German G7e torpedo with FAT (Federapparat), 1943

The Federapparat (FAT) or ‘spring device’ was developed to increase the probability of hitting a ship or escort in a convoy. The idea was to make the torpedo zig-zag across the convoy if it missed the initial target. The FAT run structure had 2 parts - an initial straight run to close the intended target, and to ensure the torpedo was clear of the firing submarine before it reversed course, and a series of course reversals where the torpedo turned through 180 degrees after running a defined leg of 1,200 m or 1,900m.

The FAT was introduced in 1943, initially for the wet heater air-driven G7a torpedo, but restricted to nighttime attacks in case the allies observed the looping paths of the air trails. Use in the Mediterranean and North sea were also forbidden due to the clearer water.

The electric G7e torpedo was a candidate of the FAT, but was restricted initially by its limited range - 5,000m. With the increasing convoy escort convoy capability, it was becoming dangerous to approach too close to a convoy. However, the German engineers managed to upgrade the batteries by developing a 17-plate cell (previous their lead-acid cells had 13 plates) which delivered a 50% range increase for the torpedo at 30 knots. This torpedo was designated the TIIIa, and is the subject of this animation.

The TIIIa used compressed air to drive its control surfaces: the Gyro, Gyro servo to operate the rudders and depth servo to maintain the desired depth. The FAT was also powered by comprised air. Because of the increased range capability (and run time), two additional compressed air flasks were fitted at the front of the battery compartment. The control air did not leave a significant trail, however, so the torpedo could be used in all theatres and time of day.

The FAT had 4 control disks, one for each of the run paths - long loop, initial turn to the right, long loop turn to the left, short loop right and short loop left. The run path and initial straight run distance were set using a single input shaft. 4 turns of the shaft would firstly select the initial run distance, and then physically raise the control disk set to the next run path type. Thus 16 turns of the shaft would cycle through all 4 run types and return to the default setting - 15,000m initial run for Long loop path, turn first to the right.

When the setting cam set was being raised (or lowered) the indicator shows the marking ‘flasch’ (wrong), since if the torpedo was fired with ‘falsch’ showing, the initial run would be zero, and the tun to left or right ambiguous, thus endangering the firing submarine. The torpedo tube-mounted setting device physically prevents a setting when ‘falsch’ is showing.

The control disks have a set of ridges, valleys and mid-levels. A control piston rides on these cams. In the mid-level, the gyro controls the rudder servo in the normal manner. However, when a ridge or valley is encountered, the air driven FAT piston forces the rudder servo to the left or right, and so turns the torpedo through 180 degrees. The Gyro outer gimble now has two control cams, one to steer the selected fire control gyro angle, and the other (lower) to steer the reciprocal course.

The FAT was relatively short lived. With a FAT, the convoy would pass over the looped run path. However, the next variant, LUT, was arranged to advance the loop pattern along a convoy’s course and speed. This is too complex for me, and I shall not attempt to animate it.

See also my YouTube animation of the G7e torpedo:

The animation was made using Cinema 4D and Quicktime Player v7

Music tracks are:

References:
‘Die Torpedos der deutschen U-Boote’ Eberhard Rösseler, 2005, ISBN 3-8132-08427
RM6-3149 Federapparat für den Torpedo G7e, 1942

[timenginemannd]

I just realized that an additional benefit of the FAT Torpedo is that the explosion can occur on the opposite side of a ship than the sub is on! This means the Destroyers and Destroyer Escort would like go off in the wrong direction looking for a U-Boat that wasn't there!!!

[cccooooooolllllllll]

Impressive engineering, the FAT is basically a mechanical programmable computer, impressive to see such a system fitted inside a torpedo.

Thanks for the video and the explanation on the usefullness of the system.

[wladi]

The alternative meaning of FAT was the "Flächen-Absuch-Torpedo", translated to something like "Area-Search-Torpedo".

Its successor was the LUT-Torpedo, "Lageunabhägiger Torpedo", translated to "Orientation-Indrpendent-Torpedo", which could make even more complicated turns and patterns.

In my opinion, the FAT, the LUT and the passive-acoustic, self-homing torpedos "Zaunkönig" and "Falke" were most impressive for the war era in the Kriegsmarine.

Also be noted, the Kriegsmarine did experiment with an alternate proplusion system for the U-Boats and the Torpedos, called the "Walter-Antrieb" based an hydrogen peroxyde. Very impressive speeds were reached while testing.

[kiwihame]

I don't know what's more impressive, the German's amazing engineering, or your incredible video. So well explained. I'm deeply impressed with your work Rob. Thanks for sharing this with us!

[pieterveenders]

These mechanical simulations of the torpedo mechanism and the maritime simulations of them being fired into a convoy are amazing, especially how you did all the torpedo internals! You must have put a lot of time and effort into finding out how they worked and producing the simulations, I doubt it could have been easy finding that information? Your video's deserve to be on something like Discovery Channel or National Geographic!

[christopherjames]

Bloody Hell, never knew they where that complicated !

[TheHepsterproduction]

The effort put into making this video must have been huge. Great job

[helipilotuh]

Had to be terrifying the first time the sailors witnessed that torpedo turning around.

