is it a 21 pitch 3 blade prop? if so, i personally would stay with a 21p but go to a 4 blade or even a 5 blade.. i run a 5 blade on my 200 merc on the back of my fx18dv and love it. but that being said, my boat is not a ranger so results will most likely vary. if you know anyone that has a 4 or 5 blader ask them if you can try.
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.