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Material Evidence

By: Mike Hilne

Aluminum, bronze and stainless steel have long been standard materials for propeller construction. Brad Stahl begs to differ. And he's willing to wager a weekend in California that we'll agree. "OK, I'll bite," I reply, when offered a chance to see how Piranha's propellers work, in California late last spring. Brad Stahl, president of Piranha Propellers, has something to prove and he's not above enticing boating journalists to San Luis Obispo, home of his propeller maufatcuring facility, to do it.

I thought I'd get the Piranha joke out of the way as soon as possible. As for Stahl, there's something he'll get out of way early in your acquaintence. That is any impression that he builds the kind of props that will only ever appear on trolling motors or small-horsepower outboards. Don't ever call Piranha props "plastic" --they're actually injection-moulded from a composite material called Verton which is a mixture of Nylon 66 resin and glass fibres -- and don't ever imagine that Pianha props are "toys." California may be a laid-back place, but the props that are produced at Piranha's compact manufacturing facility are serious powerboat equipment.

Available in a range of three- and four-bladed styles suited for outboards from eight to 150 hp and sterndrives ranging up to 260 hp, Piranha's props have been well-tested and are finding acceptance among boaters. It's been a slow battle for Stahl, since he developed the line of props about a decade ago. Piranha's annual test days, which show how a wide variety of propellers perform, are one way of speeding up that progress. He's up against years of tradition, in one of the areas of boating performance where the term "black art" could describe a research and development process. Prop performance is always a tad mysterious and messing with the material used to build props is close to sacrilege. So Stahl's got plenty to prove.

Test day dawns a bit cloudy, but the lake beckons. We've got the full range of Piranha's props from world-class manufacturers such as Mercury Marine, Michigan Wheel, OMC, Sola, Hill and Mach. For test boats, there's a 17-foot Sea Ray sterndrive, an outboard-powered 18-foot Bluewater and a 21-foot Ski-Pro sterndrive water ski boat, all at our disposal at lovely Lake Nacimiento. There are a wide variety of pitches available, with 13-inch diamater props for the outboard and mostly 14-inch diameter props (plus a few 14.25-inch diamater) for the sterndrives.

Using a Stalker radar gun, connected to a laptop computer with a performance graphing program, we are able to determine top speeds for various props, then plot the performance curves. I also use a handheld GPS to gauge top speeds.

While the radar and laptop are valuable technical tools, testing props is still largely a question of trail and error -- trying a prop, testing its performance by feel as well as by the numbers, taking the prop off, installing another one and doing it all over again. Fortunately, there's a crew helping out with the prop-swapping.

If it's working well, a propeller should allow the engine to rev up well into the midst of its maximum full-throttle operating range. For the 3.0 Litre MerCruiser in the Sea Ray and the 5.7 Litre MerCruiser V-8 in the Ski Pro that's 4400 to 4800 rpm. The 140 hp Johnson on the Blue Water has a peak operating range of 5,000 to 6,000 rpm.

As a rule of thumb, reducing the pitch of the prop will let the engine rev higher (about 200 rpm for every inch of pitch), while increasing the prop's pitch brings down the engine speed (once again by about 200 rpm per inch). Every engine and every prop behave differently, of course.

While all of the propellers I tested on the Sea Ray performed adequately in acceleration testing -- as could be seen on the acceleration curves. I found more than a three-mile-per-hour gap at top speed. While the 19-inch pitch stainless steel Mercury prop lets the engine spin up to 4,750 rpm for 39.9 miles per hour, Piranha's four-bladed propeller helps the boat top out at 37.8 miles per hour, slightly faster than the equivalent three-bladed aluminum prop.

While performance curves for the outboard also show similarities between props, with minor differences in subjective performance and handling tests, OMC's aluminum 19-inch pitch prop results in the top speed, coming within less than half a mile per hour of an 18-inch stainless steel Hill Lightning. While a 20-inch four-blade Piranha achieved a top speed over 40 miles per hour, at 6,000 rpm, changing to a 22-inch pitch may have squeezed out a bit more performance. The aluminum prop's performance, however, shows that a well-matched propeller can make a big differnece.

No question, material also influences prop performance. Aluminum is the alloy of choice in most original equipment propellers, for durability at a reasonable price. Stainless steel props are much more expensive, but the blades can be built thinner with less flex for more speed and performace.

Verton, however, also has some unique qualities that make it ideal prop material. It's half the weight of aluminum and a quarter the weight of stainless steel, with a tensile strength that's 15 percent greater than aluminum. Because the blades flex, they can often take a beating, such as being ground into sand or gravel, without actually breaking. One of the boating journalists present for test days did some impromptu testing of his own, grinding the prop into a gravel bed; he was surprised to find no damage. Happy too that he hadn't embarrassed himself by destroying a prop a full day before planned destructive testing was to begin.

