Aluminum and Stainless Steel Boat Propellers

When buying a propeller for outboard or sterndrive engines, you have two primary choices – aluminum or stainless steel.   There are considerable differences between these two in terms of their cost, performance characteristics and durability. In this article, we’ll delve into a comparison between aluminum and stainless steel propellers to help you make an informed decision about which type is best suited for your needs.

Aluminum Propellers

For aluminum propellers, there are two primary advantages:

Cost

The price of an Aluminum propeller is about one-third to one-fifth that of stainless steel.  For those on a tight budget, this may be a key benefit.

Is Sacrificial

If you hit a hard object such as rocks, the blades of an Aluminum propeller will giveaway either by bending or breaking.  This actually helps to avoid damaging your prop shaft or lower unit which may be very expensive to repair.

Stainless Steel

By comparison, stainless steel propellers are superior to aluminum in three key areas: performance, durability and corrosion resistance.

Performance

This is a broad term that often means different things to different people.  In this context, it really comes down to three things: top speed, increased “grip” and better fuel economy.

  • Top Speed: Because stainless steel is stronger, the blades are thinner than that of aluminum which creates less drag. Plus, the stiffer material flexes less and so it maintains its pitch better at high speed or under load.  So why does this matter?  Let’s start by reviewing what “pitch” is.

Pitch is the theoretical distance that the boat would move forward with one revolution of the propeller through a soft solid.  So, a 24” pitch propeller would theoretically travel two feet.  In reality, this never happens because there is always some slippage (and some slippage is actually good and important as you can see from this short article from Mercury).   A stainless steel propeller is likely to have less slip than an aluminum propeller of the same size and blade design, all else being equal, because there is less drag and flexing.  The net result is higher top speed.

How big of a difference is this?  Comparison data is really hard to come by – partially because it is hard to hold the variables constant.  That is, you are unlikely to find an aluminum propeller that is exactly the same size and design as a stainless steel propeller.  Nonetheless, I crunched some numbers that I found from a Boating Magazine and earlier Popular Mechanics article which are summarized in the table below.

Key West with Mercury 150 HP Four-Stroke*Brand/ModelSizeTop Speed (Light Load)Top Speed (Heavy Load)
AluminumMercury Black Max15 x 1741.9 @ 576641.8 @ 5820
Stainless SteelMercury Tempest Plus14 5/8 x 1745.9 @ 577943.5 @ 5534
Difference+10%+4%
Data Source: Boating Magazine
Ranger with Mercury 175 EFIBrand/ModelSizeTop Speed (Light Load)Top Speed (Heavy Load)
AluminumMercury Black Max14 1/2 x 2151.6 @ 585050.4 @ 5665
Stainless SteelMercury Trophy13 3/4 x 2155.3 @ 570553.1 @ 5650
Difference+7%+5%
Data Source: Popular Mechanics

As you can see from these examples, the stainless steel propellers achieved approximately a 5% to 10% increase in top speed over a fairly comparable aluminum propeller.  Though the comparison isn’t perfect for the reasons stated earlier, this does seem to fit with anecdotal information from other boaters.  That is, if your boat currently runs 60 mph with an Aluminum prop, you might be able to get 3 mph or more by switching to stainless steel.

  • Grip. Some would argue that the biggest performance difference in going with a stainless-steel prop is that it will give you better grip or handling.  Because of the reduced flex and better cup design, stainless-steel propellers will “grip” the water better – especially when turning or with the motor trimmed up.  One might think of this as the difference between driving a vehicle on a wet vs. dry surface.
  • Fuel Economy. Given their better grip and lessor flex, stainless steel propellers tend to be more efficient.  That is, each revolution of a stainless steel propeller will likely travel further than an aluminum prop of the same pitch.  This is largely because the increased flex of aluminum reduces the effective pitch when under load.

But what is the difference in the fuel economy of an aluminum vs. stainless steel propeller?  Again, information is very hard to come by.  However, a video posted by Solas propellers suggests that aluminum propellers flex roughly 10% to 15%.  Stainless steel is said to flex very little.  While it won’t be zero, let’s assume the net difference in flex between aluminum and stainless steel is roughly 10%.  Based on that, we can estimate a difference in fuel efficiency as follows:

TypePitch% FlexNet PitchDifference
Aluminum1910%17.10
Stainless19019.0011%

Using this example with a 19” pitch prop, the aluminum propeller would travel 1.9” less with each revolution which equates to 11% better efficiency with stainless steel.  In reality, the efficiency gain will be considerably less because most flexing occurs at high speeds and under load plus it takes more energy to spin a stainless steel prop due to its increased weight.  However, if the actual gain is half this, that is not trivial for those with larger engines and travel long distances or use their boat often.

