I am way out of my depth with all this talk of guns. A couple technicalities about USN airships I do know something about then:
....the first dirigible that they had the USS Los Angeles.
Actually, no, the USN commissioned a total of 5 rigid airships and the LA was spang in the middle of the series. The USS Shenandoah was ZR-1, although I believe ZR-2 flew first. But the latter did not last at all long, being very poorly constructed for its mission! Shenandoah was built in the USA, closely following reverse engineering of downed late German Zeppelins with some revisions. It was constructed in the then-new Lakehurst Hangar 1 which later was able to house the Hindenburg as well as at some time three of the other 4 Navy rigids. It flew in the early '20s. ZR-2 was authorized later but was completed earlier, in Britain, and purchased from the British. It was also reverse engineered from Zeppelins but with far more extrapolations, and failed because it was extrapolated from late model German "height climbers" designed to attack Britain from a very great altitude to evade interceptors and AA, and therefore the height climber designs were extremely lightened, to reach high altitudes. They were adequately strong for low air densities found there, but had to be handled very carefully at low altitudes. The British designers did not entirely understand how and why the Germans did what they did, and believed something so lightly built could operate aggressively near sea level. The USN sent a contingent of trainees over to Britain to learn more about airship operations (using the well-built and innovative if small R80 designed by Barnes Wallis working at Vickers) and then evaluate and take possession of a late model big design the British decided to unload. The thing split apart and burned up over the Hull river with great loss of life of American and British personnel aboard, long before the Americans could try to figure out how to get it to America or give it a ship name.
But although the experience was negative, this was the first rigid American Navy people operated. The second being Shenandoah, flown for some years before being lost in a storm over Ohio, although Rosendahl, then a Lt Cmdr IiRC and second in command under Cmdr Landsdowne, who stayed in the control car and perished when it fell loose, organized the survival of a lot of crew free ballooning the severed nose section down to the surface.
LA was the third USN rigid, ZR-3, and not acquired until some useful flight experience was accomplished in Shenandoah. It was made by Zeppelin in Germany, and flown over the Atlantic inflated with hydrogen to Lakehurst where it was turned over and refilled with helium for all future operations. At the time the Navy's helium supply was so scanty that most of the gas for LA was taken from Shenandoah which spent some time laid up at Lakehurst until more helium could be acquired gradually. Thus the Navy had two commissioned rigids on the books, but could only fly one at a time.
Later, in the early 30's, the new and larger American-made sister ships Akron and Macon, ZRS-4 and -5, were built at Akron by Goodyear-Zeppelin, a spin-off partner company whose design staff was largely recruited from former Zeppelin engineers. LA overlapped each in service, and was modified to prototype their otherwise unique operation of hook on aircraft (that is, other airships of both rigid and pressure type did occasionally hook on or launch airplanes or gliders, but only the two ZRS ships were designed integrally for that function). LA outlasted each, and Akron was lost off New Jersey just before Macon was ready to fly. So LA served as "the other airship" alongside three American made Navy rigids.
It certainly deserves memory as arguably the most successful USN rigid, and certainly by far the longest serving, and definitely one on which many though not quite all lessons were learned.
But it was not the first! Even not counting doomed and nameless ZR-2 it was anyway the second.
Goodyear does it today with the Blimps over sporting events, the pilots have training manuals dating back to the first airships (both rigid and semi rigid) that instructors use in training them on how to stay around a single point using the wind and engines in a minimalist mode. Using a simple expedient of running up wind and then using the wind to blow you over the target while using your engines just enough and flight controls to run over the target. Remember they are using their engines in most cases to charge their batteries at night and they are not exactly quiet.
If the wind is too high for the blimp to be station keeping the wind is to high for it to safely fly in that area except in a running mode.
I think you mean to argue that with light engine use, and considering the U-boat is busy charging its own batteries, the noise of light operation might not be noticeable, especially with the possible tactic of positioning upwind and allowing the wind to blow the blimp over the sub.
