On the back of my 6m Humber, the Suzuki 140 4-Stroke seems to be using around 20litres of fuel every 30miles. That's cruising at about 28knots, 2 people on board, and 90 litres of fuel in the tank (full).
The boat itself maxes out at just over 40knots (43 knots with no tide running), at which point the engine should be using about 33 litres of fuel an hr.
As yet, I see no fuel consumption difference between the new stainless prop and the original ali prop that came with the engine.
On the other hand, my old 5m rib with 70hp two stroke would use 25 litres of fuel for a 20 mile trip, cruising at around 20knots.... this is why I am very happy with the 4-stroke. Although, saying that, you don't buy a big petrol engine if you don't expect to pay out big bucks for fuel - damn that tax man
I have one of them plastimo fuel monitor thingies on the list for a winter project, at which point I should be able to get the best fuel consumption out of the boat.
Looking through the new rib international mag which I got this morning, I'd be interested to see the fuel use now for the new Yamaha 150 4-stroke.... it's 20kg heavier than the 140 Suzuki, and also has a bigger engine block, but probably newer electronics again, so, you never know.
Glad I don't have a boat like Charles' down here - would be on the 2nd loan by now