Prop question

[Flatsman]

I have a 99' Polar 1996 model skiff with a Yamaha c90. I havent owned the boat too long. I have rebuilt the carbs and got it running really good, but it doesn't seem to have the power that I thought it would with this matchup. In the front storage area is a beat up 13x19 alum prop. On the boat is a 21p prop. The dia isn't stamped on it. The boat doesn't have a tach, so I dont know rpms. It doesn't seem to rev out like my old johnson 115 did though. Any suggestions on the right size would be greatly appreciated. I know that I need to install a tach too for proper sizing.
Thanks

[99yam40]

look here to see what prop they used on a set up like yours
Performance Bulletin Home

[Flatsman]

Thanks for the help.