COMPOSITE MATERIALS, combinations of resins, plastics and space-age fibers, are replacing metal in everything from bicycle frames to car engines. Now composite construction has entered the mainstream marine world in the form of the COMPROP, a composite propeller.
The COMPROP was introduced several years ago as a three-blade design intended as an inexpensive “get-home” spare wheel. Now the COMPROP has been redesigned as a primary-use, four-blade propeller, and it will be offered by Regal, Wellcraft and Corona as the original-equipment propeller on a limited number of new boats.
One such boat is the Regal 202SE. To see how the COMPROP stacks up against the aluminum alternative offered by Regal on this boat, we headed to the boat-builder’s test center in Orlando.
The COMPROP can handle up to 200hp (outboard or stern drive) aboard boats as big as 21′ and/or 3,000 pounds. Our 19’10″ bowrider test boat, with its 180-hp MerCruiser 4.3LX V-6 engine and an Alpha One outdrive, fit those parameters easily.
COMPROP has a 13 1/2″ diameter, and it’s offered with 18″, 20″ and 22″ pitch. Composite Marine Propellers, which manufactures the COMPROP, says changing from three blades to four smaller blades spreads the load out over more area, eliminating the excessive blade flexing that hampered the performance of earlier models. CMP says the COMPROP blades now flex about as much as an aluminum prop’s blades.
CMP refuses to divulge the exact material used in the prop, other than to say it’s a composite—let’s call it Unobtainium. It’s very light—the 22″ model weighs just 2.5 pounds, compared to 4 pounds for an equivalent aluminum prop. The entire prop is one piece, with splines for the prop shaft molded into the hub. There’s no rubber hub cushion.
The alternative offered by Regal on this boat is that long-time favorite, a three-blade Quicksilver aluminum prop with a machined spline insert and a rubber hub. In our test, the aluminum propeller showed the strongest midrange speeds while the COMPROP provided the quickest acceleration—as would be expected of a four-blade prop—and top speed.
But what about the COMPROP’s durability? CMP claims the COMPROP stands up well in general use. In a high-speed, solid impact, the COMPROP blades are designed to break off the hub, thus protecting the shafts and gears of the drive unit.
This is why the COMPROP doesn’t have a rubber hub center. CMP’s point is that it’s better to break an inexpensive prop than an expensive gearcase. In the same circumstances, aluminum blades would likely bend, absorbing some shock, while the rubber hub would (you hope) handle the rest. If bent, the aluminum prop could perhaps be repaired, not necessarily replaced. And unless it is severely damaged, the aluminum prop remains a propulsion device.
According to Quicksilver marketing manager Bill Gius, “Our rubber hub and the ductility of the metal material will protect the gearcase. It’s still more economical to repair an aluminum prop than to continually replace a composite prop.”
The cost (or even possibility) of repairs, of course, is determined by the extent of the damage to the propeller. Perhaps the COMPROP’s strongest feature is its price. The 22″-pitch COMPROP lists for $94.95, compared to $140.75 for the Quicksilver aluminum prop.
The COMPROP is available with hub splines to fit MerCruiser and OMC Cobra stern drives and all brands of outboards. It also can be ordered from marine dealers and through catalog houses like Overton’, Bass Pro Shops and West Marine. Outboard Marine Corp. sells it under the Prop-Tech label through its OMC Parts and Accessories division.
The original three-blade COMPROP also is available in several sizes for smaller outboards. It’s the COMPROP’s low price that has attracted the attention of boat-builders. But performance can’t suffer to save a buck, and Regal is certain its customers will be happy with the COMPROP. Clearly, space-age materials are coming of age.
- Four-blades for improved acceleration, cornering and speed
- Costs 30% less than aluminum
- Composite construction allows blades to break upon impact
Why worry about propeller size?
While your prop pushes your boat, it also acts as a single speed transmission. Your boat is not like a car with many speeds” or “gears” that can be used while underway. The prop size (diameter and pitch) determines the “gear” you’re in all the time- whether pulling up a skier, pleasure cruising, or going for all-out speed. Most boat manufacturers use a simple performance test that determines the correct prop size for almost all applications. It is explained below. We have also included a few definitions and hints.
If you have further questions, call us. We’re open 8:30-5:00 central time. Good luck!
There is no “master chart” that shows what prop your boat needs.
You need to know three things:
- The size propeller (diameter and pitch) currently on your boat. Pitch is usually stamped on the prop somewhere. You can sometimes determine this if you have the prop’s model number or some other identifying marks from the prop.
- Your engine’s recommended RPM limit (red line). This information is in your engine owner’s manual. Common RPM (revolutions per minute) limits are; stern drives (I/O) 4200 to 4800 RPM; outboards 5000 to 5800 RPM.
