Stainless vs Aluminum Propellers

[Starscream]

My E-tec dealer has offered to supply a second propeller at "cost" price. To me that means about 20 to 25% of the retail price but we'll see what he really meant.

So I have an opportunity to go stainless at a reasonable discount. See what Mercury Marine states below (only applies to Mercury propellers):

"If you are looking to improve the performance of your boat, upgrading to a stainless steel propeller is the most cost-effective way to get better handling, acceleration or top speed out of your boat.
Will a stainless steel propeller damage my lower unit in the event of an underwater impact? Mercury engineers, conducting underwater impact testing, found no substantial difference in the amount of torque transferred to the propeller shaft between an aluminum and stainless steel propeller."

Interesting. BS or real?

[Russ]

I don't see the value in a SS prop. Even at cost it's pricey.
If you had a ski boat, I might consider it.

Also, if you ding it up, it costs a LOT to repair a SS prop. Repairing an aluminum prop is relatively cheap.

What I found was ensuring the correct pitch/size was important. I changed my pitch down a bit and got much better performance.

--Russ

[Tomfoolery]

I'm going with Russ on this one. I can get 3 or 4 aluminium for the price of one stainless prop, using normal retail prices. And being that Macs are slow boats as go-fast boats go, I rather doubt there would be a noticeable difference in performance.

I'd rather spend the bucks on a few different aluminium props, like a 4-blade for pulling tubes and skiers, or running heavy (with better reverse and slow speed docking thrust).

I'm also suspicious of the 'same torque' claim. If that's based on the rubber insert limiting the torque to the shaft, that may be true, but there's also prop inertia reflected back to the shaft, and axial and radial forces when you hit something. A relatively soft aluminium prop will bend with less effort than a stainless prop, and the thing it's reacting against when it's being bent (the shaft and lower unit in general) will likely experience less force, since most of it is reaction force (intertia being the exception, to some extent).

I would ask him for the same discount on another aluminium prop, like a 4-blade, or higher/lower pitch once you've tested what you get. If it were me.

[mastreb]

The reason a stainless prop would have better performance compared to aluminum is stiffness, aka low deflection. Higher stiffness means more motor torque transferred to moving water because all flexibility in a prop translates to loss. I would expect to see something like a 5% improvement overall due to the inherent stiffness difference of the two metals. So maybe 1 extra knot out of 20 for a SS prop, best case scenario, at WOT.

Our motors spend 90% of their time doing 7 knots or less, where the difference in stiffness is completely negligible.

Where you pay for stiffness is in a strike: Striking a rock or other object is what kills props. Aluminum just gets eaten by a strike--which is exactly what you want. The problem with SS is the shock of that strike gets transferred to your drivetrain rather than absorbed by bending, so instead of prop damage you get lower unit damage or a spun shaft, which you can't fix easily in the water.

Props are cheap. Motors are expensive. Aluminum is the way to go.

[Starscream]

love this forum. Aluminum it is. Any predictions on the 13 pitch performance? I read another post where the OP had a 17 pitch and was overpropped so maybe 13 won't be so bad.

[mastreb]

Be sure the ETEC dealer greases the prop shaft so you can change the prop in the water, and buy a plastic prop wrench from him. It's a rarely used part but absolutely essential in an emergency, so I stash mine between the hull and the deck liner forward of the aft table seat. Basically anywhere there's otherwise unused space where you'll remember it. Also put a correct cotter pin AND EXTRA NUT in your rig box. Because you're totally going to drop that nut in the water the first time you replace a prop at sea.

[Billy]

Do you guys tear up that many props? I have near 600 hours on my ss prop (18 in.) and not even a small nick. I've been swept over a reef under power and once chewed up one of my rudders in shallows and everything is still fine.

In 50 years of boating I've only seriously damaged one prop. It was aluminum. When it struck a submerged log, not only did it rip one of the blades it shattered the internals on the outdrive. No guarantee going aluminum will save your engine.

The other good thing about ss--it stands up to sand and mud much better than aluminum. Less replacing.

I'm not trying to persuade one way or the other. Just wanted to share some actual experience. Best of luck on your decision.
(But I do like my ss.)

[mastreb]

I tore up my original prop on my first outing in the boat. dunked it at a terribly current affected ramp with no experience, got turned around, and yes I bent all three lobes on the ramp. Yes, the ramp. I'm sure it was amusing for everyone, and then of course all maneuvering sucked after that and I'm lucky I got it out and away. But I didn't damage the outboard, so that's my experience.

So.. 1st prop: About 2 minutes. 2nd prop: About 300 hrs.

[Erik Hardtle]

My motor came with a SS prop... and i knew there was no way I was keeping that on.... especially the way I drive.


I keep a spare aluminum prop on board at all times... and have used it several times. It is easier to change a prop on the beach than a lower unit (which I do not have a spare for)


This was a under water rock hit, which I am sure would have wasted my lower unit if I had the SS prop.

[vkmaynard]

Erik actually damaged his prop while driving up on this dry beach. Made it pretty far before his prop gave up

Victor

[SENCMac26x]

Vick, if Erik had a stainless steel prop do you think he could have taken the highway home. lol

[vkmaynard]

He probably would try just for the challenge.

Victor

[fishheadbarandgrill]

Maybe Erik always wanted to be a farmer.... maybe he was plowing?

[dlandersson]

I have found that SS (I have one) are approx. 2X the cost of aluminum. I have 3-4 props (ebay), and the SS is currently mounted simply because 1) I just got it and I'm trying it out. 2) It's my most expensive so I may as well use it at least some of the time.

I'm going with Russ on this one. I can get 3 or 4 aluminium for the price of one stainless prop, using normal retail prices. And being that Macs are slow boats as go-fast boats go, I rather doubt there would be a noticeable difference in performance.

I'd rather spend the bucks on a few different aluminium props, like a 4-blade for pulling tubes and skiers, or running heavy (with better reverse and slow speed docking thrust).

I'm also suspicious of the 'same torque' claim. If that's based on the rubber insert limiting the torque to the shaft, that may be true, but there's also prop inertia reflected back to the shaft, and axial and radial forces when you hit something. A relatively soft aluminium prop will bend with less effort than a stainless prop, and the thing it's reacting against when it's being bent (the shaft and lower unit in general) will likely experience less force, since most of it is reaction force (intertia being the exception, to some extent).

I would ask him for the same discount on another aluminium prop, like a 4-blade, or higher/lower pitch once you've tested what you get. If it were me.

[Starscream]

Had a look at the dealer-installed prop today, which got me 20mph at a maxed-out 3,000 RPM with the Etec90 (2 adults, full ballast). It's a 13.75 x 17 pitch and obviously too much prop for the motor & boat combo. The dealer will change it out for free of course. After reading all the previous posts I will try a 13 pitch prop next.

I also ordered a Piranha 14x11 3 blade composite prop through the net. Partly because the sales rep sold me on the idea, partly because I just spent a year developing an F.R.P. commercial air handling unit and having a composite prop on my boat will give me some talking points when I start to sell the product. The marketing guys tell me that a personal story creates a "hook" that is invaluable in advertising. Anyway if the theory that 1" pitch = 200 RPM then the 14x11 should get somewhere in the mid 4K RPMs...still doesn't seem like enough but all the other posts seemed to indicate that it will work out.

I hope that one of these will get me up to 23 or 24 mph with full ballast. With my old BF50A I got 8mph with full ballast and 15mph with empty ballast, so maybe that elusive 30mph without ballast is within reach?