Foxer

The noisy Foxer had the disadvantage that it also prevented the ship's own ASDIC from detecting any other U-boat nearby that could approach the convoy.

The water flowing between the two bars caused them to flex and bang together generating far more noise than the towing ship above 1 kHz.

The single noise-maker however would work effectively only if the torpedo was equally sensitive to sounds from both the front and rear.

The water rushing through the holes and the pipes banging together created cavitation noise, much greater than that coming from the ship's propeller.

[6] The drums also wore out quickly and the sound could be heard underwater over a long distance giving away the position of the towing ship to U-boats searching for convoys.

In the closing stages of the war the Germans developed the T11 Zaunkönig II torpedo, designed to ignore towed decoys and noisemakers.

Foxer decoy float resting on the top of the depth charge racks of HMS Hind (U39)
The effectiveness of a single noise-maker towed by a ship travelling at 12 knots.
Typical trajectories of German passive acoustic homing torpedoes relative to a ship moving at 16 to 18 knots with two towed noisemakers (shown in red). Similar trajectories from the starboard side.