ASW damage from AC

[Lokasenna]

I've had several I-boats sink from a single 500-lb bomb hit. They never made it more than 4-5 hexes away from where they got hit.

[Bullwinkle58]

ORIGINAL: Lokasenna

I've had several I-boats sink from a single 500-lb bomb hit. They never made it more than 4-5 hexes away from where they got hit.

I've had USN fleet boats sink from it too. But after moving away, as you say. I've never had one sink in the attack hex, immediately. As before, not saying it can't happen.

[AW1Steve]

ORIGINAL: Chickenboy

ORIGINAL: Lokasenna

In my IJ game against the allied AI, I've lost 2 subs to 500-lb. bombs off the west coast. In deep water, too. I have a better grasp of detection level mechanics and such now...

This came up in another thread recently. Usually, ASW aircraft aren't very effective at actually damaging subs. Where they succeed is in raising the DL. I see that ops report message all the time, but I figure that maybe only 1 in 50 actually represents a successful attack. Especially from Catalinas...though that could just be a result of their longer range. I believe the IJN subs that I lost to ASW aircraft were to SBDs.

If you actually manage to hit a sub with a 500-lb. bomb, it's hurting. Noticed that from the above losses as well as several port strikes in various games - one decent bomb hit typically results in boom/glug-glug.

I've lost a small handful of subs to Allied ASW air attack-Catalinas. 500 lb. bombs did the trick.

It's difficult in the game as it was IRL to get a sub kill with an aerial bomb. From Clay Blair's Hitler's U-Boat War (Volume 1): a 500lb. bomb needed to land within about 10-15 feet of a surfaced submarine to cause lethal damage. Very difficult to do IRL.

So, OP, I'd assume that 75% of the "sub is reported HIT" messages are FOW. 25% are probably real. Most 500lb. (or 250kg bomb) "hits" (probably near misses IRL) will cause some pain requiring repair of the submarine. Rarely will you get an outright kill.

In order to increase the likelihood of a hit (or damaging near miss), trained bomber or patrol crews with an ASW rating of 70+ are advisable. Airframe weapon used is important too. A 500lb (or 250kg) bomb will do more damage than a 100kg (or 250lb) bomb, as you might imagine. Thus, select the best balance of airframe, range, ASW skill, altitude (<2000ft.) and co-ordination with other forces (ASW H/K groups, naval search, mines, etc.) for best effect.

Andre , in fairness, Blair was discussing type VII and type IX U-boats. EXTREMELY tough boats. Much deeper diving then USN or IJN boats , and I'd bet tighter seems too. I was always taught that a depth charge within 30 feet would probably be a kill. And btw , depth charge is a misnomer. Any bomb fitted with a hydrostatic charge is a depth charge. (I recall discussing with a couple of ordancemen the possibility of fitting a 2000 pound mine with such a fuse. They winced and said "Yeah that'll leave a mark.")[:D]

If I recall, not only will machine gun and air cannon mess up a sub, the RAF was real big on using Mosquitos with rockets as sub killers in the Bay of Biscay. And on that subject , I recommend "Submarines and Slide Rules", a real egg head study on ASW in the Bay (Some of you math geeks will drool over the studies, me? They gave me a migraine.)

I'd think the Moose will confirm that Shock is what kills subs , and opening the seems (and causing high pressure leaks) and not so much a direct hit.

My point is that I suspect the strength and condition of the boat has more to do with the kill of a sub then the size of the bomb or if it penetrates the hull. In that regard , aspect might be more important than explosion. A smaller bomb near the shaft seems or the torpedo seams might do more damage then one amidships. But I defer to the Moose. He's the subject matter expert.

[Chickenboy]

Hi Steve,

My comments (Blair's quote) was on aerial bombs dropped on submarines per se. These didn't have hydrostatic fuses and-based on his comments-made comparatively inefficient submarine killers. Note the accuracy comment when using aerial bombs with instantaneous fuses.

Most of the Coastal Defense Liberators, Cats and other (mostly) British ASW air assets operating against the type VIIs, IXs and XIVs used depth charges-these were much more efficacious. Particularly after 1942, Coastal aerial ASW racked up ungodly numbers of u-boat kills. Usually while the boats were surfaced. Usually with a well-laid field of depth charges near the boat during an overflight. These DCs were 'shallow-set' to explode at 25-50ft. depth. Usually the low flying aircraft would clear the area before the blast, but premature detonations did occasionally fatal damage to the bombing aircraft. When combined with the standing 'stay and fight' orders and a rapid fire 37mm Flak gun, these sorties were not without risk.

Yeah, the Mossies in the Bay of Biscay operation were occasionally successful with rockets (or their 57mm 'stinger'), but the underappreciated killers were the B-24s and other heavy aircraft with multiple shallow-set DCs. A typical pass would see 4-6 dropped atop the submarine. A very bad day for the submariners with 4-6 shallow-set DCs erupting around you. The boats (as strongly built as they were) would splinter, hole and huge sections stave in.

Incompressible fluid hydrodynamics-fascinating stuff.

ETA: As far as the game goes, I'm OK with assuming that the ASW aircraft are using depth charges per se. With that in mind, I'm mostly comfortable with the implementation of aerial ASW, the damage wrought and expected progress of ASW capabilities.

ETAII: It would be cool if the game could mimic use of aerial ASW "Fidos" for the Allies or some of the special anti-escort homing torpedoes ("mines") used by the Allies very late in the war. Both were used in the war, with the former being credited with a number of U-boat kills. I don't see why they wouldn't work (even better) against slow-diving, poorly manuevering IJN submarines by USN Avengers.

