RPMs too high?

[Captain Caveman]

Race car engines don't last more than a couple of races either!

 

[Lowlysubaruguy]

Searay 17 ft bowrider with a 1999 Mercury 115 2-stroke outboard,

A sticker on the engine shows the RPM limits to be 4750-5250, im getting 5500+ RPM with speedo showing 42mph, it would actually climb higher if not for the rev limiter. Im thinking the prop is underpitched? The stamping on my prop says Quicksilver 48 16988 18P, its in great condition with no dings at all. If i trim it down it will slow down to 5000 but the speed drops significantly, which tells me its not trimmed up too much to cause the overrev. I would like the RPMs to peak right in the middle of the engines Max range at 5000, how much of an RPM reduction can i expect out of each degree of prop increase? such as a 19P or 20P prop?

My beliefs has been to prop for the best overall performance regardless of what red line is unless your running flat out at wide open throttle all the time.
if it really gets out of the hole loaded go with a bigger prop
if your most common cruise speed rpms seem high or low or your burning a ton of fuel at cruise speeds prop up or down. No bigger than your boat is an aluminum prop should get you there without throwing a ton of cash at props. In the end you might end up with several.

and you do need to address the accuracy of your tach or the issues your engine has if it won’t idle below 900 rpms for real something is wrong.
but I do think you should hit 45 to 47 mph at peak rpms so my thought is you need a bigger prop unless your pulling heavy skiers or your floorboard is water logged