A useful discussion about propellors has been going on on the 2024 Rally WhatsApp group which contains some useful info worth preserving on the forum - so I have cut and pasted some of the exchanges below:
[Richard Woods] I have 6hp Yamaha. The factory fitted prop was no good for Cape Cutter as started to cavitate above half revs. I changed mine - made big difference, now usable over whole rev range, less speed more grunt.
[Malcolm Sadler] I had the different prop fitted to my mariner a few years ago. As Richard says, it’s sooo much better. I’ll go and check invoice for description
[Simon Head] It’s the saildrive prop. I transferred it from my old Mariner to my new one.
[Malcolm Sadler] Yes. My invoice says 48-812951 A02
[Richard Woods] Original prop was 7 1/4 × 8 replacement 7 1/2 × 7. First number is prop diameter in inches, second is pitch, again in inches. Therefore replacement is 1/4" digger diameter (cannot go bigger or will catch on anti cavitation plate) and pitch is 1" less. Diameter and pitch is stamped on one of prop blades and it is genuine Yamaha part no 6E0 - 45943-01-00.
Diameter and pitch are stamped into rear of prop blade. First picture is the replacement prop, now on engine. Other photos are of two propellers side by side. Photo does not show clearly, but putting one over the other you can see diameter of replacement is bigger. Pitch is all to do with blade angle, side view does show the difference in angle of the two blades. Working it out is a bit complex, pitch is how far one revolution of prop moves boat/engine forward with no friction, drag etc. Also interesting to note is the shape of propellers is different, replacement has a straight trailing edge, original has curved trailing edge - former will give more thrust.
Here is how to calculate propellor pitch - good luck if you want to try it!
https://naval-architect-jobs.blogspot.com/2011/10/marine-propellers-pitch-and-how-to.html?m=1It is not easy and even more complex if the blade has a compound geometry - best to look for the marking on the propellor.
[Andrew Scullion] Most three blade props have one blade start aligned on the circumference where the previous blade finished, so you can estimate this quite easily by measuring the length of the prop cylinder shaft (forwards-backwards direction) from where the blade starts to where the blade finishes and multiply by three, one for each blade and this is approximately how far the pitch distance is.
Interestingly my Mariner engine is a 6 Hp, (again I think it is the same engine as the 4 or 5 Hp with an alternate carburettor) and mine appears to have a 205mm (8inch) diameter 150mm (6inch) pitch so even higher thrust.