I guess that this is probably a fairly numpty question so apologies in advance.
I have always used the standard prop which came with the engine over the years on several boats so have no working knowledge of prop choice.
I bought a second hand Zodiac 9 man in the winter with a Mariner 50EFI. The previous owner had been struggling to teach his son to water ski with it and said he had changed the prop, including the old prop with the boat.
The boat didn't seem quite as quick as I expected, no speedo, but was definitely revving high at 7000 RPM.
The prop reference on the boat is 856/32A45P11 and the one in the box is 731/36A40P13
I am guessing that the last three digits are the pitch and if that is correct I guess switching to the higher pitch will give more 'resistance' and lower the max revs.
Am I thinking all this through correctly and if so what would the change do to the top speed?
Correct, p13 is 13" pitch, the 11" is way too small and isn't doing your engine any good at all revving to 7k. Get the 13 on there and take a note pad out with you and a GPS and record your top speed and rpm's with various numbers of persons to build a picture of the props performance. Then you need to decide what your average number of persons/weight load is most likely to be and work out what size props is going to give you the best performance within the correct max rpm range.
Or just bang the 13 on and leave it, it'll be miles better than the 11.
As a general guestimate, 1" of pitch is approx = 200 RPM.
So if you go 11 to 13" = 400(ish) rpm drop. The book of words for the engine will tell you what to aim for in max RPM.
Other thing to watch is that diameter also affects things, but not in a linear manner. For example If you stuck with 11" pitch and upped the diameter by 0.5" (assuming of course there is space under the cav plate to do that) it would also drop the RPM, but only a trial run will tell you by how much. Bigger diameter will give you more "grip" - but if you don't need it (i.e the prop isnlt letting go on every third wave) just alter the pitch.
As many boaters have experienced, factory delivered props performs very well in most conditions. You are 1 K killing over reving that horse. Test 13 standard prop at wot with your current load and check if prop/engine is delivering at least medium rpm readings betwen min-max factory rpm parameters stated for that engine for good overall performance.
You can make it a long job if you need to.......a few beers, music, maybe some wrong tools to help lengthen the project, depends on whether your in-laws are over.
Otherwise it's a 5 minute job, new split pin and grease must be had and use a block of wood to hold the prop against the leg as you undo the bolt.
