The reports into the damage to both Adventure and Blanche conclude that non-contact mines were the cause. Mr Offord estimates that the mine which damaged Adventure was about 10ft from the hull and therefore not a ground mine (i.e. a mine on lying on the seabed) and was of about 500lbs.
There is an Internet site which has a detailed list of German mine designs. This tends to indicate that a ground mine is the most likely explanation for two reasons:-
- Most, if not all of the non-contact mine types used in 1939 were ground mines
- The moored mine types appear to be intended for much deeper water than that found in the Thames estuary
The most likely types (assuming that the mines were laid by destroyers) is either RMA or RMB, both magnetic ground mines. RMA has a 1,764lb charge and RMB 1,014lbs. Both types were in use in 1939. The picture below is of an RMA mine. The RMB was very similar.
I do not at present have accurate information about the depth of water at the point where Adventure was mined, without which it is not possible to know how far away from the hull a ground mine would have been. Without this information it is not possible to estimate how far apart the theory based on the listed mine types and Mr Offord’s 1939 estimates actually are.