RG what is your WoT speed?

 BH what are your calculations to get that?
 Does take into account a well setup efficient petrol internal combustion engine at best will work around 80% efficiency of the stored potential of the fuel used?
 Then around another 10% in the drive train
 And that most modern outboards product between 5 and 10% more than advertised hp in the midway point in the WoT specs So at the crank there would be around the 260/ 275 + hp plus ad the efficiency factor.. looking over 300hp worth of fuel.
 And regular unleaded petrol is approx 11hp/hr per L

And just because a engine (or at prop) is rated at say 255hp, it only develops the hp to move the boat/ weight at the speed one wishes to travel at... hence why faster you go the more fuel per hr one uses.
Funny thing, with a plaining hull thu, the faster you go, the less hull in the water and the hull friction co efficient drops.

That extra litre is probably the motor injecting additional fuel to cool the cylinders. 

I have recently brought a 225hp honda and basically I'm far from impressed. I'm getting 15.8Lph at 7kn with a 17r prop on a 6.3m challenger. Top speed 38kn. I got better fuel economy out of my old 150hp mercury optimax 2005 with a 15p prop as I have been told the 15p is for boats that are under powered and I got the same top speed. Know two other people same engine getting 7kn to 7Lph could someone shine some light on this for me. Dry the boat is 2T same weight as the other two guys boats.

Yeah, that doesn't sound right. What RPM is the engine doing at 7 and 38 knots?

So it's doing 7.1kn at 2600rpm an it's brand new like this is all happening on the 1st few trips. The dealer is a absolute tugger typical sales person got your money now go away don't want to hear about your problems. And I'm stuck with in my mind a very expensive peace of s@$t. Has anyone got one of these engines without a honda prop ??

I have a 7.2m challenger with a 225 Honda on it. I get 40kn in absolutely perfect conditions. 38kn in most other conditions. I'm not convinced I have the right prop. I suspect it has too much slip at most RPM's, but need to actually record them and check. I also think I'm a bit under powered. Solas 15,1/4"by 17.

Hi Gibby - if you're running at 2,600rpm for 7knots it would indicate to me that your propeller pitch (17) is to low.  I would try a 19.

I would recommend getting rpm and speed in 500rpm increments and letting Steps have a look at slip etc before changing to 19"pitch. It's very possible your diameter is miles out. Changing pitch without sorting diameter, won't give the best solution.

Do it the way it is meant to be done professionally.
 And MUST be done in this order
1/ correct engine height
2/ have a current prop that is withing the manufactures WoT max rpm range.
3/ Take ACCURATE rpm/speed data from around 3000 rpms to WoT in approx 500 rpm increments
3/ Calculate out the slip (prop diameter/number blades) for that weight Hull if not correct
4/ Once slip sorted  THEN sort the pitch.

When one has done so many times one collects a data base so the slip and pitch can very often be done at the same time from experiance.

Or there is the 'normal method'  try this prop, try that prop get it in the middle of the max rpm range...and usually the slip and pitch are wrong for best performance and economy.
 From memory the how to data needed is on the previous page of this thread.

Its not not rocket science. A planning hull will burn more at low speeds due to the wetted surface,not designed for low speeds unlike a displacement hull.Yes a displacement hull will plane with more HP and a bigger fuel burn.