any time zone as long as I know which or remember what time zone the place it's set to uses and I'd figure it out. I could get close with a star atlas in place of the nautical almanac. Sagittarian Milky Way (talk) 00:10, 6 February 2023 (UTC)
 * I thought of another reason, there were predatory pirate-based economies till after the 1820s, Confederate privateers till after the war ended, piracy continued long after its golden age. Sagittarian Milky Way (talk) 01:27, 6 February 2023 (UTC)


 * Until the mid-20th century, just looking and guessing a distance, course and speed was usually good enough. Unless your goal was to attack the other ship, in wich case you wouldn't expect it to cooperate. Nowadays most ships (here it's mandatory for any vessel longer than 20 metres) are equipped with the automatic identification system. You don't even have to ask, the ship automatically broadcasts its identity, position, speed and heading every few seconds, which can be plotted on a map. It can even broadcast destination and special details like: “We're laying a cable, so we can't deviate from our course and please stay away from our stern.” PiusImpavidus (talk) 11:12, 6 February 2023 (UTC)
 * Prior to such systems, most such information could (and still can) be conveyed with International maritime signal flags, or by morse code, etc., using the International Code of Signals. Those articles only list 1- and some 2-flag (or letter) codes, but ships usually carried a reference book listing many longer combinations, covering most eventualities. See England expects . . .. {The poster formerly known as 87.81.230.195} 90.221.194.253 (talk) 17:17, 6 February 2023 (UTC)


 * This sort of thing was incorporated in gunnery Fire Control systems in warships. See, for example, the lengthy article on that fitted to HMS Hood. The Evershed Bearing Indicators could both transmit and receive. MinorProphet (talk) 01:11, 12 February 2023 (UTC)