Cruzieduzie said:For more information, please visit Steel Props.
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If you want more HP, look at the Engine specs very carefully. if there is a 9.9 and 15 hp that share the same displacement, your are not going to gain very much, if anything. The 9.9 very likely underrated(that way they can get under HP Limits and Registration of Boats in some Areas. The 15 would get it's extra power by extending the RPM range and moving the Torque Curve to Higher RPMs, with a corresponding decrease in Low End Torque. On a Very Light hull, that isn't a big Deal, but you are having problems getting on Plane, and you need more Low end Grunt. An engine with More Displacement and Higher HP is what you need.As for your Original Question...SS props being stronger can be made thinner which raises their Efficiency,it also allows for better Blade designs that enable changes to Engine Height(really not required until speeds over 45 mph). and higher Trim Angles which lessen wetted Area on a Hull. Some Props can literally lift the Entire Hull Vertically, others are designed to keep the Bow Down and were developed to keep Tunnel Hulls from flipping over backwards at high speedThe much commented 'Blade Flex' was marketing Genius, as it has sold Many SS Props. Get an AL Prop,clamp a blade in a Vice, and you will see how Inflexible(about 1/100" on a 20 " prop) it is, besides, it is pushing a Fluid, and the Fluid will yield long before the Blade does. .
Are Stainless Props Worth the Money?
Prop selection is a critical factor in getting the most from your boat. While you can get wrapped up in the technical details of prop design and performance tweaks, from our perspective, prop selection is a mainly trial-and-error process that balances off a number of factors.
Getting the right diameter and pitch, on-the-water performance and feel, and of course, price, all need to be considered when deciding which prop is right for you, and prop material figures prominently in all three areas.
Consider this: 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.
But 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.
The increase in performance is usually attributed to greater stiffness, which translates into less flexing of the blades, but in a lower-powered boat like our project boat, would we notice any difference? Most reports involve boats with much higher horsepower engines, and much higher horsepower/weight ratios than what we're dealing with.
That's why we decided to do this article. We've now tried a number of similar propellers made of these three materials on Toy Boat 2, and here are the results'
During initial setup of our project boat (Toy Boat 2), we started with aluminum propellers from our previous boat, then eventually switched to a 4-blade ComProp for our final configuration. That propeller was a 13.75 diameter x 15' pitch model that yielded wide-open-throttle (WOT) engine revs of RPM, and a top speed of 31 MPH.
This was in the middle of the recommended WOT range for the motor ( - RPM), so we felt like we pretty much had the right pitch/diameter and blade construction. (NOTE: These tests were done with our old carbureted Johnson 90 HP outboard, not our new Evinrude E-TEC.)
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 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 ComProp's 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.
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When we repowered TB2 with a new Evinrude E-TEC 90, we ran a number of tests to measure fuel economy and performance. These tests were done with a 3-blade Piranha propeller with 14 x 13 blades. The lower pitch was used to get the engine RPM into the upper end of the allowed WOT RPM range, as recommended by the dealer.
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, than to swap the entire prop.
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 compared 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:
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. The SST easily survived a close encounter of the worst kind with the pavement in front of our home, as well as a few unintended trips through some submerged trees at a local lake. Those incidents, in our opinion, would have seriously damaged either a composite or an aluminum propeller.
In the analysis below, we are going to assume the following:
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
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/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, as it is infinitely easier to carry spare blades than a spare prop.
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