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.