choosing the right stainless prop - OMC Cobra 4.3L

[nutnboltguy]

i have an 88 17' Reinell 173E with a newly rebuilt OMC Cobra 4.3L motor and outdrive with the stock 15x17 aluminum prop and i want to upgrade to a good stainless prop. i should add that the motor is bored .040" over and now has 272ci as opposed to the stock 262ci and is equipped with a Holley 550cfm 2bbl (marine of course).

with 6 hours on the new rebuild the boat comes right out of the water and planes within seconds, and cruises nicely at 2300-2500 rpm... i absolutely love it, but i understand the benefits of the stainless prop (strength, durability and efficiency) and so now im on the search for what the best and most reasonable stainless prop would be for my boat.

the stock aluminum prop that im running now is a 15x17 and im running this boat at 2500' elevation in the sierra foothills and 6200' when i take it up to Tahoe.

your guidance and expertise will be greatly appreciated on what size, pitch and brand.

thanks in advance!

 

[Scott Danforth]

read the stickies at the top of the prop forum. there are 6 things you must answer.

you may also want to PM a mod to move this to the prop forum.

 

[nutnboltguy]

thanks!

 

[nutnboltguy]

how do i notify a moderator? dont know of any just yet... lil help.

 

[nutnboltguy]

okay, im now on the prop forum so im asking how to choose the right prop for my 17' Reinell 173E with a few more vital bits of info.

boat is 17' long, weighs 2000lb, has a freshly built GM 4.3L equipped with a Holley 500cfm 2bbl, OMC Cobra out drive with the stock 15x17 aluminum prop. it holeshots and planes great as is.

my biggest question is: should i just keep the same diameter and pitch or go to a larger diameter like a 15.5"x17 since the stainless prop is more efficient? i wouldn't mind a bit more cruising speed for when im out on Lake Tahoe so it doesn't take forever to get across the lake.

thanks.

 

[Chris1956]

What is your max RPM with a "usual" load? That will tell you whether to change the pitch.

Generally, you will choose the design, material and pitch of the prop, and the manufacturer will choose the diameter.

 

[nutnboltguy]

im cruising at 2300-2500rpm with 600-800lb (including the cooler) and it feels like its hardly under load.

even pulling a tube last weekend with a big person on it it was more than fine as far as engine load goes.

fyi, i bored .040" over when i rebuilt the engine so its now 272ci rather than the stock 262ci. 10 more cubes and a tuned Holley 500cfm 2bbl.

 

[H20Rat]

im cruising at 2300-2500rpm with 600-800lb (including the cooler) and it feels like its hardly under load.

even pulling a tube last weekend with a big person on it it was more than fine as far as engine load goes.

fyi, i bored .040" over when i rebuilt the engine so its now 272ci rather than the stock 262ci. 10 more cubes and a tuned Holley 500cfm 2bbl.

Cruise doesn't matter. Need wide open throttle rpm's and speed with GPS with usual load or slightly less. (I like to do WOT with just myself and 3/4 or so gas.) Adjusting a prop to how a boat performs at cruise speed is a good way to ruin an engine. In car terms, you could end up driving around at 20 mph in 5th gear, lugging the engine and putting lots of load on the connecting rods and bearings.

 

[nutnboltguy]

copy that. im kind of cautious about running it at WOT at only 10 hours on the new motor so i think ill let it break in before i try this method...

but i think for now is it safe to say if i choose a stainless prop with the same diameter and pitch that it will be a pretty solid upgrade and more efficient?

my aluminum prop is in bad shape and i want to change it out and i was hoping to upgrade when i did.

 

[bruceb58]

No problem running WOT on a new motor. You actually want to do that occasionally with a new motor so that the pistons and rings reach their maximum stretch point. Don't want to develop a ridge in the cylinder while running super slow and then that first time you open it up the rings hit that ridge.

Run the motor at varying RPMs which includes high RPMs.

My experience with stainless props that are identical shape to an aluminum prop, you drop a little pitch since the stainless props hold their pitch with less flex than aluminum during acceleration.

 

[jkust]

I'll be interested in the results of this. I've always been under the impression with some experience and reading some stories here as well that low hp engines such as a non-vortec 4.3 won't be able to harness the potential benefits of an SS prop. I've always thought though that just their thinner blades alone would have to provide some benefit. Curious why didn't you add a 4bbl carb when you were doing the mods? (I say that again, as i've read mixed results with the 4bbl on 4.3's)

 

[nutnboltguy]

Curious why didn't you add a 4bbl carb when you were doing the mods? (I say that again, as i've read mixed results with the 4bbl on 4.3's)

tell ya, i run the same carb on my built chevy 302 that's im my 63 IH Scout and the thing i like about it is it is quick and driveable. when you lay into it (WOT) that power valve opens up and its more than enough power for that little truck. i figured the 4.3 bored .040" over is 272 cubes (30 less than the 302 in my Scout) so more than enough carb for that little boat. hole shots and planes nice n quick and cruises real easy. works well is all i know.

 

[jimmbo]

Well, the Holley 500 is more carb than the engine actually needs, but it is a good simple little carb. You could lean the main jets out a couple of sizes when at the higher altitude, but would have to remember to put the bigger jets back in when you leave.

Stainless Steel props are heavier than Aluminum props and much stronger. Being stronger the blades can be made thinner. which raises their efficiency. Running a stainless prop is not without risk, the stronger blades, unlike AL do not break off/bend as easily absorbing impact stresses, and often lead to bent propeller shafts, and with the advent of rubberless hub systems, more damage to internal parts of the drive train.

Don't be too concerned about diameter, let the propeller engineers/designers worry about that.

When it comes to recommendations regarding brand and model, everyone including yours truly, has their preferences. I am partial to OMC/BRP Rakers and Mercury Enertia.

A prop that puts the engine in the WOT range at the lower altitude, is going to be too big at the higher altitude. You will be needing more than one prop.

Since you haven't provided WOT rpm numbers at either altitude with your 17" prop, any recommendation would be a wild crap shoot.

 

[nutnboltguy]

Well, the Holley 500 is more carb than the engine actually needs, but it is a good simple little carb. You could lean the main jets out a couple of sizes when at the higher altitude, but would have to remember to put the bigger jets back in when you leave.

in a perfect world Holley would offer a 450cfm 2bbl for these 262ci V6's so that's pretty much the plan - jet according to altitude - which im pretty good at and well equipped for.

good info on all the prop info you shared. owning a few props is no big thing - just want the boat to run right when i take it up the hill (6200') and back down again (2500'). thanks for your input.

ps: WOT on these motors (GM 4.3L V6) is supposedly 4300rpm.

 

[jimmbo]

4300 sounds low. 4200 - 4600 is what most older I/Os ran

 

[nutnboltguy]

give or take im sure, im just not into pushing it too hard just yet.

 

[QBhoy]

I'd reckon 4300rpm is a little low on this motor...especially if it has been rebuilt. I think the more recent 4.3 carbs run 4400-4800 ?

 

[nutnboltguy]

really?

 

[QBhoy]

really?

Yeah....