Why stainless steel prop?
 

I've been boating a long time, but only just started thinking about stainless steel props. My question is what is the advantage? My understanding is that if you hit something with a SS prop, it has the potential to do internal damage to the lower unit (and I know people that hit rocks at slow speed thinking there was no damage, only to realize later that they caused serious damage), while a non SS prop will tend to bend or break. I know the obvious solution - don't hit anything! - but am curious as to advantages...

I've been told they flex less and you get a slightly higher speed.

From an internet article:

"The advantage of a stainless steel propeller over aluminum is durability. Stainless props can withstand more of the damage caused by small rocks, sand, or other loose objects in the water.

However, the disadvantage to a stainless propeller is that there is “minimal give" to the blades, so if you hit an object hard enough, there is the possibility of causing major damage to your lower unit. With aluminum props, the blades will most likely sacrifice themselves before any damage is caused to your lower unit. "

Also the high cost of stainless steel makes repair and replacement VERY pricey. New Aluminum prop around $100, Stainless $400 - $500.

Supposedly any performance difference is only minimally (10%) due to prop material. It's more the design of the prop. Well designed aluminum beats mediocre designed stainless.

I have dual stainless steel props (Bravo 3 drive) and have the same ones for 24 years. I dinged one early on and had it reconditioned. No trouble or work required since then.

Gain you some advantage in performance boats and if you ski a lot. If you are just a recreational boater, fisherman and occasionally ski or tube people they aren’t necessary.

Also human nature being what it is, thinks that if it cots more it has to be better, faster, stronger... In some cases it's true, but imho, when it comes to props, I would stick with aluminum if you are just a recreational boater. If you own a boat, it's not IF you hit something, it's when... I run a 4 blade aluminum prop on my 22' boat (7.4L - Bravo 1) and it comes out of the hole much faster, (fully loaded), and is significantly more economical on fuel than my standard 3 blade ss prop was, and my top end speed is just fine with the aluminum, keeps the boat right at 45mph...

Thanks all! My next question, is that I am about to buy a used 19 foot Whaler that has a SS prop. Thoughts on putting on an aluminum one instead?

I would get an aluminum prop and carry it as a spare for when you zap the SS prop.

It comes down to how well you know the areas you will be boating in. If you are new to the areas, I would buy an aluminum prop and use it for a few years. Just my opinion.

R2B

Thanks all! I know the waters Well - but boat will be used bu many others who might not. I’m going to look into a prop change.

If it comes with stainless use it. You want to come out of the hole fast get trim tabs.

Save the gear
 

I purchased a SS prop set for my Volvo Duo prop outdrive. I got a little more top end 62 vs 59 but you can't use that top end on Winni. I ultimately put a Aluminum prop set on the boat again and keep the SS for a spare. I'm sure this decision saved my outdrive one day a couple of years ago when I hit some rocks in a location I'd never been. No doubt in my mind that I would have lost the drive if a SS prop set was on. Some will argue that the insurance company will cover your drive if it gets wiped out on rocks, this may be true but you still will lose weeks or months with downtime waiting for the drive repair.

My $0.02.

Did you say how much HP you have? If it's under 150HP save your money. Aluminum won't really flex under 150.

It is 115 HP. My friend who nicked a rock and lost the lower unit had a 200 HP outboard. All very helpful advice!

Advantage Stainless Steel
 

If you nick a rock with stainless steel, you are sure to cause lower unit drive damage. Usually far exceeds the deductable on the boat insurance. Could be an advantage in some eyes.

In reality the difference between the "impact damage" from hitting something with an aluminum propeller and a stainless propeller is minimal. If you hit something at speed you can wipe out the lower unit regardless of what material the propeller is made of. Stainless steel has a higher strenght than aluminum and allows for a thinner blade profile relative to the same propeller in aluminum resulting in a more efficient propeller when properly designed.

