First boat 1969 starchief restoration.

[Watermann]

Propeller question. Currently the boat came with a 19 pitch propeller and I was wondering if this is the most efficient for my boat? Wide open 4000 RPMs around 31 miles per hour

It depends quite a bit on your load out with that motor/boat combo, also the age and type of prop. The MC 120 is affectionately referred to around here as "the cast iron lump". The props common for those are a larger diameter and have a big coon dog ear blades. If you go to a newer style prop that has a more aggressive rake and substantial cupping then you could loose RPMs in the same size / pitch as newer props really bite the water much better than older designs. The old rule of thumb 200 rpms lost for every inch of pitch increased or the opposite for a decrease in pitch doesn't hold true unless your replacing with the exact same prop. You could also have trouble with the trim tab being in the way if you get a newer prop with the high rake, a lot of times they'll hit the big old tab.