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COMP TIME

By: Jim Barron

Composite plastic propellers continue to improve, but do they measure up to stainless props? We test three brands to find out.

So-called "composite" propellers have been around for more than two decades. These 'glass-reinforced plastic designs were initially marketed as inexpensive, spare "get home" props, to be used only when the main stainless steel or aluminum propeller was damaged during a day of boating.

Over the years, however, the durability and performance characteristics of composite props have improved dramatically. For example, in our previous testing ("Composite Props: How Good Are They?," June 1993), we found that the performance offered by composite props can equal or exceed that of aluminum props.

Since then, new "comp" prop brands have entered the market, and the engineering has been refined. Composite props also offer some of the most interesting designs on the market today. Advantages are low price, light weight and good durability in normal use. The disadvantages of a composite prop? They are not repairable if damaged, which has led to prop designs with easily replaceable blades. This had the effect of reducing the cost of prop repair to as low as $20 if only one blade is damaged. Also on the negative side, comp props have not been able to optimize performance of an outboard or sterndrive. For this, stainless props rule. Yet with the new composite designs, we wondered if the pardigm still held true.

So we decided to stage a new round comp prop performance testing. The results suprised us. Without giving away the results, we will tell you that at least one comp prop brand is closing in on the performance standards of stainless wheels.

TEST SUBJECTS

Our prop evaluations consist of both objective and subjective measurements. Objective measurements consist of rpm and fuel consumption at top speed. We also measure the time to reach 30 mph from standing start, as well as rpm and fuel ecoomy at 30 mph. Subjective testing includes an evaluation of how well a prop holds in hard turns, how well the prop fits and carries the bow, and overall handling.

Our test was conducted on California's Lake Castaic with Sylvan 1600 Adventurer DC powered by an 80 hp Yamaha four-stroke outboard. This 1600-pound aluminnum fish n' ski package comes standard with an 18-inch-pitch Yamaha Performance Series three-blade stainless steel prop, which we used as a baseline.

ProPulse is the newest of the comp props we tested. This is a four-blade design with a flat (chopper type) trailing edge with progressive pitch and a mild cup. The ProPulse is unique in that the pitch is adjustable over a range of five inches. We ordered an 18-inch-pitch prop, and this covered a range of 16 to 20 inches in pitch. Adjustable in one inch increments, it allows you to match pitch to engine rpm and load conditions of the boat. The pitch is adjusted by twisting the blades in the hub. An allen wrench is required to make the adjustment. We ran the prop at each pitch setting, and it is interesting to note how a 1-inch change affects the overall performance of the boat and engine.

The second composite brand we tested was Piranha. This prop also is adjustable, but in a different way. The hub is designed to accept interlocking blades. To change pitch, you change blades, and the only tool required is a prop wrench. Piranha props come in three- and four-blade configurations.

The four-blade Piranha hub features a built-in thrust washer. Unfortunately, it appears that the engineers at Piranha mismeasured, because the thrust washer comes in contact with the bearing carrier before it seats on the prop-shaft shoulder of Yamaha's 80 four-stroke. Because of the mismatch, we could not run the four-blade Piranha on our test boat.

The three-blade Piranha hub requires the use of a stock thrust washer and comes with a choice of two blade styles. The "B" style is a round blade and the "XB" style had a flat (chopper type) trailing edge. There is a sizeable difference in the performance between the two blade types: the Piranha with the XB blade configuration is better. We ran the three-blade Piranha hub with 19-inch-pitch B blades and 19- and 21-inch-pitch XB blades.

The third brand of prop we ran was the Comprop by Composite Marine Products. This company was one of the first to market a composite propeller, and it too has made design changes and improvements over the years. The Comprop is noteworthy for its low price, but it does not offer the option of replaceable blades. If the prop is damaged, you'll have to replace the entire propeller. We ran Compop's four-blade 17- and 19-inch-pitch models.

Stainless Steel Baseline

With the stainless prop, Yamaha 80 pushed the Sylvan to 41.4 mph at 5200 rpm. The engine burned 6.8 gallons of fuel per hour (gph) at wide-open throttle (WOT), and the boat accelerated from 0 to 30 mph in 11.4 seconds. The 30 mph cruising speed was accomplished at 4000 rpm with a fuel burn of 4.8 gph.