[Bullwinkle58]

ORIGINAL: AW1Steve

Andre , in fairness, Blair was discussing type VII and type IX U-boats. EXTREMELY tough boats. Much deeper diving then USN or IJN boats , and I'd bet tighter seems too. I was always taught that a depth charge within 30 feet would probably be a kill. And btw , depth charge is a misnomer. Any bomb fitted with a hydrostatic charge is a depth charge. (I recall discussing with a couple of ordancemen the possibility of fitting a 2000 pound mine with such a fuse. They winced and said "Yeah that'll leave a mark.")[:D]

If I recall, not only will machine gun and air cannon mess up a sub, the RAF was real big on using Mosquitos with rockets as sub killers in the Bay of Biscay. And on that subject , I recommend "Submarines and Slide Rules", a real egg head study on ASW in the Bay (Some of you math geeks will drool over the studies, me? They gave me a migraine.)

I'd think the Moose will confirm that Shock is what kills subs , and opening the seems (and causing high pressure leaks) and not so much a direct hit.

My point is that I suspect the strength and condition of the boat has more to do with the kill of a sub then the size of the bomb or if it penetrates the hull. In that regard , aspect might be more important than explosion. A smaller bomb near the shaft seems or the torpedo seams might do more damage then one amidships. But I defer to the Moose. He's the subject matter expert.

I'm no expert for sure. I looked up some sites on underwater explosion physics and the maths are pretty deep. They are also highly dependent on a lot of factors not modeled in the game.

German boats were advertised as deeper-diving. The late-war fleet boats could and did go a lot deeper than speced. QA was also far superior, and fleet boats after the P-class ( I think) were 100% welded, not riveted. U-boats were cranked out in sections, sometimes by forced labor, and put together in a big hurry. I'd take my chances going to 700 feet on a Balao a lot quicker than a Type VII.

Also, you and I both know that training is worth a lot when it comes to casualties. USN submarines got stronger in that area across the war since by 1943 1/3 of every returning boat's crew was detached for new construction. The men got a rest which the U-boat crews did not, and every new unit was already seasoned when it left the yards. The U-boat force got more and more green as the war went on. Nothing beats a response to flooding like having done it before.

When DCs were invented their role was seen as a mission-kill weapon more than a vessel killer. Sonically blind the sub, cause depth excursions, start flooding, let the skimmers fly past. If the sub sank it was a bonus, but a mission kill would do. By mid-WWII killing the sub itself was of at least equal importance. The hedgehog and similar contact weapons reflected this mindset. Either dead or a miss. No damage.

Lethal range depended on a lot of factors as I said, but a hull which was contacted by the steam bubble from the DC explosion was probably going to tear. Punching holes through pressure hulls was almost impossible except by shaped charges. DCs depended on incompressibility of seawater and the bubble's energy to deform the hull enough to either open a seam, or most often shear off hull fittings or shake loose hull fittings inside the boat inboard of hull isolation valves. If that happens the boat has little choice but to go shallow to reduce flooding rate and pound in a DC plug and shore it to close the hole. Takes time, makes noise. If the shock wave sprung a hatch, a shaft seal, or a tube door those were even harder than pipe breaks. Hatches were doubled but a sprung shaft seal was often fatal. The ER in most subs exceeds reserve buoyancy in the pressure hull when its flooded.

Thirty feet might be a good number. But you have to remember that's only five Vikings'-worth. [:)] And it's in 3D. A depth change is as good as an x,y change in getting over 30 feet of clearance. The steam bubble from the DC is roughly a sphere.

[crsutton]

ORIGINAL: Lokasenna

In my IJ game against the allied AI, I've lost 2 subs to 500-lb. bombs off the west coast. In deep water, too. I have a better grasp of detection level mechanics and such now...

This came up in another thread recently. Usually, ASW aircraft aren't very effective at actually damaging subs. Where they succeed is in raising the DL. I see that ops report message all the time, but I figure that maybe only 1 in 50 actually represents a successful attack. Especially from Catalinas...though that could just be a result of their longer range. I believe the IJN subs that I lost to ASW aircraft were to SBDs.

If you actually manage to hit a sub with a 500-lb. bomb, it's hurting. Noticed that from the above losses as well as several port strikes in various games - one decent bomb hit typically results in boom/glug-glug.

Hits are very rare in 1942 but it is not really the plane types so much as which ones carry search radar. That seems to increase the hit rate some. Most Allied naval attack aircraft have radar by 1944.

Numbers and experience count too. Viberpol had plenty of success with his planes vs me by training lots of Japanese pilots and saturating areas with patrolling aircraft. in 43 I would say that I had on average 20 subs either RTB or in the yard at any given time with damage from bomb hits. And a few sunk. A nuisance but not a game changer like the super E type ship was.

[crsutton]

ORIGINAL: Bullwinkle58

ORIGINAL: heibernt

Thx for the reply. Yea i know i can only find out via the ship loss screen, but i was wondering what kind of typical dmg such an attack would cause. Will the sub still be operational, or does it have to return to port, or will it likely sink on the way home? Im just wondering so i might be better able to evaluate his sublosses, and thereby there presence in my shipping lanes.

In the game my limited experience has been a typical 500lb/250kg bomb strike on a sub will generate between 30 flooding and 95 flooding. In most cases system damage is in the 40-60 range. Some engine damage. There are many randoms involved. I do not recall an "off screen" sinking of a sub by air ASW in a first attack. Not saying they can't happen.

In every case it has happened to me I have needed to send the boat home to the yards. If flooding is over 70 and system in the range 40-60 there is an excellent chance it will not make it. If flooding is in the 50s with that system damage it has a fair chance if speed is reduced to Cruise.

In my experience that's the envelope more or less. Your results may vary.

Yes, it a different story for Allies subs-even the small ones. A single bomb hit will never put one under, and most Japanese ASW planes have fairly small bombs so some hits do not even put the sub out of action. It takes multiple hits (rare) to sink an Allied sub. I had a lot of damaged subs heading home but Japanese air attacks were never a real threat.

[sandman455]

ORIGINAL: Bullwinkle58
Thirty feet might be a good number. But you have to remember that's only five Vikings'-worth. [:)] And it's in 3D. A depth change is as good as an x,y change in getting over 30 feet of clearance. The steam bubble from the DC is roughly a sphere.

