Help in prop selection

[1sloopti]

I have a 18.8 center console. I have a 1998 opti 135 on it. The weight is 1200 empty w/no motor. I tried a 21 pitch alluminum prop today. I was only able to rev to 4900 W/motor tilted. Than I tried a Mirage 19 and got 5050. I now have a 17 mirage on it but haven't gotten a chance to bay test it. Any suggestions? On the original prop i was able to get 43mph out of it. Any idea on prop selection, 3 vs 4 blade? And what kind of mph i should be able to see proped right. I think the rev limit on this motor is 5700rpm. Thanks in advance.

[Fast Fred]

one inch of pitch is worth just about 400 rpms. so goin by what ya said
should spin right around 5850.

if that turns out to be true on your hull, for a four blade go down in pitch 2"more

[1sloopti]

one inch of pitch is worth just about 400 rpms. so goin by what ya said
should spin right around 5850.

if that turns out to be true on your hull, for a four blade go down in pitch 2"more

I'm kinda surprised I didn't get more rpm going down to the 19 from the 21 pitch. Hopefully i can be close to rpm limit with the 17.

[Fast Fred]

try it see what happens

[1sloopti]

Fastfred you were right but my tach seems to be 200rpm off from the motors rev limiter which i believe to be 5700. Today with the 17pitch stainless I hit the limiter. So my other runs were all 200rpm off. Anyway im going to put a jackplate on the boat, getting to much spray. With the jackplate on, 1 inch up is =200rpm.
Am I correct? So i might end up with a 21 pitch anyway, i think.