- Test your current prop. How much above/below are your engine’s RPMs vs. the red line? This test should be performed at full throttle, maximum speed at full trim and with a light load. Your engine should be in tune, your prop should be in good shape. Don’t count on what you remember from last season! Perform this test before you buy any prop.
Interpreting the result
If your propeller is the properly sized for your boat, your actual RPM during the above test should be within 100-200 RPM of your engine’s RPM upper limit. You want your engine to be able to develop full power without exceeding the RPM limit. A correctly sized prop on your boat will let your engine develop full power, but will not allow it to go over the RPM limit.
A general rule of thumb is each inch of pitch change will result in a change of around 150-200 RPMs. If your test shows you need higher RPMs, select a prop with lesser pitch (shifting down). If your test shows you are over the RPM limit, choose a prop with higher pitch (shifting up).
- Most applications are a compromise. Most boaters want good acceleration and top speed.
- Using a higher pitch won’t always increase top speed. If you ”over pitch”, your engine won’t develop full power at the lower RPM and you could lose speed while dangerously laboring your motor.
A 21 pitch prop turns 4200 RPM on your boat, but 4600 is the upper RPM limit. Try a 19″ pitch prop to shift down” and increase your RPMs. If your outboard is turning 6100 RPMs and your red line is 5600, “shift up” and try a prop with 2″ more pitch and test again.
Propellers are labeled diameter x pitch. (A 13×19 prop is 13″ in diameter and is 19″ pitch)
- diameter – the distance (in inches) across the circle made by the blade tips as the propeller rotates.
- pitch – the distance (in inches) a prop moves forward with one 360Â° revolution (assuming zero slip).
If you’ve dinged a propeller really badly lately, you know that a replacement prop can put an even worse ding in your wallet. A new aluminum prop can cost $200, and stainless-steel wheels start at $350. But now there’s a third alternative. The COMPROP is a composite propeller (please don’t call it plastic) that will equal or better the performance of a good aluminum prop for less than $100. Available for most brands of outboards and stern drives up to 200 hp, the COMPROP is now installed at the factory by some boat manufacturers, including Wellcraft, Regal, and Glastron.
Molded of a proprietary fiber-filled resin material, the COMPROP has a splined aluminum hub insert to mate with the propshaft. Unlike aluminum and steel props, COMPROPs do not have a rubber-cushioned hub to protect the gearcase from prop impacts–the company says its blades will flex to absorb minor impacts or simply break off in a hard impact. The theory is, it’s cheaper to replace a COMPROP than risk damage to the expensive gearcase or to repair a metal prop.
We had the opportunity to test the COMPROP on Alaska’s Kenai Peninsula. The rough Anchor River beach launch we use to reach halibut on Cook Inlet often requires running through rocks and gravel to return to the landing.
Our test boat was a heavily loaded 19 1/2-foot Almar Lite aluminum boat, powered by a 1983 Evinrude 115 outboard and running a 13- by 19-inch OMC SST II stainless-steel prop. We started by measuring the boat’s performance with the old prop (see chart). We then gathered the same speed and acceleration data with a new OMC SST II, an OMC aluminum prop, and a COMPROP, all 13 by 19 inches. The COMPROP met its claims, producing about the same top speed, and better acceleration, than the aluminum prop. It was only 0.8 mph slower than the new SST II, and almost 3 mph faster than the battered original prop—proof that it pays to run a straight prop.
Note that the COMPROP is a four-blade design, so thrust is spread over the two drive blades that are always in the water. This elminates the problem of the composite blades flexing under load, not a concern with aluminum or stainless props at this performance level. And after the performance test, we ran the COMPROP through a gravelly area along shore, succeeding only in slightly chipping the blade edges.
We left the COMPROP with the boat’s owner for the season, hoping to get some long-term time on it, but when push came to shove, he stuck with the stainless prop. He reasoned that if he were running up the Ukon River, say, and the COMPROP sheared its blades, he’d be up the proverbial creek for real. Losing the blades completely is the COMPROP’s one serious weakness. Even a badly bent metal prop will likely produce some thrust, but when the COMPROP’s blades go, so does your control.
The COMPROP we used had a suggested retail price of $89.95, compared to $110.95 for the OMC aluminum prop and $355.95 for the OMC SST II. Johnson and Evinrude dealers can order a COMPROP directly from OMC Parts and Accessories. As an alternative to aluminum and stainless props COMPROP is worth consideration by boaters who don’t demand the ultimate performance and don’t operate in unusually rugged conditions. For more information contact Composite Marine Propellers Inc., 9808 Pflum Rd., Lenexa, KS 66215 (800-266-7767). —Charles Plueddeman.