Haven't a clue what 30 km ph is but with mine loaded with about 150kg total we're usually at 17 kts WOT rarely (tide assisted) touching 18 in a typical UK sea (ie not glass). A little lighter load and dead calm we might touch 19 now and again but 15/16 is a nice cruise with our rig.
My consumption seems to be all over the place depending on usage (river pootling/coastal cruising/wave jumping but rarely better than 2.5 nm per litre, more typical is closer to 2 nm per litre which I tend to stick to for calculating fuel needed.
I was looking at maybe trying a different prop on the 380/Merc 15, if anyone has changed the standard prop please comment on the below:
Y Class avon best prop?