My memory is rapidly fading but here is what I recall for single drop @ periscope depth for hull compromise -

15 kt nuke - sorry but you would be shocked and the numbers that follow would make a lot more sense
2000lb GP bomb (50% HE) 18-25'
500lb GP bomb 8-12'
100lb GP bomb 4-6'
1-99lb GP bomb 1-2'

Surface numbers would be about the same since the fuse would still put the weapon at the same depth. BTW, if you hit the target while its on the surface you'll probably get a hull breach with any bomb but should the fuse fail the sub will still have a decent shot at sailing home with a great sea story about how it blew you out of the air.

ORIGINAL: AW1Steve
If I recall, not only will machine gun and air cannon mess up a sub, the RAF was real big on using Mosquitos with rockets as sub killers in the Bay of Biscay.

Subs were pretty immune to light weapons. They would tear up the outer hull and fixtures but that's about it, no hull compromise. To get a decent chance at real damage you would need a 40-57mm gun and some steep angle hits less your rounds deflect off the outer hull or water. Since most patrol craft can't dive worth a crap, you are looking at rockets or bombs. Ships needed a 4" or better to have some good chance of success at penetrating water/hull or overcoming the high angle deflection angle.

ORIGINAL: Bullwinkle58
I'd take my chances going to 700 feet on a Balao a lot quicker than a Type VII.

Hell no. A WW2 Balao at 700' - you sir are as close to dead as can be. Please don't reference the Chopper - it was a Guppy, far removed from it's WW2 configuration and it never sustained the depth. Wasn't even on an even keel which makes the depth readings unreliable. When she surfaced, she was so overstressed she had to be scrapped. For any type VII or IX uboat, 700' was a walk in the park - documented countless times. A depth of 900-1000' would have been approx crush depth for most of them if you had the guts to go there. A few did and IIRC they were still serviceable afterwards.

ORIGINAL: Chickenboy
ETAII: It would be cool if the game could mimic use of aerial ASW "Fidos" for the Allies or some of the special anti-escort homing torpedoes ("mines") used by the Allies very late in the war. Both were used in the war, with the former being credited with a number of U-boat kills. I don't see why they wouldn't work (even better) against slow-diving, poorly manuevering IJN submarines by USN Avengers.

FIDO - it was kind of like a quick use mine. It homed in on noise - if I remember it was the band at which the diesel engines fuel injectors worked. Without that diesel running your weapon was worthless. It was the weapon of choice for a snorkled sub or any at night. Trying to hit a target with so little to aim at was pretty hard. Half the time you couldn't see the sub/snorkle in time to make a good drop. Any close aboard fly over could easily alert the sub ruining any chance of setting up again. This thing allowed the aircraft to pickle the weapon on the initial flyover or well ahead to remain unnoticed. Again, the weapon was going to be worthless if the enemy figured out how it worked.

[witpqs]

ORIGINAL: sandman455

ORIGINAL: Bullwinkle58
Thirty feet might be a good number. But you have to remember that's only five Vikings'-worth. [:)] And it's in 3D. A depth change is as good as an x,y change in getting over 30 feet of clearance. The steam bubble from the DC is roughly a sphere.

My memory is rapidly fading but here is what I recall for single drop @ periscope depth for hull compromise -

15 kt nuke - sorry but you would be shocked and the numbers that follow would make a lot more sense
2000lb GP bomb (50% HE) 18-25'
500lb GP bomb 8-12'
100lb GP bomb 4-6'
1-99lb GP bomb 1-2'

Surface numbers would be about the same since the fuse would still put the weapon at the same depth. BTW, if you hit the target while its on the surface you'll probably get a hull breach with any bomb but should the fuse fail the sub will still have a decent shot at sailing home with a great sea story about how it blew you out of the air.

So what does "sorry but you would be shocked" equal in numbers?

[Chickenboy]

ORIGINAL: sandman455
FIDO - it was kind of like a quick use mine. It homed in on noise - if I remember it was the band at which the diesel engines fuel injectors worked. Without that diesel running your weapon was worthless. It was the weapon of choice for a snorkled sub or any at night. Trying to hit a target with so little to aim at was pretty hard. Half the time you couldn't see the sub/snorkle in time to make a good drop. Any close aboard fly over could easily alert the sub ruining any chance of setting up again. This thing allowed the aircraft to pickle the weapon on the initial flyover or well ahead to remain unnoticed. Again, the weapon was going to be worthless if the enemy figured out how it worked.

This is contrary to the useage of the FIDO weapon system used against the U-boats, according to Blair.

The weapon was typically dropped by Avenger-type aircraft after the U-boat had submerged.
The attacks typically took place by daylight scout aircraft-often a pair of F4F and TBF/M. The F4F would strafe the submarine, forcing it to submerge. The Avenger would come in and drop on the swirl. Daylight attack, submerged U-boat.

The U-boats weren't using their diesels after submerging. Therefore, the guidance system of the FIDO was unlikely to be the diesel pressurization system.

[Bullwinkle58]

ORIGINAL: sandman455

ORIGINAL: Bullwinkle58
I'd take my chances going to 700 feet on a Balao a lot quicker than a Type VII.

Hell no. A WW2 Balao at 700' - you sir are as close to dead as can be. Please don't reference the Chopper - it was a Guppy, far removed from it's WW2 configuration and it never sustained the depth. Wasn't even on an even keel which makes the depth readings unreliable. When she surfaced, she was so overstressed she had to be scrapped. For any type VII or IX uboat, 700' was a walk in the park - documented countless times. A depth of 900-1000' would have been approx crush depth for most of them if you had the guts to go there. A few did and IIRC they were still serviceable afterwards.

Chopper's bow went deeper than 1000 feet. Yes, a Guppy had been modified from WWII, but the pressure hull was the same. The damage done to her which resulted in her re-designation was as much from the rapid ascent from the depth excursion as the excursion itself.

