1965 aquasport 22-2 repower

[fitz73222]

Its a 14 1/2 dia 19 pitch I believe it's the original based on the part #, my motor says 5600 rpm max would it be safe to be 5600-5800 range? I would like to upgrade the prop its a little worn out small nicks and dings on the edges but nothing terrible. The previous owner took the motor off a 98 proline said it went 43 at 5300rpm. The 1st day I went out I still had some premix in the tank from priming the oil injection about half tank, yesterday I filled up and ran it almost empty so my next should be just gas. It did run a little better yesterday wot was 37-38 using a cell phone app, and rpms were about 5300 using the gage on the dash, it may not be correct I dialed it in at idle using a digital photo sensor tachometer getting it within 20 rpm or so but did not check to verify if my tach on my dash was accurate at wot compared to the digital photo sensor tachometer. I haven't changed the plugs out either waiting for the premix to run out. The only things I have done is change the lower end oil and verify my t stat was operating correctly my starboard bank runs slightly hotter by about 10-15 degrees measured at the plug.
   

Keep the revs to 5600 then. The engine is capable of much more than that but I guess Mercury wanted to tame this powerhead. Now one thing to consider when propping a boat and engine combo for year round use is consider the average load you usually run and the time of year that you are propping the engine. I say this becuase there is a noticeable difference between summer propping (warm, humid air) and fall/winter propping (cooler dry air) and as much as a 12% power loss in the summer. It can be as much as 150-200 rpm difference at WOT. So if you're propped at 5200 with cool air and low humidity, you may be only 5000 during the warm and humid summer, which is borderline being over propped. So my advice is to prop the engine at "redline" when its cool and still be able to stay in the safe zone during summer use, that assumes that you can run in the winter like here in Florida. Mercury has an excellent publication about how atmospheric conditions affect propping. I have it from 20 years ago, I'll see if I can find it, scan it and create a PDF we can put in our archives. Also, if you are going to run with 3 people and 40 gallons of fuel on your average trip, then you need to consider that when doing prop testing. Don't get hung up on how fast the boat is going at full throttle, propping is about the best overall performance and the engine running at or near its peak rpm which is best for the powerhead life throughout it's throttle range. Yes, get the engine height as high as reasonable, cav plate even with bottom is fine with your stock aluminum prop.
Admin Edit: Farley I pulled your response outside of the quote so we knew who was saying what.

[Gt30691stang]

 Thanks for the info! Definitely look forward to reading that article. I am familiar with how atmospheric pressure affects performance from drag racing in New Hampshire it was a considerable  difference from New Jersey, and always preferred to dial the car in early in the morning or really late(track is still hot though) fall is by far the best season for performance! I will take your advice when choosing a new prop! Tomorrow I plan on raising the motor, changing plugs and making sure the carbs are clean.

[Gt30691stang]

I raised the motor to its highest location, the cavitation plate is still below the keel a little over an inch. Ran it with full tank fully loaded with gear and 2 other people today revs were just about 5100 with temps sunny 80 degrees, vs earlier this week mid 60s overcast in the evening reaching 5300rpm. So is it safe to say That I need to be pitched to 17 maybe 16 to ensure I am at max rpm at wot? Fitz would it be safe to run a SS prop with the motor that high, would I see flex at the transom due to no flex in the SS blades vs having alum blades flexing?

[fitz73222]

Since you're running a 19 pitch now, a 17 should get you were you need to be rpm wise. Typically there is a 150-200 rpm change per inch of pitch. As far as blade flex between aluminum and stainless is anyones guess, yes it is supposed to occur but I've never been able prove or disprove how, when and why it occurs and really think its pointed towards high performance applications or where props that are subject to surface running (super high engine heights) that are grabbing air where blade strenght becomes super critical. Forget about it! The amount of prop bite will have nothing to do with your transom life, but your driving habits will! Full throttle water ski starts or running it WOT in a 2 foot chop will take its toll so baby it a little, ease up on plane and slow down to a 25 mph cruise in a heavy chop and your transom will thank you for it. Your transom is 50 years old or at least appears original so if it's not cracking and flexing at the lower height it should be fine with the engine up and inch or two but keep an eye on it for new developing cracks or issues. If you have any cracks, mark the end of the crack with a sharpie and see if over time the crack starts to pass by the mark or time. Suffice it to say, your boat is probably carrying some water weight in the stringer foam, deck and transom, all of them did it so comparing speed and prop data against a lighter, rebuilt flattie really isn't a fair comparison. If you're bumping up against 40 mph with an original hull with a 150 you're doing very well.