Yanmar 370HP V8 Diesel

[mcarthur]

It's not too bad. We have a 50' powercat and what you're seeing isn't unreasonable. Have a look at Domino for good figures, and - only an example - the Aquilla for poor figures (here is my analysis on other cruising powercats' consumption).
Instead of plotting L/nm (which is useful but can hide some things like sea/wind), try just pure L/hr which shows it always increasing.

[Johan Leopard51]

We have a Leopard51 with two of these. In practical terms the fuel consumption in nautical mile per liter is about the same at 13 knots as 18 knots when disregard current and swell. For us, the most economical is around 7 knots which is also nice speed for game fishing.

A nice thing with having the speed available is when everything combines and you can “surf” fairly long distances on right swell and reach instead of that fast slow thing with heading up a swell half the time. My best (at speed) is at 11 knots recent we were getting 10.5 liters per hour per motor. So that would be about ½ nautical mile per liter. We needed to make weather gap on a 125 mile crossing that we did not want to end in darkness as marine charts in Seychelles are not wonderful so visibility key in the shallows. 05:30 depart first light, we were moored just after 18h00 sunset.