Prop. Diameter

[Firewood]

My current Prop is a 14x17 cupped and my WOT rpm is low. Its on a 90hp Johnson. Its a brand new prop its a shame it not the right one for my boat. I have a 19' Crestliner. V-hull. How much will my rpm increase if I go to a 13.25 diameter with 17 pitch? My top end is 35mph, 4600 rpm or so on the old tach.<br />What about a Stingray or Dolefin for it?<br />Thanks for your help...

 

[corm]

Re: Prop. Diameter

Hi Fire,<br />For every inch you change your prop you will<br />change your engine RPM 150 to 200 RPM. So you will increase the RPM 3-400 RPMs with the 17" prop. Prop dia changes with the pitch to get the proper surface area on the blades so the engine can turn the prop at the proper RPM with the load it is pushing.<br /><br />...have fun...corm

 

[corm]

Re: Prop. Diameter

Sorry I miss read your post. Changing the dia<br />will give some more RPMs, but not necessarily<br />more speed due to the blade area. You may get more slipage because of the size of the boat. If your engine came with a small dia prop the yr that it was made you may like it better, because it will rev up quicker into the power band of the engine and pull more.<br />If you use an uncupped prop you will also gain RPMs,but gain cavatation in the corners and you won't be able to trim up as much under power. If your tach is right it seems<br />like you need a 15" cupped prop. Wish I had my OMC prop book at home. <br />hope this helps some.<br />corm

 

[Firewood]

Re: Prop. Diameter

Yea. I need to get the rpm's up. I just couldn't decide if I wanted to go smaller diameter and less pitch, or just smaller Diameter. I think I'll go with 13.5 x 15" pitch and see how that is. I'm not looking for more speed I just want to motor to run normally and not be bogged down at WOT. It gets up and goes, I really should check the rpm's with a better tach.<br />Thanks