Durability

Stainless Steel is a much tougher metal.  In fact, according to Mercury, stainless steel has five times more stress tolerance than aluminum.  This means that stainless steel props are much more likely to withstand an impact with objects such as timber or a sandy bottom.

I know this firsthand.  Each Spring, I travel to the shallow backwaters on my home lake of Lake Palestine in pursuit of some big bass.  Like many other lakes in East Texas, it is chock-full of timber in many parts.  It is nearly impossible to avoid hitting a stump in route (and the Major League Fishing pros got to experience this during the “Heavy Hitters” tournament in April of 2022).  I likely would have had to repair or replace an aluminum prop at least ten times by this point.  However, with my Mercury Tempest Plus stainless steel propeller. I am still going strong except for a few minor nicks.

However, there is one downside to this added durability.  Unlike an aluminum propeller, stainless steel props are less likely to “give” when hitting a very hard object such as rocks.  This could potentially result in damage to your prop shaft or lower unit.  One way to mitigate this concern is to get a stainless steel propeller with a removeable hub.  These are designed to “give” on impact which might save your lower unit and potentially even your propeller.  But you will want to have a spare insert or aluminum propeller for a back-up in case you “spin” a hub.

Corrosion Resistance

Stainless steel will likely hold up better in a saltwater environment.  This is because aluminum is a softer metal and therefore is more likely to corrode.  This should not be an issue, however, for freshwater boaters.

Conclusions

Neither aluminum nor stainless steel propellers are inherently better.  There are advantages and disadvantages with each.

Generally speaking, boaters with V6 or larger engines typically opt for stainless steel whereas those with outboards of 75 HP or less often stay with aluminum.  This is largely because the performance benefits of stainless steel are less pronounced in smaller engines.  Between 75 HP and 150 HP is a bit of a toss-up.

However, apart from these generalizations, you need to consider your specific needs, budget and usage environment.

For example, if you are a tournament angler and top speed is important to you, then stainless steel is definitely the way to go.   On the other hand, if you have a pontoon and simply cruise around on a small lake, then aluminum might be perfectly adequate.  Saltwater boaters will likely want stainless steel for the added corrosion protection.

If budget is a factor, you will save money with the initial purchase by going with Aluminum.  However, your long-term cost could be higher if you think you might hit stumps or run aground on occasion.

Thinking of changing to a different sized propeller to improve performance?  Be sure to check out my prop pitch calculator to find out how changes in pitch could impact your top speed.

Sources:

Prop Bite: How To Choose Between Aluminum and Stainless Steel Props“.  Web Blog Post.  Mercury Marine.  15 December 2020.

Prop Bite: Understanding Prop Slip“.  Web Blog Post.  Mercury Marine.

Aluminum vs Stainless Steel Props“.  Web Blog Post.  Michigan Wheel.  2 February 2023.

Carmichael, Casey.  “Is an Aluminum or Stainless Steel Propeller Right For You?”  Web Blog Post.  Boat Specialists.  12 January 2023.

Talley, John. “Stainless Steel vs. Aluminum Propellers“.  Web Blog Post.  Boats.net.  

Horsfall, Sean.  “Is an Aluminum or Stainless Prop Better“.  YouTube Video.  Len’s Cove Marina.  14 June 2020.

Eldridge, Jonathan.  “Stainless Steel or Aluminum Prop? How to Choose the Ideal Metal for Your Boat’s Propeller“.  Web Blog Post.  Better Boat.  24 June 2022.

Boggs, Ron.  “Do You Really Need a Stainless Steel Prop?” Web Blog Post.  High Country Boats. 5 October 2019.

Burden, Tom and Gordon, Brian. “SELECTING A PROPELLER“.  Web Blog Post.  West Marine. 21 April 2023.

The Evolution of the Mercury Propeller Hub“.   Web Blog Post.  Mercury Marine.  3 November 2021. 

Choosing the Right Boat Propeller“.  Web Blog Post.  Boating Magazine.  25 August 2017.

Comparison Test: Boat Props“.  Web Blog Post.  Popular Mechanics.  17 April 2006.

Jerry Mona - BoaterInput

About the author

Jerry Mona is an avid boater and angler and long-time boating industry insider. With over three decades of experience, he is often considered to be the leading research expert with boaters and has helped numerous manufacturers and trade associations to understand the needs, wants, attitudes and behaviors of boaters. He now shares many of his insights about boats and boaters for free on his BoaterInput.com website.

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