I purchased Robbe U 31 month ago. It is nice kit, mostly watertight cylinder. I have it included static dive system. Only I do not like is plastic hull from kit,I do not know, if this can handle 20 kg heavy submarine. I do not thing so, just because this I purchased excellent fiberglass hull. It is more money, but I am sure, I never will have any cracks on hull. Outrunner electric motor is required, because 6 bladed propeler has 10 cm diameter, and it need low RPM, because kavitation. Outrunner have 100 RPM on Volt, so in 12 volt system, it is 1200 RPM.