Current prop diameter
Don't assume the data that was true when you boat was built is still true today. For example, you have probably loaded several hundred pounds of gear aboard. If the boat has been re-powered, it is likely you have more horsepower than the original engine.
The correct pitch will only be achieved after experience with your boat, engine, and prop combination. Factors such as the weight of your fully loaded boat, the altitude of the water you're sailing in, and even the salinity of the water will have an impact on the exact pitch that is ideal for your boat.
For example, if you have your boat in a high mountain lake, the normal pitch might be a bit too big. At high altitudes, your engine doesn't have as much horsepower. If the prop takes as big a bite with each revolution as it might at sea level, it will over load the engine. The engine won't achieve the expected RPM's and therefore will develop even less power. Re-pitch the prop to a slightly smaller pitch and your engine will run at its rated speed again. With this simple change it will produce all of the horsepower it possibly can.
A heavily loaded boat can have the same effect on your speed as high altitude. More drag is going to slow the boat down requiring a smaller pitch. A slightly smaller pitch may also be advantageous in the more saline water found in the Caribbean. Conversely, the less salty water found in the Pacific North West will dictate a slightly larger pitch.
Use you new prop for a season. Monitor engine operation carefully. Use your ship's log to record conditions and operation perameters such as sea state, throttle position, engine RPM, engine temperature and boat speed. Compare the engine speed you achieve in various conditions against your engine's power curve. At the next haul out, have the prop re-pitched to match your needs. If engine speed is not where it should be to achieve full power when you need it, have the pitch reduced slightly. If the engine tends to over-rev, add a bit of pitch.
Caution: If your prop is over-pitched and the engine is struggling, you are causing undue stress and premature wear to the engine. Re-pitching the prop sooner rather than later may be in order.
With the detailed notes you have kept in your log throughout the season, any specialty prop shop can advise you as to how much the pitch needs to change. For a small fee they can then re-pitch your prop to optimize it for your needs.