4 premature detonations you say?
I just tried again, I lined the shot up, set a spread of 4 eels, at about 5 metres depth. Fired them (admittedly the sea was choppy, but not rough), and they ALL detonated seconds after they left the tubes. Just 4 bangs, no sign of the torpedoes.
I can only think now that I have a faulty install, as this is blooming ridiculous.