There is no depth limit however in Stiebler's patch there is a time limit of ten minutes that the torpedo will search for you before giving up.
There is also no speed at which it will not engage, however the slower the better; it will rarely engage at even two knots so one knot is even better.
Fido's looked for a diving sub's cavitation which of course all diving sub's produced since they were crash diving.
So the key is to eliminate that cavitation, at least from your boat, hence the slow speed. Throwing a BOLD (gives the torpedo cavitation to look at besides yours) and maneuvering doesn't hurt either.

Ok, thanks!

This is from readme for Rubini Air Torpedo_mod_v2.0 :
5. Fido can now hit you from 4,5m to 45m deep!

Thanks. My info. was for Stiebler's version. :up:

This thread info is 11/10

John Pancoast, can you post your mod list here, please?

Here you go, though as I mentioned earlier, this is a heavily modded 8k/stock sensors campaign with files from all over including various megamods.
I also have modded many of these mods for this install :)
In terms of the Fido, the only two you would need are H.sie's and Stiebler's patches.
If you need help setting them up, just ask.

Generic Mod Enabler - v2.6.0.157
[C:\Sh3\MODS]
ImprovedConvoys with enlarged convoys.
Hull damage made by torpedo impact mod v1.0
Marhk's Super Smoke v1
DD_OH_V3.09_20091209162038
Airpower 1.4c
Rub fatigue
MFM-Improved
Darkened stock water
GWX Hudson and air zone cfg
TheDarkWraith_Ship_Plane_Fire_Damage_v1_4_SH3
Anvart_Stiebler_TurmsR01R02
NYGM Cam ship MAD Catalina Avenger and wolfpack files
Von Dos NYGM Liner
Jimbuna's Q ship
Various gui and minor graphic files.
Scope zoom level fixes - DDucks camera file
NeuUZO&PeriV.2
SH3Speech_GWX
Convoy-Routes on Map
Late war start
Late war SNK_RWR_for all_v2
Supplement to V16B1 (JSGME)
Stiebler4C_Addon_for_V16B1

Thanks!

What's Marhk's Super Smoke v1?

No problem. Lately though, I've been spending more time with AOD since SH3's cream puff escorts stink worse than yesterday's diapers. :D

Old ship burning/hit smoke mod. I'm not to picky when it comes to graphics. :)