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Clearly, I don't mean simply keeping eyes on a target ship -- unless of course there was some method by which one ship could remain within visual range of another without being seen by the target. I just don't think there is, and I don't want to resort to magic to solve this issue.

So what I'm looking for, in a nutshell -- is there a method such that one ship could observe another without revealing itself?

I've considered (for my story) using trained birds, but I'm not sure how plausible that is.

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    $\begingroup$ Passive sonar. Sound travels a long way underwater. (A basic passive sonar is a directional hydrophone connected to an amplifier and a pair of headphones.) A trained sonar operator can identify the type of ship and it's speed from the noise of its engines and propellers; sometimes they can even identify the particular ship, especially if it's one of only a few of its kind. Ah, and by the way, radar is not stealthy at all; due to the inverse square law, a radar emitter can be detected from far beyond its range. $\endgroup$ – AlexP Jan 4 '18 at 22:41
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    $\begingroup$ It sounds like this question would be a better fit for History. Since you're asking about how things were in our past, historians are going to be better equipped to give you an answer than people who build fictional worlds. I'd leave out the bit about trained birds if you ask it there. $\endgroup$ – sphennings Jan 4 '18 at 22:50
  • $\begingroup$ I may be splitting hairs, here, but it seems to me that there's no way that one can observe something without being observed back. Even one-way mirror glass isn't truly one-way. $\endgroup$ – Jakob Lovern Jan 4 '18 at 23:08
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    $\begingroup$ One word : Submarine. $\endgroup$ – StephenG Jan 5 '18 at 0:38
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    $\begingroup$ You need to indicate what technological period you're looking at. Age of Sail will provide different answers than in the era of coal-burners which will be different from the era of submarines. $\endgroup$ – Keith Morrison Jan 5 '18 at 17:00
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Take an age-of-pirates or age-of-Napoleon ship. (Technically, ship would refer to a ship-rigged vessel, not just any large watercraft.) The masts consist of separate spars mounted on top of each other, with yards holding the separate sails. Here is a typical mast with main mast, topmast and topgallant mast, and here are the yards.

The convenient lookout positions are where the mast sections and/or yards meet. Often there would be a small platform, called a top. Note that the top is not at the tip of the mast, because there would be no conventient ropes to hold in bad weather.

In good weather, lookouts would detect the sails of another ship well before the hull of the ship comes over the horizon. But that's not certain, in bad weather the detection might come much later. So if the uppermost sails of a ship are not set, there is a good chance that the lookout would miss the bare masts, at least at long distance.

This suggests several options:

  • Send a sailor to climb as high as possible, above the height of the usual tops, and adjust the speed of the pursuing vessel so that the target's royal or topgallant sails are just over the horizon. With luck, the own royals and topgallants are just below the horizon.
  • As above, but strike the royal and topgallant sails and possibly even their yards (naval crews were trained to do that at sea). The pursuing vessel would have to keep up with the target on main and topsail alone, which makes this problematic.
  • Use a spare spar (warships would carry several on the spar deck) to extend the height of the mast and get a higher lookout post. This wasn't normal practice because the lookout position would be rather uncomfortable, especially in high winds.

All these options depend on the height of the observer relative to the own sails, so they don't depend on having a large ship. It would be possible to use a sloop or the like, or perhaps a schooner.

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Early in WW2, the German Navy experimented with a small gyrocopter that was towed by a surfaced submarine. It could reach a height of a few hundred feet, greatly expanding the visible area to the observer flying the gyro. The gyrocopter was small enough that it would have been essentially invisible to ships at any distance. Never very successful, it also committed the submarine to being on the surface, and the possibility that in the event of attack, they might have to cut the gyrocopter loose and dive, leaving the observer on the open ocean alone.

Along those lines, it is theoretically possible to use a towed balloon in the age of sail... by the early 1800's, lighter than air gases had been used to carry a balloon aloft to increase the range of an observer. If the balloon were painted light blue or gray, it would have been difficult for opposing ships to spot at any distance. As far as I know, that was never actually done in a naval setting.

In WW1, some of the Zeppelins carried a small gondola that could be lowered with an observer, for use when the airship was above an overcast sky. They'd lower the observer to below the clouds, and have them report back what they saw. It wasn't used for tracking ships, but could have been. However, that would only work as stealth observation when an overcast sky was present.

Also early in WW2, the Imperial Japanese Navy had refined night time naval operations to a very high standard, both attack and observation, aided by excellent night optical gear... binoculars with very large objective lenses to amplify the available light. Using this, they were able to ambush an Australian and American fleet in the first Battle of Savo Island, sinking four cruisers with no losses. So excellent night optics could be another way to track ships without being observed... but only at night.

Edit: to show the flip side, the Japanese reliance on night optics came back on them at Guadalcanal, during the Night Action of Nov 14. IJN battleship Kirishima, along with some cruisers, were closing to bombard Henderson field, and had set fire to a couple of US destroyers, while also battering US battleship South Dakota. The flames from the burning destroyers blinded the night optics, masking the approach of USS Washington (who was using radar to track the Japanese fleet). Washington was able to close to within 8000 yards of Kirishima without being seen - point blank range for 16 inch naval rifles - and wreck it with several broadsides. Kirishima capsized and sank the next morning. This incident is also notable for being the last time that a battleship sank another battleship in combat, with no assistance from other forces.

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What technology period are we talking about? During the first world war, naval ships 'saw' each other over the horizon by looking for the smoke plume. Coal fired boilers produced huge pillars of black smoke. So it was easy to track them at a distance.

If your ship used wind, then it could stealthily follow such a war ship.

As long as there was wind.

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  • $\begingroup$ And the direction of the wind has to be favorable, especially if your vessel lacks triangular sails. $\endgroup$ – Renan Jan 5 '18 at 6:06

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