My comments on U-boats were not predicated on the design parameters or that many did go not to 700 feet. It was on QA. U-boat COs commented in their private writings that boats from different yards were of very different quality. As the war progressed this became highly relevant. Speer ordered that U-boats be made in sections and assembled at a central yard. Many of the contractors assigned sections had no submarine, or even naval, experience. They employed tens of thousands of slave laborers. The sections were "persuaded" together at final assembly with sometimes terrible results. Further, many of the sub-assemblies were subject to sabotage by French and Polish yard workers, as were the final boats when in refit.

The pre-1943 boats were well-built. By the end over 100 Type 21s were so poorly sub-assembled that only seven made it to sea before VE Day. After the war both the RN and USN instituted trial programs to peel the Type21 design and learn from it. The programs were scrapped when numerous accidents, including battery explosions, occurred with such frequency that the boats were scuttled in deep water and the technology was adopted from blueprints.

I believe there is a documented depth excursion in USS Tang to about 670 feet. Part of the problem was USN depth gauges pegged before crush depth, so a lot of the reports in the USN came from TR pressure gauges and not control room conning depth dials.

I wouldn't want to go to 700 in any WII sub. But I'd rather trust US QA over German.

[Chickenboy]

This from the Mk-24 FIDO Wiki: Interesting...

Upon water entry, FIDO performed a circular search at a predetermined depth controlled by a bellows and pendulum system. This continued until the potential target's 24 kHz acoustic signal detected by the hydrophones exceeded a predetermined threshold level, at which point control was then shifted to the passive acoustic proportional homing system. Initially the torpedoes were set to search for a target at a depth of 50 feet (15 m), this was later changed to 150 feet (45 m). To prevent the torpedo accidentally attacking surface ships, it resumed its circling search if it rose above a depth of 40 feet (12 m).

The torpedo's relatively low speed was kept secret because although U-boats could not outrun the torpedo when submerged, they could outrun it on the surface.

Combat history

On 14 May a Catalina of the US Navy attacked and destroyed a U-boat; this was either U-657[3] or U-640.[4] On 13 May Liberator B/86 had attacked a U-boat with a FIDO, but this vessel, U-456,[5] was only damaged, sinking the following day from damage received. One of these vessels was the first U-boat sinking achieved using FIDO. During its career, the torpedo sank a total of 37 submarines, achieving an effectiveness of about 22%, compared with about 9% for depth charges.

from US Navy OEG Study No. 289, 12 August 1946 provides the following data related to Mark 24 effectiveness:

Number of attacks in which Mark 24s were launched 264
Total number of Mark 24 torpedoes launched - all targets 340
Number of Mark 24s launched against submarines 204
Number of Mark 24 attacks on submarines by US aircraft 142
Number of Mark 24 attacks by Allied (primarily British) aircraft 62
Number of German U-boats sunk by FIDO 31
Number of German U-boats damaged by FIDO 15
Number of Japanese submarines sunk by FIDO 6
Number of Japanese submarines damaged by FIDO 3
Total number of submarines sunk by FIDO (German & Japanese) 37
Total number of submarines damaged 18

[btbw]

Fido was tuned for search noise under 40 feet. So it can be used against subs entereing in convoy's order. It why better use for Fido SUBMERGED sub.

[sandman455]

ORIGINAL: Chickenboy

ORIGINAL: sandman455
FIDO - it was kind of like a quick use mine. It homed in on noise - if I remember it was the band at which the diesel engines fuel injectors worked. Without that diesel running your weapon was worthless. It was the weapon of choice for a snorkled sub or any at night. Trying to hit a target with so little to aim at was pretty hard. Half the time you couldn't see the sub/snorkle in time to make a good drop. Any close aboard fly over could easily alert the sub ruining any chance of setting up again. This thing allowed the aircraft to pickle the weapon on the initial flyover or well ahead to remain unnoticed. Again, the weapon was going to be worthless if the enemy figured out how it worked.

This is contrary to the useage of the FIDO weapon system used against the U-boats, according to Blair.

The weapon was typically dropped by Avenger-type aircraft after the U-boat had submerged.
The attacks typically took place by daylight scout aircraft-often a pair of F4F and TBF/M. The F4F would strafe the submarine, forcing it to submerge. The Avenger would come in and drop on the swirl. Daylight attack, submerged U-boat.

The U-boats weren't using their diesels after submerging. Therefore, the guidance system of the FIDO was unlikely to be the diesel pressurization system.

Who is Blair? So are you and Mr. (Ms?) Blair suggesting that you could passively track and acquire a submarine running on batteries?? You guys are a heck of a lot better than the clowns I flew with.

My guess is that your Blair is an avid historian/author and has simply regurgitated what little there is out there on that particular weapon system. And you sir are a vet from the midwest who might not think too highly of the USN's officer corp, its naval flight officers and S-3 crew members in particular.

fb.asp?m=3162068&key=last%2Cminute%2Cin ... %2Cofficer

I still wonder where you got that such a negative assessment of that small group of officers. Oh well, no ill will here. I'm not much for making judgments on stuff I know little about. What I do know a lot about is ASW. So if you would like to know more about the Mk24 FIDO Mr Chickenboy and Mr/Ms Blair please read on.

The Mk24 FIDO was a rather peculiar weapon. Look at its payload of 92lbs HE. What were they thinking? Here I am telling you all that 1000lbs of HE or even 30 million lbs of HE just isn't much when trying to kill a submarine designed to handle to the overpressure of copious amounts of water. Why would anyone ever want to carry such a weapon? There's your B-24 and they are loading it up with a FIDO. Would you not be tempted to tell those guys to skip with the FIDO and go with the 2000lb bruiser?

Well perhaps after a few more flight hours you might find that spotting snorkels and periscopes may be challenging, but targeting them is nothing short of a real bitch. There you are cruising along and someone blurps out "holy shit - periscope 3 o'clock" followed by the high banked turn, followed by the customary "where the hell did it go." Lots of shouting and confusion – ugh gives me headaches even today. And you will find the exact same issue happens at night with the whole damn boat on the surface. While it is relatively easy to locate and attack a surfaced or submerging sub in daylight, trying to reacquire something very small or in the dark can be most challenging. Simply put, your visibility on the target is approaching your turning radius. So if you can't see it from far enough out, how can you put together an effective attack run before it gets a clue and dives? This is the very reason why the FIDO was invented in the first place. Someone thought that if you just poke a hole in the bugger that forces him up on the surface and during daylight everybody can get back to the business of sinking them. It had the beginnings of being a pretty good weapon.

