JJ Donovan wrote:...I am looking for the most fuel efficient [propeller] and am not looking for high speed...
In order to assess the fuel efficiency of a engine-boat-propeller combination, you will need some means to very accurately measure the instantaneous boat speed and instantaneous fuel flow rate. Without this sort of instrumentation, you will never know the fuel economy. There are many people who lack this sort of instrumentation, but freely give advice about the fuel economy of their boat, engine, and propeller combination, based on very imprecise measurements.
If the goal of your near-future use of the boat is to test for maximum fuel economy, you will first need to obtain the necessary test instrumentation, and then gain some experience using it with the present propeller. Once you get a good data set as a basis of the current fuel economy, then you can try a different propeller, run it long enough to collect an equivalent set of data, then compare the results with the first set.
I don't think you can get the "most fuel efficient" propeller for your particular boat and engine any other way.
When you conduct your fuel economy testing, be certain to maintain other influences constant. The other influences on fuel economy will be:
--air temperature
--air humidity
--wind speed and direction
--water current and wave condition
--water temperature and salinity
Also, you might give consideration to other aspects of a particular boat-engine-propeller combination, other than the highest possible fuel economy. In some instances, choosing solely on the basis of greatest fuel economy will not produce the best all-round operation. Other aspects of performance will be
--acceleration
--minimum planing speed
--freedom from ventilation or blow out in rough seas and turns.
If you test the four propellers mentioned by Phil, I would expect that the difference in fuel economy among them will not be very large. You might find a range from worst to best of perhaps 10-percent.
If fuel economy is really the ultimate goal, get a modern engine. It will improve fuel efficiency by more than 50-percent. This will be a much bigger improvement than a propeller.