[godofdefeat]

Great to see that you are still working on stuff like this 🙂

[Aitelly]

This is well-research content. You give inspiration to small 3d Animation like ourselves.
But I wonder why YouTube is not recommending your channel to more people.
I mean this is the Real Engineering Channel.
All the best Again.

[UnknownUser-pfrk]

Me, a modern day engineer, having no idea what I'm even looking at! Who on earth would come up with something ingenious as this?

[SolarWebsite]

Amazing how they did this with purely mechanical means 80 years ago. Nowadays, a 10 cent IC would do all this, but having watched this that feels like cheating 😉

[bumblebeebob]

Fascinating! I had no idea that such a system existed! Thank you!

[JM-ntfm]

Absolutely incredible animation and attention to detail. This is so much work. Truly impressive!!!

[homesqueeze]

Glad to see your still posting these feats of engineering, via that smooth animation. You deserve a gig doing this for the history/military channel. Best of luck and thank you brother.

[gvii]

I was waiting for this. I have no idea where you find the technical data and drawings for your modeling, I have to believe they're not exactly what you'd call readily available. But I'm really glad you are able to find them. These torpedo videos are absolutely fascinating to me. I have always wondered how the FaT fish worked. Amazing bit of mechanical computing. Thank you very, very much for putting in the time to do these. You have no idea how much I appreciate what you do.

[thothru]

Wahnsinn, was ohne Computer möglich gewesen ist. Deutsche Ingenieurskunst 👍🏻

[havardhovdet]

Very nice and detailed animation of the Fat I function. However, here are some notes to consider:

There were no G7e variant using the Fat I programsteering mechanism that is depicted in the video, it was only used on the G7a(TI Fat I) torpedo.

I have a lot of documents relating the german WW2 torpedoes, including the Fat/Lut mechanims. I'm not sure about your BaMa-reference RM 6/3149 "Federapparat für den Torpedo G7e", 1942 though. I can't find it in Inevnio, and due to it's early date I wonder if it's a generic description, not specifically for the G7e(TIIIa Fat II) (I have actually seen a reference to G7e(TII) torpedoes being equipped with Fat, but if that is correct, I assume it was related the development)?

The following program-steering mechanisms were developed and fielded by Kriegsmarine during WW2:
* Fat I ("S-shaped" loop-patterns). The initial straight distance could be set from a minimum of 500 m to a maximum of 1500 m. Two loop-patterns were available: "short" and "long", and in addition, there was a choice of right or left turns (i.e. a total of 5 settings on the Fat-mechanism). For "short" loops, the straight distance between turns was 800 m, with a total width of the pattern-search of 1140 m For "long" loops, the straight distance between turns was 1500 m (PS: Note your typo in the description), and a total width of the pattern-search of 1840 m. The turning radius for both patterns was 170 m.
* Fat II: As Fat I, but with an additional program for a circular loop (a self-defence measure to be used against an attacking escort-vessel).
* Fat III, later renamed Lut I ("zig-zag" pattern): The initial torpedo course could be chosen so the torpedo platform didn't need to change its bearing to an optimal course for torpedo launch. The "Lut-speed" (i.e. the resulting advance-speed of the torpedo, considering the zig-zag pattern) could be chosen and subsequently the straight legs between turns would be adjusted accordingly.
* Lut II: Same as Lut I, but even slower Lut-speeds could be chosen.

The following G7e-variants were equipped with programsteering mechanisms:
* G7e(TIII Fat II)
* G7e(TIIIa Fat II)
* G7e(TIIIa Lut I)
* G7e(TIIIa Lut II)

In addition, the G7e(TIIId) "Dackel" torpedo was equipped with a special Lut I-version with extremely long initial distance of ca 36000 metres, but it wasn't recognized in the official Kriegsmarine-designation (unknown reason, but it might have been due to secrecy in relation to the torpedo's high vulnerability for detection and allied recovery).


In regards to the torpedo-model, intended to depict a TIIIa, the additional airtanks mounted in the front of the batterychamber was not the same 5 litre flasks as used in the aftsection. This was a round ball-shaped container ("kugelluftgefäße") of 25 litre.
Also, the warhead was always painted dark grey regardless of type (note that your head is a Type Ka or Kb series with a Pi1 pistol which isn't correct for a TIIIa torpedo).

In regards to the ausschiebbaren vertikalflossen (extendable vertical fins), some documents indicates these were first introduced with the Lut I, and not used with Fat II-equipped TIII and TIIIa torpedoes (I know Rössler indicates the latter though. I'm not sure what is correct).

The "coupling" piece mounted on the GA VIII servo engine spindle for the Fat, differs slightly from the real thing as it was adjustable and had "fingergrips" and notches for attaching a tool (you can actually see this if you look closely at the two images in Rössler's book, page 117). I don't know if it has been omitted intentionally for visibility, but the GA VIII gyroscope should have a metal shield covering the regulating airnozzle (you can see it on the drawings in Rössler's book, page 115 and the top image on page 117).

[lucabrasi]

Masterpiece of German engineering. Pure art

[redacted]

As a Silent Hunter 2/3/4 addict, this was amazing. Thank you!