When the Mk24 FIDO first came out it was really hush hush stuff. It's probably why Blair and the rest of you enthusiasts can't find much about it. Even the USN's own historians appear to be struggling for data if their website is any indication. I'm not surprised. Very classified material gets destroyed when it is no longer useful. There is never any dissemination. This weapon was TS level stuff and its life cycle was really short. When it became useless, most of the data was destroyed or remained far too classified for PR types publish.

So why did the USN put all the secrecy around this rather small, short lived weapon? Seriously, they even went to the trouble of misclassifying it as a mine just to keep prying eyes off the scent of what it really was. Now that’s hardcore. Can you think about any other weapon systems where they did stuff like that!? I can, but I'll save that story for another beer tab.

Let’s take a quick look at the hydrogen bomb which was arguably the most classified project ever to be undertaken. You didn't see them call it an "incendiary device" after they had it all ironed out and ready to go boom. Why not? The thing was one heck of a game changer! I mean it ended wars. Yet nobody gave a care about keeping it all top secret. The beans were spilled that the good guys were splitting atoms and had become death, the destroyer of worlds, etc etc. You know why? Because it worked and there was nothing the bad guys could do to change that. When a weapon system works - there is little need to disguise it other than to keep them from finding a counter. Since you can't really counter a hydrogen bomb, there is very little need to keep a super high classification. What does that say about the Mk24 FIDO? Oh yeah, this thing was so sensitive that the bad guys already had the counter installed in their subs. Bummer! I guess maybe we should go to some crazy lengths to keep them from finding that out!

Is any of this making sense?

So what does this have to do with our Mk24 homing in on subs and junk. Well I sure as heck don't want to spend the next 5 hours explaining passive and active acoustics. All I'm going to give you is that you’ve got two ways of acoustically locating a sub. Listen for its sound signatures (passive) or generate your own sound and wait for it to return off the sub (active). Guess which one is super accurate and which one is just horrible for targeting data? Go watch "The Hunt for Red October" again. You know they did a lot of things right in that movie. Remember the ending were Sean Connery is trying to dodge the torpedo - that puppy isn’t using passive acoustics. In fact almost all of our torpedoes and our enemies too have used active homing. So that must mean the Mk24 was an active homing torpedo right? Nope, it was passively homed. The plot thickens . . .

First let’s explain a little about passive acoustics. There is both broadband and narrowband. Broadband is like “whoa that thing is loud.” Our ears and brain combined to form a good broadband sensor/processor. Broadband noise is usually what we are referring to when we complain about something being too loud or too hard to hear. But we can also process narrowband stuff as well. Otherwise we would not know to turn the burner off when the tea kettle is whistling. That’s a simple example of narrow band processing by your brain and ears. Certain things make certain sounds so when we hear the frequency we know something is afoot.

So what should we use to find subs? Well the ocean is louder than your neighbor’s last party. So broadband isn't looking good. What’s more is that surface ships are far louder than subs so trying to find one in and around your TF or convoy will be hard if you are looking for a broadband acoustic signature. Lucky for ASW, narrowband works real well. In fact when it comes to man-made machinery it is outstanding. All those moving parts make noise on very specific frequencies. Knowing your enemies frequency signatures is the first step in finding him using passive acoustics.

Today (or at least back in my day - 1990's) you would use a computer processor to create a nice waterfall display of the frequency spectrum over time picked up by your microphone(s). Back in FIDO's day computers weren't an option. They opted to use a simple 4 hydrophone array (one each axis – brilliant!) operating at 24 kHz, which in turn were connect to some vacuum tube steering mechanism. I’d say that’s pretty high tech stuff for 1942, wouldn’t you? So what does the 24 kHz mean? Well that's the only frequency those 4 hydrophones were going to hear. It's actually a pretty high freq – well above what our 20 Hz to 20 kHz capabilities. So why in the world did they choose 24 kHz?

http://www.bc.cityu.edu.hk/staff/bcaleu ... 24-645.pdf

Sorry it was all I could find on the internet with very limited time. Anyway, look at page 647-648. The chart on a pg 648 should be all you need to see. I'm no acoustic expert but I'd bet dollars to donuts that all diesel fuel injectors – since they are squirting the same liquid will pump out the same frequency noise.

Are you still with me?

If you are, I’m sure Mr. Blair is just itching to ask if the screw of a submarine also turned out 24 kHz. Or for that matter, is there any other part of a sub that makes noise in this 24 kHz range? In a word – no. There isn't much left for noise other than the skipper shouting orders and if he did it at 24 kHz, only his dachshund would be hearing it. A bottomed sub on batteries makes about as much noise as a flashlight at the bottom of your pool. They’ve got pumps, but the sub just dove and they would hardly be a reliable source to pick if you are attacking as he is initiating a dive. Honestly the only thing moving on that sub beside intermittent control surfaces is the shaft and the propeller. What kind of noise do they make? Easy question; since air and water are much the same when it comes to sound propagation, go turn on your ceiling fan. It has blades just like a propeller and it has a shaft powered by electricity. Sure doesn’t look good homing in the acoustic signature of a prop or shaft does it?

Yes, even using 1990 technology you are not going to reliably track, let alone attack a sub’s using the acoustic properties of its shaft or prop. And lest you have even the smallest amount of doubt of what I say - what do you think would put more noise into the narrowband frequencies associated with a turning shaft/prop – the big slow turning one a few hundred yards away or that small fast spinning one 3 feet from hydrophones on the tail of the torpedo? Ouch – guess that torpedoed the prop theory.

