Hi,
Don\'t know the photo that you refer to but it is probably a Geräuschboje.
Here is a short passage describint it:
\"An apparatus attached beneath the forward hull section provided limited protection from acoustic mines. Named Geräuschboje this device consisted of a floating turbine, turned by the ship’s speed through the water, that caused an internal striker to hit the metal casing — the mechanism would (the ship’s crew hoped) create enough noise to detonate any acoustic mines lying in the ship’s path.\"
I found one from the wreck of M4003 in Brittany, it is a floating device about 70cm long and a diametre of about 30-40cm. Most Speerbrecher have them, often when out of use they are mounted atop the bow.
Lawrence