Year 2000 90hp. 1 pob full tank, dinghy, aux o/b, lots of anchor chain and gear. Average over a season 1.6 miles per litre at 3600 rpm 23knts. cruising speed. Rather more if I have to waggle the throttle a lot. More too if I'm at displacement speeds.
I'm beginning to do a lot of 'messing about' among rocks and stuff and I'm thinking it would be more economical to use my 3hp Johnson aux for slow speed, though manoeuvrability is a bit dodgy.