So yeah, the data you are seeing out there about the FIDO doing this or that is old and quite unreliable. The weapon was obsolete just as soon as knowledge submariners figured out it was homing on the diesel engine. Remember, they called this thing a mine. They certainly had no problem telling you Avengers would be strafing the sub to make it work - got to admit that made me laugh - because once that propeller got submerged . . . err . . . MORE submerged, it was going to be rocking out at 24 kHz. They would have told you anything to conceal the fatal weakness which was so big that just flipping an internal switch on the sub made the weapon useless.

Thanks for reading.

[Bullwinkle58]

Blair is Clay Blair.

24 kHz might be the weapon's sensor range. I don't know; I'll take your word for it. Above that was probably technically hard to deal with in the early 1940s in a pre-digital environment.

But a diving sub makes lots of narrowband noise other than the prop and control surfaces. What do you think turns the prop? If you have ASW experience in the 90s you know what I'm referring to. It's almost a certainty that the trim pump is going to be screaming too as the superstructure goes under. Also the hydraulic plant is going to be running full out to answer plane and rudder orders, especially if an evasion course is ordered as way comes on and the hull gets under.

A 24kHz sensor gets surfaced subs on diesel, and it gets snorkeling subs that don't know they've been popped. It doesn't get a fully-submerged sub on batteries (although in the Atlantic nobody was going down to sit on the bottom. In mid-ocean we were in circa 19,000 feet quite a bit.) I assume the FIDO was prototyped before snorkels were known. In that case it's great for catching a sub on the surface charging batteries, at night. It could be dropped on the run in, before the gyrations and come-around you describe.

It's also a good anti-diesel-powered ship weapon for a USN fleet boat to shoot as it went deep, pursued by a bear. But that's another story and as I recall another acronym.

[AW1Steve]

ORIGINAL: sandman455

ORIGINAL: Chickenboy

ORIGINAL: sandman455
FIDO - it was kind of like a quick use mine. It homed in on noise - if I remember it was the band at which the diesel engines fuel injectors worked. Without that diesel running your weapon was worthless. It was the weapon of choice for a snorkled sub or any at night. Trying to hit a target with so little to aim at was pretty hard. Half the time you couldn't see the sub/snorkle in time to make a good drop. Any close aboard fly over could easily alert the sub ruining any chance of setting up again. This thing allowed the aircraft to pickle the weapon on the initial flyover or well ahead to remain unnoticed. Again, the weapon was going to be worthless if the enemy figured out how it worked.

This is contrary to the useage of the FIDO weapon system used against the U-boats, according to Blair.

The weapon was typically dropped by Avenger-type aircraft after the U-boat had submerged.
The attacks typically took place by daylight scout aircraft-often a pair of F4F and TBF/M. The F4F would strafe the submarine, forcing it to submerge. The Avenger would come in and drop on the swirl. Daylight attack, submerged U-boat.

The U-boats weren't using their diesels after submerging. Therefore, the guidance system of the FIDO was unlikely to be the diesel pressurization system.

Who is Blair? So are you and Mr. (Ms?) Blair suggesting that you could passively track and acquire a submarine running on batteries?? You guys are a heck of a lot better than the clowns I flew with.

My guess is that your Blair is an avid historian/author and has simply regurgitated what little there is out there on that particular weapon system. And you sir are a vet from the midwest who might not think too highly of the USN's officer corp, its naval flight officers and S-3 crew members in particular.

fb.asp?m=3162068&key=last%2Cminute%2Cin ... %2Cofficer

I still wonder where you got that such a negative assessment of that small group of officers. Oh well, no ill will here. I'm not much for making judgments on stuff I know little about. What I do know a lot about is ASW. So if you would like to know more about the Mk24 FIDO Mr Chickenboy and Mr/Ms Blair please read on.

The Mk24 FIDO was a rather peculiar weapon. Look at its payload of 92lbs HE. What were they thinking? Here I am telling you all that 1000lbs of HE or even 30 million lbs of HE just isn't much when trying to kill a submarine designed to handle to the overpressure of copious amounts of water. Why would anyone ever want to carry such a weapon? There's your B-24 and they are loading it up with a FIDO. Would you not be tempted to tell those guys to skip with the FIDO and go with the 2000lb bruiser?

Well perhaps after a few more flight hours you might find that spotting snorkels and periscopes may be challenging, but targeting them is nothing short of a real bitch. There you are cruising along and someone blurps out "holy shit - periscope 3 o'clock" followed by the high banked turn, followed by the customary "where the hell did it go." Lots of shouting and confusion – ugh gives me headaches even today. And you will find the exact same issue happens at night with the whole damn boat on the surface. While it is relatively easy to locate and attack a surfaced or submerging sub in daylight, trying to reacquire something very small or in the dark can be most challenging. Simply put, your visibility on the target is approaching your turning radius. So if you can't see it from far enough out, how can you put together an effective attack run before it gets a clue and dives? This is the very reason why the FIDO was invented in the first place. Someone thought that if you just poke a hole in the bugger that forces him up on the surface and during daylight everybody can get back to the business of sinking them. It had the beginnings of being a pretty good weapon.

When the Mk24 FIDO first came out it was really hush hush stuff. It's probably why Blair and the rest of you enthusiasts can't find much about it. Even the USN's own historians appear to be struggling for data if their website is any indication. I'm not surprised. Very classified material gets destroyed when it is no longer useful. There is never any dissemination. This weapon was TS level stuff and its life cycle was really short. When it became useless, most of the data was destroyed or remained far too classified for PR types publish.

So why did the USN put all the secrecy around this rather small, short lived weapon? Seriously, they even went to the trouble of misclassifying it as a mine just to keep prying eyes off the scent of what it really was. Now that’s hardcore. Can you think about any other weapon systems where they did stuff like that!? I can, but I'll save that story for another beer tab.

