Prop Dillema 1974 Cobalt 19V

[Mahoney]

Ok, I understand about as much as I thought I needed to know about Props and choosing the right one for your application.

Although I am a bit confused about what I am seeing in my setup. I am currently running a Propco S24 14X18 Aluminum prop and the 225 OMC GM 307 is still able to rap up the rpms to over 4500 or so. WOT on my engine is 4400.

Cobalt is cool enough to have the original owners manuals online and recommend for Heavy load a 14X13 SST and Cruising a 14X15 SST.

I have an original perfect 14X13 SST I am going to try tomorrow, but my question is. With the 14X18 Alum I seem to be getting something rediculous like 40% slip. Top speed prolly only 37 or so at WOT.

Could the 14X18 just be a shoddy made prop? I know aluminum will deform a bit under load, but that much? do I have way to much vntalation? Could that old GM 225 be putting out more horses for some reason?

Stumped.

 

[QC]

Re: Prop Dillema 1974 Cobalt 19V

What's the gear ratio and are you cranking that into your calcs? Kinda looks like you are using 100% of your engine RPM as prop shaft RPM although I didn't check your figures . . .

 

[Mahoney]

Re: Prop Dillema 1974 Cobalt 19V

Gear ratio is listed for that setup as 1.16

 

[walleyehed]

Re: Prop Dillema 1974 Cobalt 19V

Well. I don't trust my books anymore, but if the final drive ratio is 1.16:1, that's gonna take a very low pitch prop...
To be on the safe side, I'd verify the tach.

 

[Mahoney]

Re: Prop Dillema 1974 Cobalt 19V

Thats whats odd, it recommends a very low pitch prop, but for some reason I was able to turn a 14X18. I am going to search for a handheld tach for myself to verify.

Thanks

 

[Crownie2]

Re: Prop Dillema 1974 Cobalt 19V

Are you positively sure it's not 1.61 to 1?? Could be a typo.

I ran the numbers using 1.61 and they came out pretty close...

Bob

 

[Mahoney]

Re: Prop Dillema 1974 Cobalt 19V

Overal ratio listed as
Overall
Ratio
(prop to engine)=1.16

Gear Ratio
Total
(engine to prop)=.86

 

[Crownie2]

Re: Prop Dillema 1974 Cobalt 19V

Well that works out the way they are saying it, so it may be, but I've never heard of a ratio that high... Almost seems to have no point.

Also - I'll second what Kenny said - check your tach to be sure it's accurate.

Bob