Check the specs on your engine for the rated maximum RPM. The proper propeller for your boat would be the one that would allow the engine to reach maximum RPM without exceeding it. There are other external factors that regulate boat speed like a fouled bottom, sea conditions, load, winds etc. How are you measuring speed, over the bottom or over the water? If it is by GPS you are measuring over the bottom and would have to take into account the strength and direction of any currents. Two runs in opposite directions might render a more realistic average speed. If measured by a calibrated knot meter over the water you may get a more realistic reading but also two runs should be made to factor out some of the wind effect. Remember these diesel engines are considered to be auxiliaries and are not necessarily going to get you to hull speed, that is what the sails are for. If you are unsure about the prop just check the RPM as boat speed is not a good indicator.
