If all they are doing is shutting down one engine they have two major problems.. One is prop drag on the dead engine.. Any sailor can tell you from experience that the prop drag is 1 to 2 knots, and probably more for the larger prop on a power boat... And the other is asymmetric thrust on a cat requiring the rudder to be cranked over to keep it straight, increasing the drag... Under these conditions the results are logical... They might be better off with one engine turning the prop at idle, and the other driving... What is spectacular is 10 NMPG at 8.1 knots... What is not spectacular is the nearly $800K price tag... You can afford a lot of fuel for an old boat for the next 50 years at that price... denny / k8do