Like many things, propeller design has come a long way in the last 30 years. The stainless propeller on the Whaler you're considering may well be the ideal choice for that boat. Why would you remove a perfect propeller to substitute something less efficient on the odd chance you would hit something? Don't buy into the aluminum absorbs damage myth.

We have a Whaler with a mere 90HP for reference; the stainless performance propeller on it made a massive difference, try 43 MPH top end vs 38 with the supposedly same aluminum propeller. That's a pretty significant difference, I'll pay attention to the markers and charts to avoid damaging it...

Stainless props are more durable no question - the performance argument is nonsense unless you are running a high performance high horsepower racing boat. Bear in mind that many of those performance boats are running exotic blade and pitch combinations that can only be attained by using stainless. I'm sorry but I don't buy a change of 5 MPH on the top end just because of the prop material.... change in pitch, 3 versus 4+ blade, absolutely (which would also require a pitch change), but doing a like for like not buying that. In fact I have run both SS and Aluminum on my stern drive and saw 0 performance difference. Same diameter, pitch and blade count.

I also don't buy the argument that a stainless prop will destroy your stern drive if you hit something. Hit anything hard enough the lower unit will sustain damage. For anything else I don't think there is a prop today that doesn't come with a hub that will strip out if the prop hits something. At least that's what Merc has done for years and I know my Johnson outboard has the same design. Both early 2000 vintage.

One reason NOT to use stainless is that it does accelerate drive corrosion a consideration if your boat sits in the water.

Think of it this way.... all these ski and wakeboard boats where performance is such a big deal run brass props. Brass is nearly as soft and flexible as aluminum. If stainless was all that you can bet those boats would be using them.

Pretty sure those are nibral, not brass.

The key point I was trying to make in my post above is that stainless allows for more advanced blade design. Things like the rake of the blades, skew in the blade that allows more effective control of pitch from the blade root to the tips and more aggressive cupping of the trailing edges of the blades to reduce tip losses. No question that a stainless propeller made in exactly the same manner as an aluminum one would have minimal or no performance difference.

The prop I'm running on the Whaler is optimized to run with the engine mounted higher on the transom. The reduction in drag from raising the gear case higher is probably the reason for the speed difference. The aluminum prop was probably cavitating with the engine raised up, it definitely ventilated in turns. It may have performed better with the engine mounted lower. This was all part of dialing in the new engine when we replaced the original engine a few years ago. Also, what I'm seeing on a relatively lightweight 17 foot skiff doesn't necessarily translate to a heavy deep V or a pontoon boat.

Stainless is substantially stiffer than aluminum and flexes less. Thus, for the same or more strength it can be made with much thinner blades, and thinner blades means less drag and less opportunity for cavitation, which translates to more efficiency. If your boat is light and tops out under 40 MPH, it probably won't be worth the expense of replacing aluminum with stainless, based solely on efficiency, but it might be worth trying for the improvement of the feel of the boat. Stainless props tend to grip better and feel more connected to the water. If the boat already has an ss prop, enjoy it. FWIW, I like to gunkhole and have touched bottom many times (at low speed) with my two Bravo 3 SS props. The prop shafts are still both straight and the props have never needed any repairs. I'm confident that aluminum would props would not have been as forgiving of my need to explore. If you hit a rock at low speed with SS, I think you'll be fine. If you hit a rock at high speed with aluminum or SS, well, you kinda deserve some damage...

we can do back and forth all day, and you can get SS ones with Blow away hubs in the hopes that if you hit something it will pop the hub and not cause lower end damage.

OEM recommendations is what I would go with. I hit a rock at headway speed memorial day weekend. Marker was missing and I was in a new area and mist judged the next marker.

I had the original prop from my boat in the hull, but had a dent in the side of the hub so I was not putting it on, i limped back on the damaged replacement prop that the previous owner had on and i used for 4 years.

Had marina order me a OEM prop - Hole shot better, better top end speed, and overall much smoother ride, even my 12 year old daughter noticed the differences.

