pitch and diameter suggestions, Volvo 270, 215hp

[propmonster]

Ok so I just bought me an IMP Inca, 23', Waukesha 302, 215 hp, 270 Volvo, boat weight ?? depends on where you get the info, 3550-4800 dry weight. I've got the motor running real well and just serviced the outdrive including new pds bearing and u joints. The prop has been repaired at one time and it doesn't seem to have been a pro job, plus there are some nicks and a couple small bends in it which in turn is causing vibration. Took the prop off today and all I can find for #'s is on the outside there are a few rondom numbers that don't match anything I can find in my searches, then on the backside I see it's a Michigan prop with several #'s, most of which I can't make out, but I can planely make out the number 15 which I confirmed is the diameter, and a 18P which I assume is the pitch. I thought pithes were in odd #'s???
Anyway, here is my issue, the boat takes forever to plane even with trim tabs, wouldn't plane with four adults. I can only get 3,500 rpm wot, recomended max rpm of motor is 4,400 rpm, plus will only top out at 25mph. So I need to get 900 more rpm, from my research I find I should gain 3-400rpm per 2" of pitch I go down. So by my math I need to go down about 5" on pitch which would put me at a 15/13? Does this make sense?? And what if I get a 14.5 dia. ?
I don't have a bunch of money to go buying and testing, and I don't know anyone with Volvo props to test with, so If I'm going to go out and buy one I want to get it right the first time. Any educated suggestions would be greatly appreciated
Thanks, Propmonster
P.S. Got the handle after I got my first boat, ate a few props up:facepalm:

 

[Maclin]

Re: pitch and diameter suggestions, Volvo 270, 215hp

Is it a 3 blade or 4 blade prop? A 17 or 18 pitch would be about right in my opinion. I would not go any lower in pitch than 17 and keep the same larger diameter. This advice is based on the drive having the proper ratio gearset.

You need to be sure the engine is running properly, in my opinion that engine with an 18p prop should be able to get up to the top rpm.