Hi,

Though i don't have exact information on this right now (i can get the approx Db levels of the 2 subs though...might take some time though) i hope i can shed some light on the acoustic signatures of both these U-Boats.

For a start both U-Boats would have been very noisey by modern standards with
propellor cavitation being a major source of noise at high speed (either surfaced or submerged....the small high speed propellors bith were fitted with would have generated huge ammounts of cavitation at high speed).

Type VII.

The loudest os the 2 U-Boats by a large margin for several reasons:

1. Un-streamlined shape plus deck gun and AA fittings would have generated very lhigh levels of flow noise as the sub moved through the water.

2. Small high speed propellors would have generated significant cavitation noise at high speed.

3. Internal fittings....most internal noise making fittings i.e all amchinery were mounted directly on the hull.In modern practice any peice of equipment which generates noise is mounted on a resilient 'raft' to isolate its noise from the hull and thus not transmit its noise to the water.That said the Germans began from 1942 onwards to mount some equipment in their U-Boats on rubber pads to help cut down on radiated noise.

Type XXI

The 'electroboot' would have been quiter overall than the Type VII but would still have been loud by modern standards:

1. While schnorkelling the Type XXI would have been VERY loud.All the noise generated by the diesels would have been put into the surrounding water.The US Navy found this out when they converted their fleet boats to the GUPPY standard after the war.The US tried 2 responses to the problem of this noise:
On the GUPPY'e they installed the first version of the 'Prarie-masker' system.This system uses compressed air to surroung the hull of the schnorkelling sub with a 'bubble screen' which absorbes the noise of the diesels and the propellors.

The second method of reducing noise was to 'raft' the offending diesels..which the British first tried in the 50's.This is the preferred method in modern subs.

2. At high speed the Type XXI would have generated 2 type's of very distinctive noise...the cavitation from its propellors and 'gear whine' from its large reduction gears.The type XXI's electric motors were based on electric train units and ran at high speed (+2000 rpm) and needed reduction gearing to bring the rev's down to a level the screws could handle.


In general both subs would have generated stable readily identifiable acoustic signatures.Modern sonars using droadband and narrowband processing would have no trouble tracking these subs.

I hope this is of some help,

Sniper