Let’s take a quick look at the hydrogen bomb which was arguably the most classified project ever to be undertaken. You didn't see them call it an "incendiary device" after they had it all ironed out and ready to go boom. Why not? The thing was one heck of a game changer! I mean it ended wars. Yet nobody gave a care about keeping it all top secret. The beans were spilled that the good guys were splitting atoms and had become death, the destroyer of worlds, etc etc. You know why? Because it worked and there was nothing the bad guys could do to change that. When a weapon system works - there is little need to disguise it other than to keep them from finding a counter. Since you can't really counter a hydrogen bomb, there is very little need to keep a super high classification. What does that say about the Mk24 FIDO? Oh yeah, this thing was so sensitive that the bad guys already had the counter installed in their subs. Bummer! I guess maybe we should go to some crazy lengths to keep them from finding that out!

Is any of this making sense?

So what does this have to do with our Mk24 homing in on subs and junk. Well I sure as heck don't want to spend the next 5 hours explaining passive and active acoustics. All I'm going to give you is that you’ve got two ways of acoustically locating a sub. Listen for its sound signatures (passive) or generate your own sound and wait for it to return off the sub (active). Guess which one is super accurate and which one is just horrible for targeting data? Go watch "The Hunt for Red October" again. You know they did a lot of things right in that movie. Remember the ending were Sean Connery is trying to dodge the torpedo - that puppy isn’t using passive acoustics. In fact almost all of our torpedoes and our enemies too have used active homing. So that must mean the Mk24 was an active homing torpedo right? Nope, it was passively homed. The plot thickens . . .

First let’s explain a little about passive acoustics. There is both broadband and narrowband. Broadband is like “whoa that thing is loud.” Our ears and brain combined to form a good broadband sensor/processor. Broadband noise is usually what we are referring to when we complain about something being too loud or too hard to hear. But we can also process narrowband stuff as well. Otherwise we would not know to turn the burner off when the tea kettle is whistling. That’s a simple example of narrow band processing by your brain and ears. Certain things make certain sounds so when we hear the frequency we know something is afoot.

So what should we use to find subs? Well the ocean is louder than your neighbor’s last party. So broadband isn't looking good. What’s more is that surface ships are far louder than subs so trying to find one in and around your TF or convoy will be hard if you are looking for a broadband acoustic signature. Lucky for ASW, narrowband works real well. In fact when it comes to man-made machinery it is outstanding. All those moving parts make noise on very specific frequencies. Knowing your enemies frequency signatures is the first step in finding him using passive acoustics.

Today (or at least back in my day - 1990's) you would use a computer processor to create a nice waterfall display of the frequency spectrum over time picked up by your microphone(s). Back in FIDO's day computers weren't an option. They opted to use a simple 4 hydrophone array (one each axis – brilliant!) operating at 24 kHz, which in turn were connect to some vacuum tube steering mechanism. I’d say that’s pretty high tech stuff for 1942, wouldn’t you? So what does the 24 kHz mean? Well that's the only frequency those 4 hydrophones were going to hear. It's actually a pretty high freq – well above what our 20 Hz to 20 kHz capabilities. So why in the world did they choose 24 kHz?

http://www.bc.cityu.edu.hk/staff/bcaleu ... 24-645.pdf

Sorry it was all I could find on the internet with very limited time. Anyway, look at page 647-648. The chart on a pg 648 should be all you need to see. I'm no acoustic expert but I'd bet dollars to donuts that all diesel fuel injectors – since they are squirting the same liquid will pump out the same frequency noise.

Are you still with me?

If you are, I’m sure Mr. Blair is just itching to ask if the screw of a submarine also turned out 24 kHz. Or for that matter, is there any other part of a sub that makes noise in this 24 kHz range? In a word – no. There isn't much left for noise other than the skipper shouting orders and if he did it at 24 kHz, only his dachshund would be hearing it. A bottomed sub on batteries makes about as much noise as a flashlight at the bottom of your pool. They’ve got pumps, but the sub just dove and they would hardly be a reliable source to pick if you are attacking as he is initiating a dive. Honestly the only thing moving on that sub beside intermittent control surfaces is the shaft and the propeller. What kind of noise do they make? Easy question; since air and water are much the same when it comes to sound propagation, go turn on your ceiling fan. It has blades just like a propeller and it has a shaft powered by electricity. Sure doesn’t look good homing in the acoustic signature of a prop or shaft does it?

Yes, even using 1990 technology you are not going to reliably track, let alone attack a sub’s using the acoustic properties of its shaft or prop. And lest you have even the smallest amount of doubt of what I say - what do you think would put more noise into the narrowband frequencies associated with a turning shaft/prop – the big slow turning one a few hundred yards away or that small fast spinning one 3 feet from hydrophones on the tail of the torpedo? Ouch – guess that torpedoed the prop theory.

So yeah, the data you are seeing out there about the FIDO doing this or that is old and quite unreliable. The weapon was obsolete just as soon as knowledge submariners figured out it was homing on the diesel engine. Remember, they called this thing a mine. They certainly had no problem telling you Avengers would be strafing the sub to make it work - got to admit that made me laugh - because once that propeller got submerged . . . err . . . MORE submerged, it was going to be rocking out at 24 kHz. They would have told you anything to conceal the fatal weakness which was so big that just flipping an internal switch on the sub made the weapon useless.

Thanks for reading.



[/quote
Sorry , I totally disagree with most of the points you made. Blair is a ww2 Submariner who became a historian , in fact many ww2 submariners that I've interviewed refer to him as THE WW2 Submarine historian , largely to some very good connection with the post ww2 submarine brass. He did several official histories.

I didn't realize that the FIDO was in service at the same time you were , or that it had a S-3 NFO connection.

Maybe you can't track a submarine on batteries, but I know many people who can and have. Crews that I've served on have on many occasions.

Clowns? Nice. You treat your former shipmates as well as people on the forum.

I'd go into more detail , but don't want to 1) go into techniques that may or may not still be classified. 2) want to waste any time as it's only a matter of time before you called me liar again , no matter what I say 3) lose my temper and say something I'd regret and get banned from the forum.

So let me simply say that I totally disagree with you on most everything you've said here. Not surprise, I generally do. [:(]

[witpqs]

I'm also questioning Sandman's dismissal of "92 lbs HE", which seems to ignore the existence of shaped charges.

