RPMS just not there

[16Pilot]

I cant get past about 3500 RPMS. 4.3 OMC 21 pitch prop.(which I think may be the issue but I wanted a second opinion) 17.5 ft open bow stratos about 3300 lbs. When I trim way up it gets up there close but it jumps out of the water.

 

[Don S]

Re: RPMS just not there

Did this just happen? Have you ever gotten over 3500 rpm with that prop? Trimming it too high won't help, the prop has to be in the water to check rpm.

 

[Manipulator]

Re: RPMS just not there

I have the same problem. The guy I bought my boat from changed the original prop for a stiletto prop. I have an OMC 5.7 and my boat is a Sea Ray SR225. I can't get the RPM above 3600 rpm at WOT and trimmed out. I figure I should be able to at least get to 4000 rpm. What do you think? Is my prop the wrong one? I really don't know what the redline is on the engine but it has to be above 4000 rpm. Would decreasing the pitch be a good idea? Would it make my boat go any faster?

 

[QC]

Re: RPMS just not there

We need some more info from both of you. If all else is right, then probably yes the pitch is the problem.<br /><br />In the case of 16Pilot is the 21 inch prop new? As Don said, did this just happen? Or are there other things you changed prior to seeing this RPM? What was the RPM before?<br /><br />With Manipulator, I beleive the wide open throttle (WOT) range is 4400 - 4800, but it may have been 4200 - 4600. Anyway, most tests I have seen show that a prop that runs closer to the bottom of that range at WOT will be a little faster than one that runs towards the top of that range (fuel economy is usually better too). However, hole shot will be best with the prop that runs the higher RPM. 1 inch of pitch change usually results in around a 200 RPM swing.<br /><br />Also, make sure that you rtachometer is set to run the correct number of cylinders. There is often a switch for 8, 6 or 4 cylinders. See the topic for WOT and Propeller selection in the Engine FAQ section of this board it will help with more detail.

 

[Manipulator]

Re: RPMS just not there

So more RPM with less pitch will get you up on a plane quicker but won't do much for top end and will burn more fuel? I get Boating Magazine and sometimes these questions come up. I thought if you get some more RPM by changing the pitch you will pick up some speed. Well this is no hot rod and getting up on a plane is no problem. I'm guessing it will do 40 MPH (hoping). It has a speedo but I really can't get the speedo over 33 MPH or so at WOT. I hoping I'm moving faster than that but can't really tell. It seems like she is really cranking to stay up to speed with most new boats. Then again, this is an old Sea Ray Cuddy (1983) and is probably heavier than today's equivalent. <br /><br />As far as condition goes, it's in good great shape. The 5.7 has a 4 bbl and is rated at 260. I rebuilt the carburetor and put new points, rotor, cap and plugs in. Mechanically, were in good shape. What about putting a 4 blade prop on? Any advantage to that? <br /><br />I will check the Tach. It's a factory one so not sure if it has the usual switch on back.

 

[QC]

Re: RPMS just not there

Manipulator, You need to do two things. Verify that the tach is right and get the WOT RPM. Without that, prop discussion is pretty hopeless. Generally four blades will get the bow up a little more and MAY increase top speed, but there are $1000 dud propellers and $150 aluminums that perform well on some boats. Costs a lot to figure all that out. Read, read, read is all I can say.

 

[16Pilot]

Re: RPMS just not there

what info do you need? The prop is brand new. It is a Hustler 21 Pitch. Tach is set correctly. Engine has only 5-10 hours on it.

 

[Don S]

Re: RPMS just not there

If the tach is correct, and since you only have 5-10 hours on the engine, I guess that means everything is perfect with the engine. Have you tried this new engine with the old prop to make sure the engine works.<br />Lots of things have changed, no one answer will fit your problem.<br />If you have your old prop, change it and see what happens. If the rpms are right, then you have the wrong prop. It's that simple.<br />If the rpm's stay the same (low) then there is something wrong in the engine department.<br />But we have to start narrowing it down, not just makeing a lot of guesses that are going to have to be narrowed down anyway.

 

[16Pilot]

Re: RPMS just not there

don, with the old prop it got up to 43-4400. for some odd reason it wont with this one. They are both the same pitch. The old one was a Michigan match that was chewed up pretty bad from the idiot that had it before me.

 

[Don S]

Re: RPMS just not there

Then you have a prop issue. The engine will achiev the rpm, the prop is keeping you from it.<br />Get a prop with around a 15 or 17 pitch and try it.

 

[16Pilot]

Re: RPMS just not there

Thanks Don, are the Hustler Props ok? I dont want to spend more money on it if its junk. I ask this because I know I can use the same hub