On a tour through Piranha's plant where moulds as well as the props themselves are built, Stahl tells the story of Piranha's genesis. An aeronautical engineer, Stahl had been working on a composite propeller program for a military plane that needed a "low radar profile." (Sorry, some of this gets a bit vague, it's classified information.) In any case, the airplane prop program was canceled, but Stahl recognized a non-secret, non-military boating application for the prop technology he helped develop, after his brother kept complaining of breaking countless propellers each summer on an Oregon river. Stahl developed and patented a modular composite propeller system with an aluminum-reinforced hub and removable, easily replacelable blades. So Piranha's uniqueness goes beyond material.

As propeller blades pop out of an injection-moulding machine, Stahl notes that the process "generates an extremely consistent, homogeneous blade." With "less than a gram difference between blades," props run with less vibration and "dampen energy and don't transmit the energy to the lower end and gears."

And when they break, Piranha's composite props give way without damaging gearcases or lower units. Many aluminum and stainless-steel props are equipped with composite or rubber-lined hubs, designed to prevent gear damage. But none can match Piranha's modular construction. But to put Piranha's protective capabilities to the test and have some prop-busting fun, we head back to the lake.

A destructive test boat is ready -- a Ranger runabout with the sterndrive's protective skeg sawn off to ensure a solid hit on a target log. This is obviously the kind of thing these prop builders love to do, just for fun. With buoys in place, I blast over the log and limp back to the dock. I manage to do a fair bit of damage, but because the blade has given way in a predictable fashion, the hub is undamaged. Over the years, says Stahl, he and other testers have destoryed hundreds, perhaps thousands of props to ensure that the hubs won't fail.

As for the damage, it is very easy to repair using a plastic (oops, I mean composite) prop wrench, knock off the top diffuser ring, use the wrench to knock the damaged blade out of its blace in the aluminum-core hub. Slide in a new blade, whack it into place, replace the diffuser ring and install the renewed propeller.

Piranha sells new replacement blades costing less than 20 percent of a whole new prop. So, for much less than the price of any type of prop, you can take along a full replacement set, or carry a set of a different pitch. You may want better acceleration for water skiing, for example, or a prop that will optimize the top end.

Because the side of me that might want to bang up a propeller has been deeply repressed and I don't get to destroy props on purpose every day I try again, and again, and AGAIN.

Every time, I see how easily the blades are replaced, except once when we try the test on a single-piece composite prop built by a competitor. There's hardly enough left of the prop to stay secured to the hub of the sterndrive.

Then there's more objective testing with the help of Skippy the Ski Drone, a contraption that simulates the effect of pulling up a water skier, to let us feel and plot the various props' performance for towing. While the stainless steel props came out on top, the Piranha out-pulls an aluminum Solas propeller of the same diamater and pitch, taking Skipp up to 30 mph in a shorter distance.

In top speed testing as well, Piranha's three- and four- bladed props are pushing the 21-foot ski boat as fast as the aluminum props and only giving up between one and two miles per hour to the fastest stainless steel props.

But is there an ideal propeller? Like most truths in boating, it's all realtive. Any single prop is a compromise, but the boater who wants low end pulling power or planing capability will select a different prop from the boater who simply wants the highest top speed possible. As Stahl explains, even prop-builders work through a process of trial-and-error, where minute changes in blade area, cupping, or size of diffuser ring seperate a prop that works well from one that doesn't work at all.

So, when selecting an all-round propeller that's best for their purposes, boaters should try a few different props. Dealers often co-operate; certainly no reputable dealer would refuse to exchange a prop that simply doesn't work for you.

But what's that ideal prop made from? Well, for you, the ideal material could be Verton just as easily as aluminum or stainless steel. The tests showed me that Piranha's capable of spinning with the best. And if it can't always deliver the top-end speed of a polished stainless steel prop, you can count on excellent performance at a fraction of the price. Bill Jennings of Vancouver-based Canada Propeller doesn't expect Verton to replace stainless steel or aluminum competely.

"Naturally, you can have a little increase in performance with stainless steel poropellers, but they cost two to three times as much," says Jennings. "The Piranhas were a surprise; they're here to stay and they're running with the best of them." For Jennings, though, Piranha's "practical side," that allows waterfront repairs and re-pitching, is what really makes the props winners.

Both in computer-graphed testing and more subjective evaluatuon, Piranha props have proven themselves. I feel more confident about composite material and very impressed by Piranha's engineered convenience.

In an ideal world, we'd all have a collection of about a dozen different propellers to choose from. But if you can only take one propeller with you, wouldn't it be nice to have one that allows you make repairs or even change the pitch right on the water?

Brad Stahl thinks so. And he's proven at least that his composite props can play in the same leaque as the alloy and stainless steel big boys. But if you want to benefit from the slight price advantage over aluminum or if you're the kind of boater who destroys or damages at least one prop a year (hey, don't feel bad about it) you should take a close look at Piranha.

With water-levels dropping on the Great Lakes, props and lower units are sure to take a beating there next summer. Wouldn't it be easier to just carry a wrench and a set of new blades? Something well worth chewing over.