[Bullwinkle58]

ORIGINAL: AW1Steve

Sorry , I totally disagree with most of the points you made. Blair is a ww2 Submariner who became a historian , in fact many ww2 submariners that I've interviewed refer to him as THE WW2 Submarine historian , largely to some very good connection with the post ww2 submarine brass. He did several official histories.

I didn't realize that the FIDO was in service at the same time you were , or that it had a S-3 NFO connection.

Maybe you can't track a submarine on batteries, but I know many people who can and have. Crews that I've served on have on many occasions.

Clowns? Nice. You treat your former shipmates as well as people on the forum.

I'd go into more detail , but don't want to 1) go into techniques that may or may not still be classified. 2) want to waste any time as it's only a matter of time before you called me liar again , no matter what I say 3) lose my temper and say something I'd regret and get banned from the forum.

So let me simply say that I totally disagree with you on most everything you've said here. Not surprise, I generally do. [:(]

I don't want to get between you two, but I don't discount everything he said. Thinking more about it.

I think he was saying that tracking a sub on batteries--from the air--in WWII wasn't happening. No sonobuoys or helos.

But the 24kHz thing does make sense to me on a technology level. Where I have trouble is in the tactics. When a D/E dives three things happen instantly in the ER when the diving alarm goes off. One of (usually) two watchstanders pulls the outboard induction valve shut. He then pulls the inboard induction valve shut. If the diesels are still running they're pulling on the people tank air. Very painful, very fast. The second guy therefore stops the diesels as fast as possible. How fast? From the diving alarm fifteen seconds would be pretty good. That gives maneuvering time to get the main motors up and going on the battery so the OOD has way to evade as soon as the hull is under. You don't turn in the initial dive if you can help it. Messes up the angle, and slows you waaaay down. Get under, then change course.

So say it's night. A FIDO equipped patrol plane finds a surfaced U-boat charging batteries. Maybe it's got radar, maybe it's using it sparingly for RDF reasons. Probably by that era the plane has radar too, but maybe it's off for the same detection reasons. The plane starts a run in. How far out does it drop the FIDO? The fish has to drop, activate, stabilize, and listen for diesel injectors that have, at best, about 15-20 seconds of noise, then they're gone. Does the FIDO get one good steer and then become a straight-runner? That might be enough if it's dropped close enough to the sub, but aspect and sub speed are going to drive the contact geometry. If too far out or not broad on the beam the sub can get out of the cone of contact pretty easily and then your expensive FIDO is just a mini-me straight-runner.

But if it does get a hit of injectors, and does a 30-degree steer to compensate for the bad drop aspect given by the plane's hurrying crew, it could increase hit percentage I'd say. Not to what an active homer could do, but better than a gravity bomb dropped in the dark.

[witpqs]

ORIGINAL: witpqs

I'm also questioning Sandman's dismissal of "92 lbs HE", which seems to ignore the existence of shaped charges.

Further on this, according to Wikipedia Hedgehog had 30 pounds of TNT or 35 pounds of Torpex. Did Hedgehog sink any subs?

http://en.wikipedia.org/wiki/Hedgehog_(weapon)

[witpqs]

ORIGINAL: Bullwinkle58

ORIGINAL: AW1Steve

Sorry , I totally disagree with most of the points you made. Blair is a ww2 Submariner who became a historian , in fact many ww2 submariners that I've interviewed refer to him as THE WW2 Submarine historian , largely to some very good connection with the post ww2 submarine brass. He did several official histories.

I didn't realize that the FIDO was in service at the same time you were , or that it had a S-3 NFO connection.

Maybe you can't track a submarine on batteries, but I know many people who can and have. Crews that I've served on have on many occasions.

Clowns? Nice. You treat your former shipmates as well as people on the forum.

I'd go into more detail , but don't want to 1) go into techniques that may or may not still be classified. 2) want to waste any time as it's only a matter of time before you called me liar again , no matter what I say 3) lose my temper and say something I'd regret and get banned from the forum.

So let me simply say that I totally disagree with you on most everything you've said here. Not surprise, I generally do. [:(]

I don't want to get between you two, but I don't discount everything he said. Thinking more about it.

I think he was saying that tracking a sub on batteries--from the air--in WII wasn't happening. No sonobuoys or helos.

But the 24kHz thing does make sense to me on a technology level. Where I have trouble is in the tactics. When a D/E dives three things happen instantly in the ER when the diving alarm goes off. One of (usually) two watchstanders pulls the outboard induction valve shut. He then pulls the inboard induction valve shut. If the diesels are still running they're puling on the people tank air. Very painful, very fast. The second guy therefore stops the diesels as fast as possible. How fast? From the diving alarm fifteen seconds would be pretty good. that gives maneuvering time to get the main motors up and going on the battery so the OOD has way to evade as soon as the hull is under. You don't turn in the initial dive if you can help it. Messes up the angle, and slows you waaaay down. Get under, then change course.

So say it's night. A FIDO equipped patrol plane finds a surfaced U-boat charging batteries. Maybe it's got radar, maybe it's using it sparingly for RDF reasons. Probably by that era the plane has radar too, but maybe it's off for the same detection reasons. The plane starts a run in. How far out does it drop the FIDO? The fish has to drop, activate, stabilize, and listen for diesel injectors that have, at best, about 15-20 seconds of noise, then they're gone. Does the FIDO get one good steer and then become a straight-runner? That might be enough if it's dropped close enough to the sub, but aspect and sub speed are going to drive the contact geometry. If too far out or not broad on the beam the sub can get out of the cone of contact pretty easily and then your expensive FIDO is just a mini-me straight-runner.

But if it does get a hit of injectors, and does a 30-degree steer to compensate for the bad drop aspect given by the plane's hurrying crew, it could increase hit percentage I'd say. Not to what an active homer could do, but better than a gravity bomb dropped in the dark.

BTW, are you accounting for the fact that FIDO was dropped so close to the sub? The closeness might have forgiven a lot of shortcomings in the sensors.