1988 Liberator with OMC RPM and Prop question

[dellsboater]

I have a 1988 OMC Cobra Drive, 350 Chevrolet. It Idles fine and the tach reads 800-1000 RPM's.
At 35 MPH at plane it reads 4000 RPMS, but at WOT it Reads 5900 RPM's at 52 MPH. That tells me I should run a 25 pitch prop not the SS 21 I have now. Taking into account each prop pitch increase lowers RPM's by 200, that should put me at 5100 RPM's and more top end. The specs say it should run at 4600-5200 RPM's.

So It seems either I am under proped, or my tach is faulty. I did restore this and replace the tach, but the idle does seem correct. It runs about 140 degrees at 35, but does go to 180 when running wide open and doing some quick turns. It does come back down to the 140 range when running around 35 MPH, and when in no wake areas runs even cooler.


Any advice appreciated

 

[HT32BSX115]

Re: 1988 Liberator with OMC RPM and Prop question

I have a 1988 OMC Cobra Drive, 350 Chevrolet. It Idles fine and the tach reads 800-1000 RPM's.
At 35 MPH at plane it reads 4000 RPMS, but at WOT it Reads 5900 RPM's at 52 MPH. That tells me I should run a 25 pitch prop not the SS 21 I have now. Taking into account each prop pitch increase lowers RPM's by 200, that should put me at 5100 RPM's and more top end. The specs say it should run at 4600-5200 RPM's.

So It seems either I am under proped, or my tach is faulty. I did restore this and replace the tach, but the idle does seem correct. It runs about 140 degrees at 35, but does go to 180 when running wide open and doing some quick turns. It does come back down to the 140 range when running around 35 MPH, and when in no wake areas runs even cooler.


Any advice appreciated

Howdy,


I think we may have had this conversation before............

but, Using your numbers with 15% slip and a 1.43:1 drive which I think you have......... (AND which is common for a single prop drive in a planing runabout) you should be going 70mph!

(but with that engine you're not going to go 70mph!!)

I would suggest keeping the 21p prop and replacing your tach!


I think you're turning around 4500-4800 RPM or so at WOT.

I had (still have actually) a 19p SS prop (that's repitched/cupped to 20p) that I used on my previously installed 460 King Cobra.

It would go 60 MPH (radar) at 5000 RPM........ (after a REALLY doggy hole-shot) .

I now do 57 MPH at 4600 RPM with 26p props......... and I can beat most boats out of the hole!



OH.......AND......if you are REALLY idling at 800-1000 you NEED to fix it or you will eventually damage the drive.

But I suspect your tach is WAY off.

 

[dellsboater]

Re: 1988 Liberator with OMC RPM and Prop question

What should Idle be?

I have not heard that a steeper pitch prop will increase hole shot before.
On my Checkmate, I went from a 21 to a 23 and hole shot is slower but top end is faster.

 

[HT32BSX115]

Re: 1988 Liberator with OMC RPM and Prop question

I have not heard that a steeper pitch prop will increase hole shot before.

It did on mine because I went from a 340hp engine, 1.41:1 King Kobra drive and a 19P prop to a 330hp engine, 1.81:1 Bravo III drive and (2) 26p Props.

I cut the prop-slip almost in half! (by doubling the number of blades in the water)

 

[HT32BSX115]

Re: 1988 Liberator with OMC RPM and Prop question

What should Idle be?

Around 600-650 RPM. But considering your tach is off, you might be right there......