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Prop for 5.7l omc cobra with king cobra outdrive.

O

Omcking

Member
Hey everybody!!

so I have got a 1991 imp Europa 205 with the 5.7 liter omc and a omc king cobra outdrive. I understand that this is not the ideal setup to have but it’s what I got. On the flame arrester cover it specifies the boats optimal operating range at 4200 to 4600 rpm at wot. The current prop that is on the boat does not specify pitch or diameter however at wide open throttle it’ll pull about 5400 to 5600 rpm. I am hoping someone would be able to tell me what pitch prop I would need. What diameter would be best and where I can purchase a prop for it. The help would be greatly appreciated.

thank you!!
 
The current prop that is on the boat does not specify pitch or diameter however
Click to expand...

Ayuh,..... Take it off, 'n look closer,...... Props are stamped, or the pitch is cast, into 'em,......

Many times, right under the prop nut,......

When ya find the pitch, go up 4",.....
 
Omcking said:
Hey everybody!!

so I have got a 1991 imp Europa 205 with the 5.7 liter omc and a omc king cobra outdrive. I understand that this is not the ideal setup to have but it’s what I got. On the flame arrester cover it specifies the boats optimal operating range at 4200 to 4600 rpm at wot. The current prop that is on the boat does not specify pitch or diameter however at wide open throttle it’ll pull about 5400 to 5600 rpm. I am hoping someone would be able to tell me what pitch prop I would need. What diameter would be best and where I can purchase a prop for it. The help would be greatly appreciated.

thank you!!
Click to expand...

NOTE: WOT RPM is a test RPM ONLY, in that the engine must be able to reach the OEM specified WOT RPM range in order to determine if the final reduction, propeller, etc are correct.
It is not an RPM that is intended to be sustained for any duration.... unless you want to shorten the life of your engine!

Since you can reach a WOT of 5,400/5,600 RPM, and if the tachometer is accurate, you are under-propped.



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Omcking said:
So do I need to go to a higher pitch prop or a lower pitch prop
Click to expand...

As Bondo and I suggested, first check to see if your tachometer is accurate.
If so, and if the engine can reach a WOT of 5,400/5,600 RPM, you are definitely under propped, in which case you would go up in pitch and possibly in diameter.


The WOT RPM test and propeller selection is not necessarily a perfect science.
It may involve trying an array of propellers before you get it right.


If you error, be sure to error on the side of a higher WOT RPM range, rather than a lower WOT RPM range.
In other words, if your engine cannot reach the OEM specified WOT RPM range, it will become over-burdened at all RPM.



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"In other words, if your engine cannot reach the OEM specified WOT RPM range, it will become over-burdened at all RPM."


Well, that's a bit misleading. The engine is ALWAYS under load at ALL rpm. It's not over burdening to have an over-pitched prop spinning at, say, 2 or 3000 rpm. You just won't be very efficient. The engine will run out of horsepower too soon with the higher pitch so it's not gonna wear out the engine or anything, you just won't be able to go as fast as you could with the proper pitch.
It's like gearing in a car.
Also, it's not really a science it's more art. In order to get the correct prop dialed in you're supposed to test various props over a measured mile on flat water with no wind and the tanks full. This involves swapping propellers, testing, swapping, testing, until you hit the perfect one. Nobody really has the time to fool around with all that so you take your best wild-ass guess.
Generally speaking your engine and drive ratio (assuming it's the King Cobra for small block not big block) can most likely do pretty well with a 14.5 diameter by 24 inch pitch propeller. If you're winding it out at 5K plus then you probably have a 19 inch pitch prop. Just guessing tho.
If you are in waters that are deep enough that you aren't going to be whacking your prop on rocks or the bottom, then go with stainless steel for its stiffness and corrosion resistance. Otherwise go aluminum. They are cheaper and soft enough to take a hit without blowing up your drive. Don't use plastic props. They are too soft and will deflect.
You won't find any info on the prop outsides. Esp if it's aluminum the size will be cast into the hub so you have to look in where the nut holds it on. Sometimes it's just a bunch of numbers so getting the info on it may be a challenge.
You can spend several hundy on a prop, even thousands for a hand tuned racing prop. Luckily we have craigslist and ebay. Try a couple used ones to see if you're in the ballpark and splurge on brand new when you think you have the right one.
 
o2batsea said:
Ricardo said:
"In other words, if your engine cannot reach the OEM specified WOT RPM range, it will become over-burdened at all RPM."
Click to expand...

