Take action to address the following safety issue - the engines on 3 Real Journey vessels operating in Milford Sound and Doubtful Sound that manoeuvre close to the shoreline and other features during the normal daily operation were prone to stalling when placed from ahead to astern at speeds over 7 knots, which raises some doubt as to their fitness for purpose. The operational procedures that the operator has put in place to mitigate the risk of stalling have not addressed the core design issue concerning the original drive train.