Swapping prop to counter improper outdrive gears

[Stuntheavy]

Hi All,

First off, let me say that this is an EXTREME budget build, to the utmost. Just a couple college kids trying to get on the water. Not looking for the best, or fastest.

That said, it's a 19' chriscraft concept bowrider, 5.0 Liter (302 V8). Just picked up an omc cobra outdrive off of a 3.0L which I believe is a 1.71:1 gear ratio if I'm not mistaken. I realize this isn't ideal gear ratio, but the price was right for our budget.

What can I do to get this boat to pull a couple tubes decent? Again, not looking for the baddest. Just looking for something to be able to haul a couple tubes occasionally, fish off of, and make it out to the beach and enjoy a tasty beverage in the fine Florida sun.

The outdrive currently has a 13 3/4 X 15 prop. What if I were to go to something in the 17-20 pitch range? Would that compensate enough for the gears?

Thanks again everyone for all your help. It's truly appreciated!

-Chris

 

[Frank Acampora]

Re: Swapping prop to counter improper outdrive gears

Is the alpha capable of handling the torque and horsepower of the 5.0? I suspect not. You may just tear the gears out of it no matter what prop you use. That is the first question I would ask before asking about props.

 

[Stuntheavy]

Re: Swapping prop to counter improper outdrive gears

Hi,
it's not an Alpha outdrive, but rather a OMC Cobra. Not sure where Alpha came from...

The previous owner had the outdrive behind a 305 GM engine, but had some wild contraption involving an automotive radiator, and transmission.

thanks again for your time though!

 

[Frank Acampora]

Re: Swapping prop to counter improper outdrive gears

Sorry, late at night and my brain had a spasm--same thing for the cobra though. An outdrive designed for a 3 liter 4 cylinder engine will likely not have strong enough gears or shafts for the added power of a 5 liter V8. An analogy would be: when you swap in a 350 into an S10 chevy truck, you must also change the transmission.

SO: the first question you need to ask is: "Will the cobra be able to handle the power."

 

[steelespike]

Re: Swapping prop to counter improper outdrive gears

Based on their budget problems I guess they will need to run the 5.0 throttle like it was a 3.0. Not knowing the years I picked 93 and the 3.0 is I believe 1.89 the 5.0 is 1.59.
I'm guessing the 3.0 would run about a 19" pitch for speed on a 19' boat.
Very roughly you need about 18% more prop.Again very roughly a 23" prop.
I don't think you can get any meaningful rpm readings from the 15" prop perhaps you could borrow a size closer to what you need for some tests.One thing you absolutely have to have is a tach.Even with the "right" prop size performance will likely be a little wonky.All said and done Im not familiar with the OMC setup so this is all really approximate.

 

[Stuntheavy]

Re: Swapping prop to counter improper outdrive gears

thank you sir for giving me a ballpark. I know its far from the ideal equipment, and probably won't give the best performance by anymeans, but making due with what I have, and what I can afford. I'm sure you can all relate to that at some point or another.

the boat....she's not named "Rent Money" without good reason!

 

[jay_merrill]

Re: Swapping prop to counter improper outdrive gears

Sounds to me like you should go on a hunt for the right outdrive either immediately, or right after you get the boat going. Sooner or later, your going to tear up the one you have, if you insist on using it.

I also don't see why there should be a huge difference in price between a used outdrive such as the one you have v. the one you should have. It kinda sounds like you latched onto the smaller outdrive, without fully exploring alternatives.

I'm not trying to beat you up over this, but I see a short, expensive boating season in your future, if you don't get this setup right.



???

 

[Bondo]

Re: Swapping prop to counter improper outdrive gears

5.0 Liter (302 V8). Just picked up an omc cobra outdrive off of a 3.0L

Ayuh,... In my own experince,...
You just Can't buy a prop Big enough to overcome the difference between a 3.0l gear ratio, 'n a V8 ratio...

 

[steelespike]

Re: Swapping prop to counter improper outdrive gears

Is the 5.0 an OMC? I did notice that some 5.0s are 185 hp some 3.0s are 150. Obviously a bunch more but not like some 5.0 versions at about 200hp.You will need to be cautious with the throttle and watch the rpm.
Props up to 28" are availavble for some 3.0 applications.
Do you know the year of the outdrive?

 

[elkhunter338]

Re: Swapping prop to counter improper outdrive gears

I would put a 21 pitch on it, make sure your tach is reading right and run the thing. You risk over rev. the engine. A 21 pitch at that gear ratio will give them enough speed. My 3.0 at 4800 rpms will run 28mph, at a 16pitch 4 blade prop. If they run a 19-21 it will run fine, just not optimal.
I knew of a guy who put a 400cid ford v-8 in a truck that had a inline 6 that had just about enough power to get it's self out of the way. With the v-8 it could burn 31x12.50 tires and the old 3 speed tansmission held up ok.

 

[Stuntheavy]

Re: Swapping prop to counter improper outdrive gears

. It kinda sounds like you latched onto the smaller outdrive, without fully exploring alternatives.

