Hi
i do do not think you can reach 18 kts with 6,5m and 1,5 T. Nevertheless this engine should run between 4100 and 4400 rpm. If it doesn't there is clearly a problem but remember the engine give a torque and the hull shows an hydrodynamic resistance . These two parameters meets together at a common point for a given speed.
Anyway the problem can result from many sources: fuel circuit, air circuit, fuel pump, exhaust circuit, over displacement, clean hull, wrong propeller etc...
first of all you need to be able to reach as necessary a State of reference: clean tank, clean filters , well know displacement ; you need to know the history of the maintenance and you need to know weather there is oil consumption or not.
if everything is clean and the prop is the right one, then ( if not enough fuel) the problem can come from the injectors which need to be examined. If injectors are ok then it can be the fuel pump which need to be renovated. You have also to check the cylinder compressions. If nothing from this point of view, Another possibility could be the exhaust circuit if to much rusted (the engine cannot breeze enough).
you have also to check the air compressor: the engine need to have enough fresh air to burn the fuel
for example I have a ocqueteau Espace 626 ( 6m - 1,2T- 4 blades prop 16x15) with a TMD 22. After a good engine reconditionning and hull cleaning with 2 pax on board I reached 3950 rpm and 15,5 knt
At 3000 rpm, I am around 10 knt also
another point is the hull. When the hull is not clean I lose about 200 rpm at max engine throttle
if you are not engineer the best you have to do is to find one and to check all these points with him.
hope this can help
eric
