Welcome to the world of boating Bloody. Expect about 1 ltr per klm with a 2 stroke. That is 100 ltr's per 100 k's in car speak. If you get a 4 stroke motor you'll get a bit better than a whopping 2 ltr per klm.
4 strokes must be different where you are Jack- using twice the fuel of 2 strokes. I think you meant 2km/litre right?