Thanks all for the replies. Not sure on the year off the top of my head, Steelspike. I'll have to look at the tag again before I open my mouth and say the wrong year. This outdrive took a couple months to find. Like I said, our pricerange is slim to none. I spent less than half on this outdrive what most of you spend on a single aluminum prop. Heck, I paid less for the entire boat and trailer than most spend on a stainless prop! I'm not exaggerating when I say it's a budget build.

As for RPM, I'm sure I can scare up a set of roller rockers and some beefier valve springs for the ol 302. At that point I have no qualms about turning it to 5k consistently, and 6k under short bursts. The engine is the least of my concerns. This'll be my 6th 302 I've owned, so I've got a good understanding of what the engines are capable of, and what they are happy with. Engines seem to have people running for the dealer, but all these old 302/351/350/454 inboards are about as simple as they come.

From my research (which could lead me astray.... the 302 I know inside and out...the outdrive, not so much), seems to be that the only difference between the v8 and v6 outdrives is simply gear ratio, and not the physical size difference of anything. That said, the outdrive should hold up just fine.

On a side note, I had a stainless merc 2.5 23pitch prop fall into my hands, and happened to notice the splines are the same..spacing will require an idea or two, but it will at least give me an idea if a 23 pitch is even close to what i'm lookign for, then find the proper 23 if need be. Also came across a OMC 15 and 19 pitch for 25$ for the pair, so between those three, I should at least be able to get a ballpark of where to go from there. I know she'll never be ideal...haven't any of ya'll made due with what you've had to enjoy building a project?

Thanks for the numbers ElkHunter, gives me an idea of what the outdrive should do on the proper engine... I've had everything from 400whp mustangs that had a 302 (or what started life as a 302), coupled to the factory 4 cylinder trans, running drag slicks, to 1/2 ton chevys with independent front suspension turning 40" mudders that were 140lbs a piece. If boats are anything like cars and trucks, It's just a matter of being aware of what you're working with.

Anything CAN be broken, it's just understanding what the weaknesses are of what you've built.

 

[86 century]

Re: Swapping prop to counter improper outdrive gears

I stumbled on to a drive this past summer that had been re-geared or had the wrong decal. Was a 1.92 instead of 1.65 After installing it I was off to the lake it made a huge difference using the same prop. I could maybe use half throttle once it was on plane. It came out of the hole like mad but that was about it. This was an old 165(chevy I6) so I would imagine it will be a good time but as others have said watch that tac or what the heck its a ford and will make a real good anchor.

 

[steelespike]

Re: Swapping prop to counter improper outdrive gears

Don't know if you have done I/Os before but keep in mind a marine engine is working like its going up hill all the time.I believe the 5.0 is rated for 4,400 rpm
Even though you can modify for more rpm routine operation over that rated will surely shorten motor life.In a car the motor operates at max rpm for seconds at a time.

 

[Stuntheavy]

Re: Swapping prop to counter improper outdrive gears

Don't know if you have done I/Os before but keep in mind a marine engine is working like its going up hill all the time.I believe the 5.0 is rated for 4,400 rpm
Even though you can modify for more rpm routine operation over that rated will surely shorten motor life.In a car the motor operates at max rpm for seconds at a time.

Absolutely Steelespike. Thanks again for the tips. The boat is pretty light, but I understand what you mean that the engine is constantly under load. The 5.0 is pretty happy living revved up. I've turned some stock shortblocks up into the 9k range in cars. That's a little extreme, and power is falling off, but possible with a little headwork. Engine longevity isn't really a concern here... I've got spare 302's...

Thanks 86 Century, did you ever end up trying a different prop on it? How'd it work out? What was your top speed when you first put it on?

Heck, its a ford anyway. My ol' chevy duramax is used to hauling junk ford around anyway, no difference if its in a boat!

 

[86 century]

Re: Swapping prop to counter improper outdrive gears

I took it off the next day and sold it to someone with a 140.
It made maybe 22-23 down from 34-35.
The old thing pulled a 200# guy up from a deep water slalom start barely got his hair wet.
Oh I should mention she is a mid 70's 22' walk around cuddy.

Another thought just hit me you might run into problems with prop losing efficiency at those rpm's.
If you can get us some video of what happens to the drive at 8k it may not last long but I bet it's ganna break in a cool way.
Good luck let us know how it turns out

 

[Starcraft Enterprise]

Re: Swapping prop to counter improper outdrive gears

If the gears in the outdive for the 4 cylinders are weaker than for the V8's, putting a higher pitch prop on it will also put more strain on them. Think of it as putting taller mud tires on compact truck axles mounted under a full size truck, powering it with a V8 and going hill climbing. The taller tire raises the final drive ratio putting more strain on the differentials (outdrive) that are now overpowered by a V8.
Stick with the lower pitch prop, show good sense / restraint with the acceleration, watch the tach and live with the top speed that you end up with.