RPMs and Prop Pitch

[kbhatti]

I'm new to boating, so please excuse my terminolgy if incorrect.<br /><br />I just purchased a 16ft Fisher Marine Bass Boat with 1992 90HP Evinrude VRO V4 Motor. It has a 14X21 prop on it and at best runs between 4400-4800 RPM when I have the throttle buried (I'm not sure if this is the same as WOT).<br /><br />From what I've read on your board, RPM for motors of this type at WOT should be in the 5500 range. <br /><br />I have a few questions regarding next steps. One, should I be worried that motor damage exists due to previous use at this lower RPM at WOT ? Second, can you recommend a course of action, with respect to next steps, i.e get a lower pitch prop ? Lastly, I have a spare prop that is 12/34X 21. I've heard that props can be re-pitched. Is it a good idea to do this ?<br /><br />Thank you.

 

[Dhadley]

Re: RPMs and Prop Pitch

Welcome!!<br /><br />Honestly, a 21 on a 16' boat with a V4 doesnt sound like its that far off. I'd look at the motor to make sure its healthy and doing all it should. Being 1000 rpm off leads us to think something else is going on. To gain 1000 rpm youd have to drop to about a 16" pitch. That aint right. <br /><br />Yes, wot = wide open throttle.

 

[patenglish]

Re: RPMs and Prop Pitch

I have a 1989 88hp Evinrude on my bass boat. The only thing different is that I pre-mix my oil and gas. 4500-5500 is wide open throttle on this engine. I am currently using a 13.5 x 18 prop and am changing to a 13.25 x 17 to get a better hole shot. I currently run 5200 rpms wot.

 

[Drydocked]

Re: RPMs and Prop Pitch

FWIW, I used to own a 17' Tracker Tournament V-17 with a 1991 90HP Johnson V-4, and I used to run a 13.25 x 17 aluminum prop. The motor turned about 5100 rpm, boat ran 47-50 mph.<br /><br />The TV-17 was a pretty heavy boat, as I had 3 batteries + a lot of gear in it. I would think a lighter 16' hull would be right in there at 5200-5500 RPM.

 

[walleyehed]

Re: RPMs and Prop Pitch

DryDocked, in this case, I believe his prop is a Raker...a specialty prop when compared to a std 17, in fact they don't compare.<br />As for your Tracker, 5100RPM is lugging the engine, and it looks as though your tach or your speedo is WAY off, good chance the speedo.<br />17 pitch on the V4 at 5100RPM with an excellent slip ratio figured at 6% would only produce 38-39MPH. I realize you no longer own that boat, but it isn't capable of 50 with the 17".

 

[the rebel]

Re: RPMs and Prop Pitch

Originally posted by noviceangler:<br /> I'm new to boating, so please excuse my terminolgy if incorrect.<br /><br />I just purchased a 16ft Fisher Marine Bass Boat with 1992 90HP Evinrude VRO V4 Motor. It has a 14X21 prop on it and at best runs between 4400-4800 RPM when I have the throttle buried (I'm not sure if this is the same as WOT).<br /><br />From what I've read on your board, RPM for motors of this type at WOT should be in the 5500 range. <br /><br />I have a few questions regarding next steps. One, should I be worried that motor damage exists due to previous use at this lower RPM at WOT ? Second, can you recommend a course of action, with respect to next steps, i.e get a lower pitch prop ? Lastly, I have a spare prop that is 12/34X 21. I've heard that props can be re-pitched. Is it a good idea to do this ?<br /><br />Thank you.

Hi and welcome. I would check to see if your carbs are actually opening fully at WOT, if not, a cable adjustment will be in order. The 21 pitch does sound close for the 90 horse engine, a 19 would probably be more optimum to get to your target 5500rpm. What is your MPH? And yes, you can re-pitch a prop 2 inches in either direction; ie: 21 pitch can go to 23 or back to 19. <br /><br /> I have a 16' Fisher Marine Marsh Hawk with a 75 horse Johnson running a 17 pitch prop. My tach isn't working at the moment, but I can tell it is not lugging.It spins up pretty nice with no over rev. My biggest problem is porpoising. I am getting ready to get a set of Smart Tabs to aleviate that problem and hopefully can get it out of the water a bit more. It is clocking right at 42 on the speedo. A solid 45 should be doable. A Raker prop might do it after i get my set-up where I want it. Good luck novice.BWJ

 

[walleyehed]

Re: RPMs and Prop Pitch

And yes, you can re-pitch a prop 2 inches in either direction; ie: 21 pitch can go to 23 or back to 19. <br />

Re-pitch??....No.<br />Add or remove cup???....yes. Problem is, when we remove cup, we also remove efficiency (Bad).<br />Best case is to have to add cup which will in most cases, increase efficiency.<br />To Truely change pitch, the blades have to be removed and welded back on....not practical or cost-effective by any means.

 

[kbhatti]

Re: RPMs and Prop Pitch

Thanks to all for your feedback. I put on a 13.25X17" and now the motor revs at 5100rpm WOT.<br />It should still be higher, right ? How does compression factor into the overall equation ?<br />Again, sorry for what may be rudimentary questions.

 

[walleyehed]

Re: RPMs and Prop Pitch

Compression is a basic verification that certain internal components are in a condition to provide good power and sevice life.<br />If you have low comp., it will be harder to start and have lower high-end power.<br />5100 isn't high enough in RPM...we need to see atleast 600-700 more.<br />Do you know what the compression is??<br />How high is the engine monted???