1988 Force 125 HP Questions

Ok guys I just bought my first boat of my life which is a 1988 Bayliner Cobra 18ft. The boat has a Force 125 HP motor which as far as I can tell has been taken care of. The boat run great. I have had it out on the water 4 times now. My question is this. My motor RPMs is only 3500. My top speed is about 30mph. I have been told by someone that I need a new prop with a trim of 13" or 15"??? That should bring my RPMs up??? So not knowing any better I thought I would do some home work before spending the money. So here I am guys tell me what you think??? I would like to get some more speed out of the boat and I would like the boat to get up out the water a bit faster. Thank you guys for any help you can give me.

Also just a side note on the matter. I live about 5000 above sea level. I think I have a 19" prop now.