Figures sound bad. At 4000rpm, correctly propped you should be closer to 33lph than 40lph. Currently you appear to be a bit over propped based on your fuel burn. A WOT of 5200rpm tends to confirm this. Having said all that, your speeds also seem low for that hp on a 7.5m boat. I don't know what engine you have, so can't check the gear ratio, but if it is a Yamaha F225 with a 2:00:1 box then your 'no prop-slip' speed at 4000rpm would be ~31knts (might want to check my maths). At 19knts you have ~40% slip which says that something is badly wrong. 

19 to big for that engine boat IMO.
I have a 7.5m billfisher with an f200 on it. Had a play with a few different props including a 19 which gave similar figures to what. You are stating in your first post ( maybe a little better) currently settled on an 18 4 blade prop which will see me get 5900rpm at wot and cruise at 22knts doing just under 4000 with fuel burn just under 30 Lph.

Have a chat to the guys at propellor services they are pretty helpful.

Yes south auckland, down whitianga way at till next week tho. would be happier with those figures. Had f200 on my last boat 650 sea nymph walkaround with same size prop, was well suited. Which makes me think its too big for this one??

Tagit wrote: Figures sound bad. At 4000rpm, correctly propped you should be closer to 33lph than 40lph. Currently you appear to be a bit over propped based on your fuel burn. A WOT of 5200rpm tends to confirm this. Having said all that, your speeds also seem low for that hp on a 7.5m boat. I don't know what engine you have, so can't check the gear ratio, but if it is a Yamaha F225 with a 2:00:1 box then your 'no prop-slip' speed at 4000rpm would be ~31knts (might want to check my maths). At 19knts you have ~40% slip which says that something is badly wrong.  Assuming that the hull is clean etc, my first check would be engine height. A too low engine could explain most of what you are experiencing. I doubt that the problem is as simple as having the wrong prop. The other simple possibility is that your prop is damaged in some way. After that the reasons get harder to work out.

tagit - can you provide some more info about calculating engine/boatspeed though gearbox and propeller measurements/efficiency

perhaps a link to something?

I'm assuming by 19x15 and a quarter - you mean a 19"pitch and 15 and a 1/4" diameter - (wouldnt fit the other way around anyway)  these numbers mean a big prop, is it a Mirage? A big diameter prop, moves a lot of water with minimal slip, can be a bit slow off the mark, thats no probs on a fishing rig but it isn't a 'top speed' prop. You need lots of torque to turn it too and a 225 yam is a bit anemic in the torque dept. The 19" pitch is also big, too big (we say big but we should say coarse, we're talking about the angle of the blades not the size of the prop) for the weight of your boat- but as Tagit says at 4000rpm with a 19" prop you should be cruising around 25 - 27knots - could it be that you have read the numbers wrong and  you have a 15" pitch? - and 14" diameter is the norm

BH - Shrekky saved me the trouble of tying all that. Thanks Shrekky.

yip thats Lone stars boat here he does some big trips so he will know all the figures etc think hes running a 19 

Appreciate all the help guys, Did consider enquiring about the prop on the wc on trade me, but that

agree - again, the yam 225 is a short stroke rever, doesn't produce max power untill its almost hitting the rev limiter. ie even at 5600rpm it's still not producing 225hp. SO don't be afraid to rev it's nuts off, you're already suggesting a 4000rpm cruise, thats a good minimum. As Tagit says to get the max out of that motor a 16" prop is probably going to be the best option 

factory props can be next to useless at times.
Yamaha ones are some of the worst in my opinion.
I swapped the solas prop on my honda for a mercury vengeance prop, unbelievable difference.

Many Yamaha and omc owners end up with mercury/quicksilver props on them.