"Urban Myth"
 

Aluminum takes poorly to flexing. An aluminum propeller would flex for a very short time before it work-hardened to its limit--stretched further, it would "yield"--to break apart!

In general, stainless steels' biggest asset is its ability to "survive" flexing: it is a very forgiving alloy, renowned for its "toughness" among metals.

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Next time you fly, watch how much those big aluminum wings flex...

Modern Jetliners...
 

Modern jetliners have wings made of composite materials. Flex improves the ride, and can exceed even remarkable loads. :eek2:

https://qph.fs.quoracdn.net/main-qim...ae33edb1e85030

Aluminum wings can't be relied on for durability. Inspections are necessary, and even then, aluminum wings can't take overstress circumstances. :eek:

So all those old aluminum-winged airplane are just falling out of the sky?

Another thing that has not been mentioned that you need to take into consideration is the rpm's of your engine. Change in prop size/ and/or pitch changes rpm's of motor. KNOW your motor's maximum rpm. It's not the red line! They vary from one motor to another, not hard to find out though. I believe it's ok to get a prop that will exceed your motors rpm, just don't do it. You may want to confirm that for peace of mind though. I wouldn't even put a prop on my boat that exceeds the maximum rpm's if you have multiple drivers. Someone WILL forget, and blow the motor, just a matter of time. You don't want to have to put in a new motor to go with your new prop... (unless the other driver's are paying for it!!) Sometimes you will lose rpm's in which case you have no worries.

Ideally they are pulled out of service well before catastrophic failure occurs.

Composite materials are far more resilient, lighter and have a longer expected service time so it makes sense they are widely used in such applications.

Modern fighter jets are primarily made exclusively of composite materials and have been for some time.

Understood. My point was that aluminum can and does flex without breaking. Aluminum airplane wings are a perfect example of that. Commercial airplanes were successfully flying with flexible aluminum wings long before carbon fiber was ever used in a plane.

Aluminum Has Its Place: Bending Isn't One of Them...
 

While viewed as "aluminum" wings, that only refers to the outer skin, which is made of sheet aluminum. It's the structural elements, also made of aluminum, that are subject to catastrophic breakup.

In the early days of aviation, aircraft had wings made of wood covered with fabric—later, with canvas. In WW1, a French inventor seeking to fly an invisible aircraft designed and flew a wood aircraft with outside surfaces covered with a plastic predecessor to Saran-Wrap! :eek2:

By WWII, much of aircraft "skin" was redesigned for aluminum. (Canvas was retained on some outer surfaces). Structural elements, subjected to most of regular inspections and replacements, were likewise changed to aluminum. (Although some rebuilders of WWII aircraft have changed structural wing elements back to wood). :eek2:

Aircraft older than 18 years were no longer subject to lawsuits in a bill signed by President Clinton. Older aircraft were criticized for weakened lifting surfaces subject to "flutter", which fatigued aluminum parts.

One month following the C-130 disaster, a second aircraft (a Consolidated Privateer) also lost wings, attributed to aluminum metal fatigue.

My Dad piloted commercial amphibians for Chalk's Ocean Airways until about the year 2000. In 2005, a wing broke off one of their aircraft. :(

The fatigue noted was in an internal aluminum structural strut. painted over, and could not be seen.

Wing loss due to aluminum fatigue is not limited to US aircraft:

Aircraft wings are hardly a good example of flexing, as they are a construct of integrated trusses, and not a solid casting or forging—as conventional boating propellers are.

While all metals are subject to flexion and failure, aluminum is particularly susceptible. As stated earlier, stainless steel is a far better alloy for desirable propeller "yield".

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So you are saying SS is better because it flexes ("yields") more than aluminum? Sounds backwards to me.

I flexed my aluminum prop until it yielded. Glad it will cost only 90 instead of 300.