From a handling standpoint, the prop maintained a better bite in turns than any of the others, and provided enough lift at WOT that it was possible to overtrim the boat. Top speed was accomplished with the trim set just a few degrees less than maximum. At full throttle,the hull ran steady in calm water, but hitting a slight wake would cause the boat to start porpoising, which was quickly cured by trimming down, but with subsequent loss in speed.

This hull seems a bit touchy in that regard. We suspect that part of the propoising problem is due to the weight of an 80 hp four-stroke outboard. The Yamaha weighs 356 pounds, while a conventional two-stroke in the 80 hp range weighs approximately 100 pounds less. That is quite a difference on the transom of a 16 1/2-foot boat. This hull is rated for a maximum of 90 hp, and if the use of a heavy engine is contemplated - with strong consideration given to 90 hp unit. Porpoising poblems were greatly diminished once the hull passed the 40 mph mark.

ProPulse

As mentioned earlier, the ProPulse four-blade adjustable prop has a range of pitch from 16 to 20 inches. We ran the Sylvan at all five pitch settings. The objective test results are best expressed in chart form, and so we will limit our comments to subjective opinions.

The PropPulse holds well in turns, but not as well as the stainless Yamaha prop. It also allows the use of maximum trim, but doesn't provide the bow lift of the stainless prop. There was a slight tendency for the boat to porpoise at a maximum speed, a condition that was easily controlled with a bit of down trim and a resulting loss in speed.

The ProPulse figures give a graphic example of how prop pitch affects engine rpm, speed, fuel consumption and acceleration. The highest top speed does not always come with the prop that lets the engine rev to the max. Highest top speed usually occurs with a prop that holds the engine near the middle of its recommended operating range. Propping for top-end rpm, however, has the advantage of improving holeshot, which is clearly shown by the figures. And, it helps prevent overloading the engine when carrying a heavy load.

Comprop

From a performance standpoint, the Comprop 17 gave us the best acceleration without allowing the engine to over rev. Comprop's 19 gave a much better top speed. However, neither model held as well in turns as either the stainless prop or the ProPulse, but Comprops did give us the best control over the boat's tendency to porpoise. This four-blade prop seems to have quite a bit of stern lift, which makes it easier for the boat to carry the weight of the four-stroke outboard. Those who are having handling and ride problems with a three-blade prop should try this relatively inexpensive four-blade to see what it can do. If the Comprop helps, a properly sized stainless four-blade will probably work even better. If you don't like how it works, you still have a lightweight spare to bring you home if your main prop is lost or damaged.

Piranha

A big surprise in this test was the Piranha. Available in three- and four-blade configurations, we didn't get to test the four-blade because, as we discussed earlier, it didn't seat properly on the propshaft shoulder.

The 19-inch-pitch Piranha with the round-ear B blades is not the best performer on this boat. If doesn't have the blade area to provide enough stern lift to get the bow free or control porpoising. The ability to maintain a bite in turns was not exceptional, either.

However, repalcing the 19 B blades with the cleaver-style 19 XB blades was an eye-opener, not for the increase in performance, but because of the difference in rpm the engine turned. The tachometer went nearly to the red line, while speed increased slightly. Handling was better, but still not up to the ProPulse or the stainless prop.

The next run was with the 21 XB blades, and they did the trick, posting the best top speed of the composite props tested. The 21 XB blades still didn't hold as well in turns as the four-blade props, but on a straight run the boat was fast enough to be up and free-running. Porposing was not a problem. The three-blade Piranha with 21 XB blades came within 0.2 mph of matching the top speed of the factory stainless prop.

Smaller diameter is partially responsible for the improved acceleration times of certain composite props. Some composite manufacturers are quite varied. Comprop offers a limited-lifetime warranty. This is a warrranty against material and manufacturing defects, but does not include damage or breakage due to striking underwater objects. The ProPulse prop has a three-year warranty against faults in the propeller hub due to materials or manufacturing defects. Prop blades are not covered, but are inexpensive and easy to replace. The Piranha has a lifetime warranty on the hub, and the blades are also inexpensive to replace.

With the cost of premium stainless steel props ranging from $500 to $700 the composite props are beginning to look good to boaters who run shallow or stump-infested waters. The cost of repairing a stainless prop can run several hundred dollars, and except for very minor nicks or dings, neither stainless or aluminum props are repairable in the field. With composite props, the entire prop can be rebuilt from anywhere between $20 and $100, depending on the number of blades damaged.

If we were building stainless props, we would think seriously about creating a line of composites to match, for comp props may be the wheels of the future.