1988 Baja force 235 speed/prop
 

I have a baja force 235/454mag motor bravo one outdrive. mercruiser mirage ss 21p prop top GPS speed 58mph 4500rpm. Consistent 57mph. Would like to break 60mph was wondering if I could go up to a 23p prop without under reving the motor. BTW didn't matter if we had 6adults or two.

Most likely it isn't going to hurt the engine, but remember, if it then turns less rpms the increase in pitch isn't going to show an increase in speed. All you can do is try it. I believe a 23 is a more common prop on your setup. If you haul a lot of people around you might just want to stay with the 21, especially if all you are seeing is 4500 rpm.

You are already lugging the engine with the 21 you have. Merc recommened WOT is 4600-5000rpm and the closer to 5000, the better.
All things beng equal, no way you can turn a 23.
That being said, you should be able turn more rpms with that boat,power and prop.
I could turn close to 5000rpms with a 454mag/23 Mirage plus on a 24 Outlaw.

get a good timing light and make sure you have 34 degrees total timing. way cheaper than another prop. also the engine makes more power at 5000 than it does at 4500. 2 inches of prop at 20 inches is 10%. so you will be forcing motor to max at around 4000. where it has less power. so, it is not going to go faster. it'll be slower, work harder, come up on plane slower. all for you spending a couple 3 hundred on the wrong prop. now, if it is timed correctly, plugs-wires good, carb good, all that and you are still at 4500 max, borrow a 19 inch prop from somebody. that is 10% the other way. revs go right to 5000. big smile.