Ok it's update time: Changed 17inch prop for 19inch and found a definite improvement but not enough. Bought a Whale Tail planing fin off the bay of fleas and fitted that today and the sea trial showed a vast improvement with us up to 26 knots at 6k RPM on a flat seas. The engine behaves much better than before as it seems to have stopped ventilating after passing over a wave top which was most disconcerting. We simulated this condition as best we could using other boat's wake. There's no doubt that we're not going to win any competitions especially on the economy front but I'm happier than before. We get the impression that we could cope with a higher pitched prop so if anyone has a stainless unit for a merc 150 greater than 19inch then I have a good home for it
Thanx again for all your advice so far. We're still on the lookout for an XL engine as the long term solution.