Update : The local "support agent" felt that the voltage as measured at the stern thruster of 11.8 volts was too high. Strange, given it is a 12 volt system and there was nothing in the Sleipner documentation about this ( the only mention of a voltage range in their documentation gives up to 16 volts as acceptable). In order to resolve this the yacht importer picked up the cost of installing a separate battery pack for the stern thruster. After this and after the stern thruster coupling was replaced again ( 3 had sheared), it has worked fine, although has not been subject to heavy use. So maybe the voltage was the problem or maybe Sleipner had re-engineered the coupling strength in the replacement units ( there has been no communication from Sleipner on this). The bow thruster has not sheared again, the apparent logic being that the long cable run from the house bank drops the voltage enough to prevent it from being overpowered. However given it had sheared previously when a solenoid lock in issue occurred, I'm still not confident in its reliability and in my opinion Sleipner should issue a communication addressing whether there is a weakness with the flexible couplings.