prop issue

Trying to get a little more out of my 90 yamaha. Its on a 17 ft express and runs about 45 wide open. Ive read where others are running in the mid 50's. What kind of prop do i need to gain a little more speed? It has a stainless steel power tech prop on it now but not sure how to tell what pitch it is. Had a guy at a boat shop tell me that i needed a 21''. What speeds should i expect from this motor?

[reelnative]

you need to keep your rpm in the 5 to 6k range, the condition of your hull and the trim angle will do a lot for the top end speed of your boat, a/e a boat with bottom paint will run 2 to 4 mph slower then the same boat with a clean non painted bottom, if you know the pitch of your prop now and the w/o rpm of your boat with that prop, you can use this as a gauge to how much more prop you can add, each inch of pitch you add will drop your rpm 200 points, so if your running 6000 rpm now and you have a 17 pitch prop and you put a 21in pitch prop on the boat it should drop your w/o rpm to around 5400 rpm, also adding a jack plate that offsets the motor 6in or so off the back of the boat will allow you to also add 1 more inch of pitch without dropping the rpm any, doing that will change your trim angle. I hope that rambling helped