Helices Pirania, con Palas Intercambiables

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Composite Props

Composite propellers aren’t new. They’ve been around for years and have provided satisfactory service when used in small outboards, electric trolling motors and as emergency limp-home props for larger engines. Ten years ago, their primary advantage was, an in many cases still is low cost. The props’ disadvantages were horsepower and performance limitations. In those early forms, composite props simply couldn’t compare (in terms of performance) with a good aluminum prop, and they were light-years behind stainless models.

In June 1993, Trailer Boats made it first comprehensive test of composite props. It was then that we realized there was indeed a performance potential for composite props. The four-blade Comprop actually ran faster than a stock, three-blade aluminum prop. Also of interest were the modular props with replaceable blades from companies like Gil and Piranha (the Gil Six Gun is now sold by Michigan Wheel). Since that initial test, Piranha has introduced a four-blade prop, and its performance it first rate.

Recently, I visited Piranha Propellers in San Luis Obispo, California, where I spent a full day on nearby Lake Nacimiento testing and comparing prop performance on several different boats, including some widely known and highly respected stainless props. I also did some destruction testing, which I will cover later. Although Piranha set up these tests, its products weren’t the hands down winners across the board. From a performance stand point, stainless props still have an edge, but the gap is closing.

The first test boat was a 21 foot Ski Pro powered by 260 Mercruiser with an Alpha 1 drive. Two Mach Magnums scored the best in top speed with this package. A 13.5x22-inch chalked up 53.5 mph and a 14.5x20-inch was approximately 0.5 mph slower. The Piranha 14x22-inch four-blade came in third at 52.5 mph, which was faster than a 21-inch Michigan aluminum, 19-inch Mercury QSS, a 19-inch Quicksilver aluminum and a 19-inch stainless PowerTech.

In this particular test, the Piranha 22 was a bit slower out of the hole. The Mach 20 was a the quickest, while the Mach 22 was the slowest with 0 to 30mph times of 6.2 seconds and 7.5 seconds, respectively. The Piranha came in at 7.2 seconds – a slower time than the other props, but not slow enough to cause any undue problems for skiers or other water sports activities.

What’s interesting is the purchase price comparisons. Three-blade aluminum props run about $160, the four-blade Piranha retails at $170, while the stainless props start at more than $300 for the least expensive and range well into the $600 price range for props with exceptional rake, cup and polish.

The second boat we ran was a 21 Stoker tunnel hull with a 200hp Evinrude. The Evinrude turned a four-blade Piranha 24-inch pitch prop and ran comfortably into the mid-60s. Again, the boat wasn’t quite as fast as with a Mach stainless prop, but the difference was slight.

The piranha prop consists of a hub with two end caps and three or four replaceable blades that slide into and interlock with the hub. The blades are all marked with their pith, and changing pitch is as easy as changing blades.

The replaceable-blade design allows for some interesting experiments with the four-blade prop. Both the Ski Pro and the Stoker would plane with two blades removed, providing the blade bases were in place to funnel the exhaust back behind the blades. If the exhaust broke over the blades (over-hub exhaust), the prop would ventilate before the boat could reach planing speed.

With the two blades missing, the remaining blades must absorb twice as much horsepower. Even with a considerable amount of throttle and hard acceleration, none of the remaining blades broke. Planing times increased because of the loss in blade area. Top speed fell and rpm was higher due to a considerable amount of blade slippage.

Next came the most difficult part of the test for me. With a great deal of reluctance I deliberately piloted a third runabout with an 898 Mercruiser into a submerged 4x6 to purposely destroy a prop. I hit that 4x6 at full tilt. The drive kicked up, the engine raced, and I settled to a stop in the water. Advancing the throttle again brought forth a great deal of shakes and shivers. Raising the lower unit disclosed three of the four blades totally missing and the fourth slightly damaged. Examining the 4x6 disclosed that one of blades was cut better than halfway through. The prop hub, however, was undamaged. The broken blades were replaced in short order, and I repeated the process again, with nearly identical results. I’m not sure whom this speaks better of – Mercruiser for a unit that has been put to incredible abuse without a failure or a prop that’s easy to break and just as easy to rebuild.

An impact of this type would destroy a $160 aluminum prop (most likely beyond repair). You might be able to repair a stainless prop after such an impact, but that would include a minimum repair bill around $100 and at least one week of lost time at a repair station. For the Piranha composite prop, replacement blades are $27 each and there’s no lost time for repairs. After the kinds of impacts I put these props through, it appears that the hubs are nearly indestructible.

It’s also apparent that composite props have come a long way. The performance of the Piranha makes it a perfectly acceptable choice for a primary prop, and its modular design makes prop repairs and pitch changes simple tasks that can be completed “in the field.” Piranha builds props for 3-, 4- and 4.50 inch gearcases, a range that covers a majority of engines from sterndrives and V-6 outboards down to engines in the 20hp range.