Couple of thoughts: Most ships do not engage in "active augury" unless they suspect there is a reason to do so.

My understanding is that a ship normally underway is automatically detected (I would assume without the use of active augury.) Basically just like you can "see" a person who is not hiding.

Most ships will not have their Master of Etherics on the console unless they are at battle stations or at least yellow alert (Whatever you choose to call it.) This means that a ship with crack crew can only detect a silent vessel approximately 30% of the time and THEN only if they have a reason to suspect the ship is there! This means our cobra class escort, well played, Might sneak in to 10 VU's from it's target and unleash it's torpedoes with no warning to the target ship! This can potentially ruin even a cruiser's day! Of course after that, the Cobra will be using thos wonderfully fast engines to get the hell back out of a larger ships range!

As to detection: Even most modern submarines are detected once they launch a weapon. The launch transient on most torpedo tubes is easily detectable and the high pitched whine of the Torpedoes engine is something every Sonarman is taught to recognise! Newer systems can mitigate this somewhat but much of that is classified (Still!).

Huh. Lot of rapid-fire back and forth here.

I've always compared augur arrays to radar, rather than sonar, myself. I'm not a specialist in the field, but this is how I picture things (and interpret the rules).

The ship conducting active auguries is comparable to a radar station or an EA-18G Growler- it's spitting out a hash of signals and seeing what comes back, doing the radar equivalent of waving a neon sign shouting "HERE I AM. I'M RIGHT HERE. YOU CAN SEE ME, BUT I SEE YOU TOO," and it may or may not be emitting signals intended to jam radar/auguries.

The ship under silent running is a stealth fighter/bomber. No more need be said here.

A ship that is neither conducting active auguries nor operating under silent running is neither spitting out signals saying "HERE I AM" nor actively concealing itself. It may well be emitting some signals, but it's not trying to conceal itself from detection arrays.

So, yeah, I tend to assume that a vessel not under silent running is visible at, say, 100VUs (barring interfering phenomena), but it's precise location may not be known. A vessel performing active auguries is visible within line of sight, regardless of distance (bearing in mind the speed of light and interfering phenomena), and it's precise location is known- if they maintain course and speed (and you account for gravitic effects), you could dumbfire torpedoes and they will hit. Since most vessels don't maintain course and speed, this is usually not a problem- my players always learn to do a bit of a random-walk after performing active auguries though. A vessel operating under silent running must be detected, as discussed.

I've also houseruled the W-240 arrays- they don't allow for Active Auguries because they are inherently passive arrays. Focussed Auguries are fine though. I also allow my players to take multiple sets of augur arrays and choose which array(s) to have powered at any given time- but each one past the first costs 1 space. They still count as external, but this means that there is at least a cost to having the Logis-Targeter, the W-240s and the super-noisy active array all in one ship.

Ah yes, I forgot:

Assuming you need to do Active Scans to see things around you (Green Knight's original position, as I understood it), then a ship on Silent Running is blind unless it has passive scanners.

Not a good situation. But not catastrophic, since Silent Running is pointless anyway under that interpretation.

Meh.

My players would quickly figure out where that torpedo came from and I don't play their opponents one lick dumber than they are. You're right, don't outright disallow it, and I wouldn't, but it still wouldn't work well without many other factors in place, afctors hard to duplicate consistently. And no, I'm not going to house rule a way for them to make it work. My players wouldn't appreciate it if I house-ruled a way for their opponents to get over on them and I try to run a fair game.

Ships can't detect each other until they are within 20 VUs + 5 VUs per success or their Augurer. That's already surprise range. That's already within torpedo range. My players get surprise often enough as it is.

There was no intent, to be negative about you or your players.

OTOH, I believe you are mistaken about detection range, let me use an historical example to explain.

The PT Boats of WW2, could do 30-45 knots ( depending on make and model ), and after the shooting started they would use everything they had ( and wish for more ). But before the battle started, they would use would use silent running to get into position and slowly close with the enemy - launching their torpedoes and continuing to close on silent running. This allowed them to frequently launch another spread of torpedoes before they were discovered. Some PT boats even have the mufflers for their engines, underwater so that even the sounds of the running engines would be reduced - so the sound signature of the engines would be less likely to give them away.

The reason they did this, is because at speed, the PT Boats would make a wake, that would be visible from a distance much further than the boats them selves could be seen - even the sound of the engines might give their presence away from a distance of a couple of miles This stalking maneuver was classic for both sides of WW2, and in some cases lead to both sides being surprised - the demise of PT-109, with John F Kennedy in command, is case in point. Kennedy was stalking another ship, when a Japanese destroyer spotted the 109 and rammed her, because the range at which 109 was spotted, the DD didn't have time to shoot - the men on PT-109 had less than 10 seconds to react to the destroyer looming up out of the darkness, and that wasn't enough to bring the PT Boat up to speed and turn to avoid.

The Rogue Trader rules, do not explicitly say so, but implies the big engines make a big signatures, the bigger the engine, the easier and further away it should be detected with casual scans - just like how a PT Boat at speed, throws up a wake that can be seen, from a distance much further than the boat it's self can not be seen. Silent Running, reduces the engine signature so that the distance at which it can be seen starts to shrink, and now you are at a point where very focused scans are needed and/or very short range, to spot the intruder, because like with a stalking PT Boat there is little to no wake to give her presence away.

Active augury represents a distinctively enhanced attempt by a ship to detect ALL contacts using all available sensors. Under normal circumstances, a ship would probably have the effect of an active augury in it's front arc but only have it's crew's skill to use in detection. This represents navigational sensors that are routinely used to detect and avoid possible collisions while in transit (Much like modern aircraft radars.) Additionally, all ships would have passive style sensors that would allow them to detect Vox and Auspex (And potentially others) emissions from any bearing. This is why a ship not on silent running is automatically detected at whatever range (Encounter range). Conversely, Ships on silent running are deliberately NOT using ANY active sensors and are minimizing their engine signature in order to evade detection. Hence, A ship not actively searching for a silent vessel will VERY likely be surprised by one unless it happens to pass through their front (Active) Arc. Note that ships do NOT routinely operate on an active augury mode relying instead on the fact that most potential threats will appear in front of them. This is actually plausible for a number of reasons:

1.) It is very difficult to intercept a vessel in transit without being detected due to the nature of constant acceleration navigation. Thus, Most attacks will occur at the beginning or end of said transit.

2.) Various stellar phenomena will make it difficult if not impossible to detect a vessel at long range.

3.) Canny Captains will typically perform a "Yankee search" (Active augury) at the beginning of a transit and at the end. Canny Raiders will seek to attack their targets immediately before or after such a search begins or ends.

4.) In civilized systems, system patrol ships will routinely patrol jump points and trans-orbital insertion points (The point at which a vessel will typically complete it's system transit and reorient towards the system's main planetary orbit). In these points a silent raider must actively seek to avoid detection or it will itself be engaged. Because of this many merchant crews that are accustomed to civilized systems may forego the search process.

5.) Simple fatigue will prevent the Master of etherics from ALWAYS manning the Augur console. Thus, The standard crew skill rating is probably more important than the PC's skill in preventing surprise!