Well, that's a bit misleading.
If you'll put your eye glasses on and read what I typed, you will see that it's not misleading at all.

I'll even loan you a pair.
shopping


The engine is ALWAYS under load at ALL rpm.
That is very true and is most likely understood by most all members.
What I was referring to was "over-burdening" the engine if it is not able to reach the WOT RPM.


It's not over burdening to have an over-pitched prop spinning at, say, 2 or 3000 rpm. You just won't be very efficient.
A Marine Engine that is performing Inefficiently (due to being over-propped) would equate to a reduced life span.

The engine will run out of horsepower too soon with the higher pitch so it's not gonna wear out the engine or anything, you just won't be able to go as fast as you could with the proper pitch.
It may not wear out the engine on that particular day or week, but give it a full season or two, and report back to us.

It's like gearing in a car.
Also, it's not really a science it's more art.
Call it what you want to. For a very good reason the OEM Engineers have determined a best suited WOT RPM for given engine.
It's right there in the OEM manual, and it's often printed somewhere on the flame arrestor.

In order to get the correct prop dialed in you're supposed to test various props over a measured mile on flat water with no wind and the tanks full. This involves swapping propellers, testing, swapping, testing, until you hit the perfect one.
This is why the OEM Engineers suggest a WOT RPM spec.

It is also necessary to load the boat as you normally would while doing the WOT RPM test.
It would make no sense at all to prop it with a light load, and then expect it to perform well with a full and heavy load.

Nobody really has the time to fool around with all that so you take your best wild-ass guess.
If a person is not willing to take the time to prop their drive correctly, when will they find time and budget to replace a very expensive engine after it fails or wears out prematurely?


Generally speaking your engine and drive ratio (assuming it's the King Cobra for small block not big block) can most likely do pretty well with a 14.5 diameter by 24 inch pitch propeller. If you're winding it out at 5K plus then you probably have a 19 inch pitch prop. Just guessing tho.
I would suggest to the OP that he use real numbers, and not just take a guess at it.
Click to expand...

OMCKing, please read what Bondo and I have suggested to you.



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OMFG. Is it your new hobby to bust on every post I make so that you don't end up sounding like a moron? Let the poor guy make his own mistakes. How else is he gonna learn? From you? Hahahahahaha
 
o2batsea said:
.............. Let the poor guy make his own mistakes.
Click to expand...

That is poor forum etiquette, and is exactly what will occur if he was to listen to the suggestions as per post #9!
Bill and I are in the OP's corner, and we do not want to see him make a mistake and have trouble.

You know full well (or should know):
...... why the OEM suggests a specified WOT RPM range.
...... why the OEM determines a final drive reduction as per the engine and hull.
...... why we select a propeller that allows the engine to reach the specified OEM WOT RPM range.
...... that an over-propped engine will be overburdened, not only at low or high RPM, but at any RPM.

In one thread (12-2019), you stated: "I don't know where you get the idea that I'm some kind of professional! Not. "
Why then do you feel the need to disrupt the OP's thread with rhetoric and misinformation?
Quite frankly, I don't get it!

This is an open forum whereby we do our best to help a member so that he/she will not make costly mistakes.

If you have a bone to pick, at least be clear about it, but please don't do so at the OP's expense!

To date, you have disrupted quite a few threads with your sarcasm and ill attitude, including Dovo3058's, Fishmaster23's, socalboatr's, Bracker's (several times) and now Omcking's thread!
Why do you feel the need to do that?




 
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