Re: large Dia less pitch? Small Dia more pitch?
There are so many variables with props it is impossible to predict what will happen when making a change. Generally however, pitch has the biggest effect but pitch must be selected so the engine can rev at or very near the upper end of the manufacturers recommened wide open throttle rpm band. That number is what you OBSERVE at WOT. Since you didn't tell us what the WOT rpm was with either prop, what boat you had, whether outboard or I/O, we have no idea what will provide the best economy. Herein lies another thought for you. You may say -- "well I don't run at WOT so why is that important?". Fact is if you prop the boat wrong at WOT it is wrong across the entire rpm band regardless how fast you run. People often think that simply increasing pitch will slow the engine down and therefore get better fuel economy. Fact is, over propping (too much pitch) lugs the engine so you need more throttle to get out of the hole and to maintain any given speed. Prop it for WOT and speed and economy will be the best it can be. As for the prop itself, diameter is a function of prop design and although it has an effect on performance, it is not major. Two 17P props for example from different manufactures can have slightly different diameters. Likewise, those two 17P props may, and usually do, perform differently.
