I do not know if this will help…
I have a Volvo D1-20 in my B32 Cruiser and I have just topped up after a week away and the first “top up of the year”.
16 Engine hours has used 24 litres of diesel…. so approx 1.5 Litres an hour.
Do not get me wrong though, I do not push it and normally only use the engine into / out of moorings and into the wind when Faith has had enough of the slog!
I tend to cruise at 5-6 kts with 2000 – 2400 rpm showing.