prop question, do rpm matter?

[Bushwacker]

. . . i remember hearing the 5000 rpm WOT prop was a bad idea in the past, im just unclear on the why.

If the motor won't wind up to it's max rated rpm at the load you normally run, you're basically lugging the engine, analogous to driving a stick shift vehicle in too high a gear. The result is increased pressure & temperature in the cylinder, which increases loads on the pistons, rings, wrist pins and bearings. This increased pressure & temperature also makes it a lot more prone to detonation/pre-ignition, where instead burning evenly, some or all of the fuel mixture suddenly explodes before the piston reaches the top of the compression stroke. Detonation is a violent event and is the "pinging" you hear in a 4-stroke, which is the valves rattling due to the shock waves bouncing around in the combustion chamber. With a 2-stroke, there are no valves to rattle, so the first sign of bad detonation, other than a slight power loss, is when it blows a hole in a piston!

Outboards have 100% needle bearings and short strokes, and the V-4/V-6 engines have very short rigid cranks, so they love to rev, and the harder you run 'em, the more oil they get, which keeps the bearings happy! But lug 'em down and get a little detonation, which you can't hear, and you can have problems in a hurry, or at the very least shorten the life of the rings and bearings! One day you'll wonder why it has low compression!

Detonation due to low octane fuel is why ethanol gas, which can drop 3 points in octane if you get enough water in it to cause phase separation and remove the octane-boosting ethanol, can be lethal for 2-strokes! If you're gonna run ethanol, it's best to buy at least 90 octane so that you've got at least 87 octane if water takes away all the alcohol!

Before you spend a lot of time/$ fooling with props, make sure the motor is mounted high enough! You should be able to see the cavitation plate above the water when you're up on plane. If not, raising the motor may get you some more rpm with the current prop!