Better Boat Two Legs Good, Three Legs Better?

There’s no doubt that three smaller engines on IPS drives allow for a shorter engine room, which benefits the lower-deck accommodation. No argument there. But the 77S’s engine room certainly feels small: three engines, two gensets, and the intrusion of the tender garage take up virtually all of the available space.

But what about weight and fuel consumption? It seems counterintuitive that for the same horsepower, three engines can be lighter and use less fuel than two, but Azimut and Volvo Penta say it’s not as clear-cut as we might think. Let’s compare a twin installation of MAN V12s (combined output 2,720 horsepower) with the triple IPS D13s in the 77S (2,700 horsepower).

Including gearboxes, the two MANs weigh 9,775 pounds. The three Volvos, complete with gearboxes and drives, weigh 15,212 pounds. Quite a difference, but it doesn’t take into account the weight of the running gear on the shaft-drive installation: shafts, P-brackets, props, and rudders. These vary from model to model, so an exact comparison is impossible, but they’re pretty heavy. And with heavier engines, like MTU V8s or V10s, the gap narrows even more.

In terms of fuel consumption, it’s an easier comparison: the two MANs use a combined total of 138.9 gph at WOT, against the three D13s’ 134 gph. At a cruising speed of 2,000 rpm, or just over 25 knots, the 77S was burning 121.8 gph during our trial, while MAN’s data sheets suggest that at 2,000 rpm, two MAN V12s would be using 126.8 gph while producing slightly more combined horsepower.

The advantages of the triple IPS engine installation are clear enough down in the accommodations, but in terms of speed and fuel consumption there is hardly a compelling case. On the other hand, all such discussions are entirely academic, because there are no power options on the Azimut 77S.