Are Stainless Steel Props Worth It?

One of the most common questions that boat owners ask is "what type of material should my propeller be made of?" The choices for most of us are: aluminum, composite or stainless steel.

A typical 3-blade stainless steel propeller runs around $350+. Compare this to around $110 for an aluminum prop. Composite props are in the same price range as aluminum, but some brands can be found for less.

While stainless steel is the most expensive material, stainless propellers have a reputation for being much more durable that either aluminum or composites. They also purportedly offer better performance, but we could not find detailed data for boats like Toy Boat 2. Most reports dealt with much higher horsepower engines, and much higher horsepower/weight ratios.

That's why we decided to do this article. We've now tried propellers made of these three materials on Toy Boat 2, and here are the results... 

We then switched to a 3-blade 13.75 x 15 Vortex aluminum propeller from Michigan Wheel This propeller produced wide-open-throttle (WOT) engine revs of 4900 RPM, and a top speed of 29 MPH. A slight degradation in performance, and contrary to the conventional wisdom that a 4-blade propeller would give better holeshot and less speed. However, if you held the two props next to each other you could immediately see why: the ComProps blades were much thinner and had a much sharper leading edge than the Vortex.

We then switched to a 3-blade composite Piranha prop. The Piranhas have interchangeable blades, so we chose a set of 13.75 x 15 blades. We originally thought performance would return to the same level as the ComProp's, but were surprised to find the performance unchanged from the Vortex prop. This is actually consistent with Piranaha's claim of performance equivalent to aluminum props. We didn't have independent measurement of the various specs like rake and cup, but visually the props looked pretty close.

When we looked closely at the blades, it appeared like the blade foil design for the Piranha looked very similar to the Vortex. The leading edge was thicker and duller than the ComProp.

Given that the ComProp was a better performer than the Piranha, why hadn't we changed back to the ComProp? The main reason was that the ComProp was not offered with a 13" pitch. Even if it had been available in 13" pitch, it was more cost effective to simply buy new blades for the Piranha.

After running the E-TEC with the Piranha propeller for a couple of months, we switched to a 3-blade Evinrude SST stainless propeller, in 13.75 x 13 size We re-ran our earlier performance tests and replotted the data. While the weather conditions were not exactly the same (warmer air and water temps), we feel like they were close enough not to have materially affected the results.

Here are some comparison charts. The red lines are for the SST, the blue lines for the Piranha. Click on the charts to see a larger version:

Speed vs. RPM

GPH vs. RPM

MPG vs. RPM

Clearly, there is a slight performance boost above planing speeds. At 1000 RPM, the apparent increase in fuel economy is probably a result of math roundoff issues. Our Standard Horizon fuel flow meter only reads to tenths of a gallon per hour, so in all likelihood the MPG at 1000 RPM are the same for both propellers.

To the naked eye, the SST has slightly more rake than the Piranha, but it has a much thinner foil design and a sharper leading edge than the Piranha. Its top-end speed is nearly identical to that of the ComProp with the old motor. This leads us to suspect that the ComProp would have performed similarly, had we been able to find it in the same diameter and pitch.

Stainless is clearly the toughest material of the three. As noted in the Toy Boat 2 Blog, it easily survived a close encounter of the worst kind with the pavement in front of our home. That incident, in our opinion, would have seriously damaged either a composite or an aluminum propeller.

• The Vortex aluminum and the Piranha composite will perform identically
• We will run an average of 1500 miles / year

Cost Comparison at Constant Speed:

Propeller	Typical Street Price	Speed	MPG @22 MPH	Annual Gas Cost @$2.50/gal	Annual Gas Cost @$3.00/gal	Annual Gas Cost @$3.50/gal
Vortex	$110	22 mph	4.7	$798	$957	$1,117
Piranha	$125	22 mph	4.7	$798	$957	$1,117
SST	$400	22 mph	5.1	$735	$882	$1,029

So at the $3.00/gallon point it would take around 3.9 years to pay off the difference in price vs. the aluminum propeller, or 3.6 years vs. the composite

Cost Comparisons at Optimal MPG

So at the $3.00/gallon point it would take around 3.0 years to pay off the difference in price vs. the aluminum propeller, or 2.9 years vs. the composite. Note, however, that our average speed would be more than 10% higher (26 vs. 23 MPH).

Perhaps more important than gas cost, however, is the impact on total range. At optimal fuel economy, the stainless steel propeller would give TB2 a maximum range (assuming similar loading conditions) of 252 miles, versus 225 miles for the aluminum or composite - a 27 mile difference!

• In this horsepower/weight/RPM range, propeller blade foil design has more of an impact than either propeller material choice or number of blades.
  
  
• Composite propellers can perform as well or better than aluminum props
  
  
• Stainless steel propellers provide better mileage than aluminum propellers, possibly more than a composite, depending on blade foil design
  
  
• The stainless steel propeller provides more speed, but in this horsepower/size/RPM range, only 2 - 3 MPH more
  
  
• Stainless is tougher, no doubt about it.
  
  

In this horsepower/size/RPM range, the small incremental increase in speed would not justify the additional cost, nor would the savings in the cost of gas. However, if you combine these factors with the increase in range and the toughness of the propeller (keeping in mind that we don't want to have to try to change a prop while at sea), in our opinion, it is worth the extra cost.

If the price of stainless steel is a bit too much, we would strongly recommend a composite prop over an aluminum prop. If you are not looking for the convenience of interchangeable blades, and can find the right diameter and pitch. the ComProps are the least expensive, and were great performers on TB2. The Piranha composites are also a great choice.