I think i understand your question - I'll do my best here:
the stock Silent Hunter 3 visibility radius is 8 km, there are mods including the GWX supermod which increases the range of visibility
In theory i suppose, if you upped your visibility to 100km and your crew were keen enough they could spot the ships at such a range
The world where SH3 is concerned, is flat
The real world as we know is quite round
so, with that in mind, in reality, with no atmospheric interferences such as haze or fog etc, on an absolutely perfect day an observer standing on a u-boat conning tower would only be able to see about +/- 10 miles anyhow because the ship would be quite literally hidden behind the earth's curvature beyond that distance.
with that in mind, increasing the visibility in SH3 only serves to detract from realism, but as the SH3 world is flat,
it is possible i suppose to mod the game in such a way that your crew could see 500 miles in any direction if one were so inclined however... with the 16km mod, i find that ships in the distance are always so far away that my crew sees them right about the same time they are barely visible through the binoculars so to increase the distance beyond 16km not only detracts from the realism but your crew would probably be spotting ships visually that you as a player would be incapable of seeing visually on your own
If you are talking about spawn radius... thats a whole other issue. Ships have spawn points as do convoys and it goes by date and time. The campaign script has random and scripted ships and convoys which are set to appear at specific dates at specific hours and minutes at a specific spot in the SH3 world... if you are within 50km of their spawn point, when their date and time stamp gets called upon they simply will not spawn. This prevents the player from plodding along through the north Atlantic and suddenly having ships come out of thin air all around him which would be quite annoying if there were a number of warships involved.
However, if you are further than 50km from the spawn point when the specific ship's date and time roll around, the game engine will spawn it and then keep track of the ships position as it follows its waypoints until such time it is within your visual range. so when you get "ship spotted" it has been "spawned" for a long time even though to you it didn't exist visually... its only just now come within visual range for your crew to see it visually.
given this information it is reasonable to assume that the ship is "Drawn" only when in visual range, despite having been spawned for quite some time in game. for this reason i am convinced that if you could modify your game files so that the atmosphere were 500 miles and the crew could "see" the same distance, there is no reason your crew couldn't "see" every ship within that distance despite the fact that you probably wouldn't be able to on your monitor and would neet to close the gap to not only formulate an attack, but bring the submarine within torpedo range of the target!
from the GWX manual - as atmospheric visibility range is increased, CPU loading is also increased. while it may be possible with a strong enough CPU it likely would crash the game as atmospheric distance is increased to some unreasonable number.
so my advice would be to stick with the more realistic 16km atmosphere or bump it up to say 100km and see what happens.
worst case scenario your game crashes
middle of the road scenario, your crew spots the ship, and it is visible on the map but you cant see it because it is a speck
best case scenario, your crew
and you can see the ship at whatever range you input (doubtful)