But remember that the L-ships at least were taken, their prototypes anyway, from Goodyear's stock of advertising blimps, of which the company operated several at the time of the US entering the war.
Goodyear very deliberately refrained from muffling their engines, because they wanted the noise of the engines to attract attention so people would look at the blimps. I can call an L-ship "small" and bearing in mind that the larger K ships only massed 3-4 tonnes all up one can see why someone might say that; compared to an airplane an L ship would be a light one indeed like a Piper Cub or Ercoupe. But physically--well, air at sea level masses just around 1.225 kg/cubic meter (versus say water massing 1000 kg, a full metric tonne, per the same volume. Airships must ascend, or in the case of blimps have enough internal balloonet volume for other reasons too, to allow ascent, some 3 kilometers, at which altitude pressure will have fallen to 3/4 surface pressure, so they can't hold more helium than would fill the ship at that altitude, the "pressure height." Putting in more gas at sea level, when there is plenty of room for it, would raise the lift but make it impossible to rise to 3 km without venting some of it out; helium is very expensive and operators take all kinds of steps to avoid that. Thus, in effect a blimp must have total displacement, in terms of sea level air density, 4/3 its total mass, and on the ground a quarter of that volume is filled with air, allowing room for the lift gas to expand as the ship rises.
Also, helium has some mass itself, so the net lift gained by displacing a cubic meter of air with absolutely pure He at sea level is 1.05 kg/m^3 at best, and realistically for somewhat impure He assumed to be pretty much 1. Thus to lift a tonne, a total hull displacement of 4/3 1000 cubic meters is needed, and for two tonnes, some 2700 cubic meters.
The length of a "small" L ship then is going to be on the order of 40-50 meters, depending on the ratio of length to diameter.
This if you think about it is bloody huge. If a "small" advertising Goodyear blimp were brought down to an American football field and handled to land more or less in the center, its length would be about half that of the field! All this for just two tonnes of airship; a bigger K ship would be some 20-25 percent longer and wider. And yet, any of us who have ever looked up in the sky to look at a blimp see something that looks subjectively a lot smaller, doesn't it? I know this was my experience living near the Sonoma County Regional airport, where for a time in the later 90s and early 2000s, a lot of blimps tended to park. Generally speaking they are up in the sky, and people do not realize how very far away they are--but people with normal hearing (not me) can hear them. Because their engines are very very very loud! Deliberately so, left unmuffled to allow people to notice them and look. But occasionally one of them would pass right over my home, flying low, and then their loudness was just amazing. One had the thunderous noise of a radial piston engine unmuffled merely hundreds of meters away. Living near an airport where airshows would be performed, every now and then we'd get something like an F-18 going into a steep 45-60 degree climb from mere hundreds of meters altitude with its engines pointed right at our house, which was pretty deafening too. And we'd have flights of WWII vintage fighters and bombers converge and maneuver over our heads.
So I knew the sound of WWII vintage radials just 300 meters or so above my head. The difference with the blimp was, it
lingered. Of course it would be puttering along at 1/10 or less the airspeed of those airplanes so naturally what we heard was a buzzing rumble but it would change position only slowly.
Airships designed to fly with unmuffled engines would require newer, more powerful engines to overcome the power impediment muffling represents due to back pressure. So mufflers could be installed but the engine spec and the fuel consumption rate would have to both go up.
Bottom line--even if they aren't advertising ships, blimps are rather amazingly loud.
I believe K ships if perhaps not L ships were equipped with variable pitch props in WWII, and these might allow the engines to rein in the horsepower, but I don't think that would reduce the noise nearly as much as you think. To have some "steerage way," to make the fins effective, the engines must run.
There is no sneaking up on the sub without totally cutting the engines and hoping the wind, however carefully judged, will not shift and blow the airship off course--passing near the target but not over it, where it will be well in range of sub-borne AA. So depth charges are out, one would have to use guns and get direct hits to counter the offset.
Again if the airship is close enough, it is a big big object in the sky--even on a moonless night it is going to be blocking a lot of stars!