:D

Flexing airplane wings
 

I was in a commercial jetliner enroute from San Francisco to Los Angeles when it hit turbulence flying over the Santa Monica MTN. Included was an air pocket in which the plane dropped vertically several hundred feet. When we hit bottom my heart sank as the wings flexed a great distance several times. Made me wonder just how much punishment it would take to separate a wing from the body of the plane. Fortunately I am here today to recall that event.

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I am a retired workaholic and continuing aquaholic

You've run rings around me, logically.

"Yield" Reduces Pitch...Then Returns...
 

That's what these folks say:
https://www.canadapropeller.com/sele...php?t=aluminum

That above site has information regarding replaceable individual blades—not a new idea, but the blades are composite! :idea:

Reminds me of the aluminum "Dial-a-Prop" of the 1960's. ('Can't find anything with Google, but the geared hub had a vernier scale to "dial-up" your pitch).

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I clicked on the link you shared and the website says this about Stainless Steel:

"Strongest
At seven times stronger than aluminum the blades hold their shape through the varying torque applied during its rotation. The strength also allows the blades to be thinner, allowing the prop to turn easier in water, and that unused horsepower can be applied to better performance shapes and heavier cupping."

That's the opposite of what you wrote earlier. :confused

Man this thread is really getting into the weeds... Unless you're a professional offshore powerboat racer running a multi-million dollar boat in the equivalent of "Nascar", where you are constantly looking for every possible advantage to squeeze one more MPH out of it, who cares what your prop is made of...??

Composite material Prop
 

OK you tech junkies! Read the following link! Kinda like a cross between SS and aluminum prop!
https://www.mercalloy.com/

The old 454 Magnum is getting tired so I decided to go down a pitch. Rather than buying a new SS, I decided to try a 4 blade Spitfire. I am very please with the result! I went down a pitch, the top speed remains the same, better hole shot, and the rpm is back to maximum. I also notice a decrease in GPH, I expected that.

With a full tank and max capacity and gear, I am very surprise how the Mercalloy prop handles the load, actually better hole shot than an SS.

If you go down in pitch, the hole shot should be better.

4 blade aluminum props are a nice compromise, IMO. That said, a few years ago, I had a boat with a 4 blade aluminum and upgraded to the same diameter and pitch 4 blade SS prop (with more rake though). My boat gained a couple of MPH on top with the same WOT RPM, and mid-range acceleration was noticeably better. Hole shot felt the same. Mid-range punch is what actually sold that boat. The first guy to test drive it was cruising at 3000ish RPM and I said "punch it", the resulting acceleration from 30ish to 50+ produced a huge smile and I knew right then and there that I would be getting my asking price. Helped that it was a really nice boat and a reasonable price, I'm sure.

Lesnor Maehr Multi-pitch
 

Thanks: Same idea, (maybe an earlier design?) but the dial was rearmost, forming a nice taper. (Like an analog radio dial, but tapered).

ETA: Also with "OMNI-Pitch" stamped into the hub.

Oh, yes it does. ;)

Not saying they're wrong.

So as not to break apart, aluminum props have to be fatter. While S/S can be much thinner, with the ability to repeatedly flex for "hole-shots", (described as "yield" in the link). Aluminum can't do it, but good for Lake Winnipesaukee, where a spare prop can be carried on board for the expected or unexpected exigencies.

Then, there's this: are replacement blades available?

http://www.marinepartsonlineshopguu9...20858608_1.jpg

☞ For example, try taking a ¼"x4" aluminum bolt (used in screen enclosures), and put it in a vice next to a ¼"x4" S/S bolt—bend both with a short pipe—you'll be lucky to get two bends out of aluminum, and S/S will take forever to break.

Most (not all) alloys of S/S are well-known among mechanics, sailors, and metallurgists for its extreme resistance to fatigue and eventual failure.

Used on a steel trailer, a loose S/S bolt will wear out the hole it's used in!

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