Yamaha has selection guides for specific boats right on their site to get you close anyhow.
I looked over at Yamaha and you are right on @ 21P but diameter is what makes and breaks just as much and you didn't supply that. Another thing is every brand is a bit different, they all have different cup angles ect, I like Yamaha props the best, I have tried a few different ones on My F150 on my pontoon and Yamaha props perform the best.
Also what engine mount height hole the engine is mounted in can make a big difference too, on plane the fin on the lower unit should be right at or about an inch below the water line for quick reference.
Here you go.