Can a plastic DuoProp perform as well as a stainless version costing twice as much? Get ready to save some bucks
When it comes to saltwater corrosion, you may be able to slow it down, but it is hard to stop. And this has been one of the major deterrents to the use of sterndrives in ocean waters, particularly among boaters who keep their craft in the water.
The primary cause of corrosion on sterndrives is not from the salt itself. Instead, it is caused by the reaction between the aluminum of the sterndrive housing and support, and stainless steel for the propshaft, tilt pins, trim rams and fasteners. Place these dissimilar metals in an electrolyte (in this case, salt water), and they act like a battery. Left unchecked, the least √��Ǩ≈�noble√��Ǩ? of the metals √��Ǩ��� aluminum √��Ǩ��� will eventually dissolve.
One way to slow down this process is to remove the metals from the electrolyte. With most outboards, for example, you can tilt the entire unit out of the water while the boat is at the dock. Unfortunately, sterndrives do not afford boaters the same luxury.
Another way to slow down corrosion is to reduce the amount of dissimilar metals exposed to salt water. On aluminum sterndrives and outboard lower units, paint is the primary barrier to corrosion.
Another means involves bonding the metal to sacrificial zinc anodes. Zinc is even less noble than aluminum, and so it will dissolve first, sacrificing itself to save other metal components. However, once the zinc is depleted, the aluminum will dissolve next.
Volvo Penta has attempted yet another solution to overcome this corrosion problem. It is the composite polymer-clad Ocean Series DuoProp sterndrive. This reduces the amount of aluminum and stainless steel exposed to the corrosive saltwater environment. However, the Ocean Series drive does not entirely eliminate metallic exposure. For example, there is no way to shield metal propellers from salt water.
So, Volvo Penta has come up with still another solution √��Ǩ���non-metallic propellers. These are also made of a composite polymer material. This plastic is not only corrosion-proof, it does not promote a reaction with metals.
The question is: How well do these composite DuoProps perform against the conventional stainless DuoProps?
We have watched the development of composite props for many years now, and some of the early designs were truly awful. Performance was poor, so they were best relegated for emergency purposes only √��Ǩ��� a get-home wheel if you happen to wreck your primary stainless or aluminum prop. The only advantage in the past was price: Compared to aluminum or stainless, a composite prop is cheap.
Over the years, however, composite props have improved significantly. Companies such as Comprop and Piranha have done great jobs in refining the manufacturing to the point where some composite wheels are approaching the performance levels of stainless steel props. This has been borne out in our comparison tests in single-prop applications. However, this was to be our first comparison test of a counter-rotating dual-prop application. Volvo Penta selected Piranha to build its composite DuoProps. As in its single-prop models, Piranha employs a system with a universal hub and replaceable blades.
For a test platform, we used our Pursuit 2460 Denali project boat. It is equipped with a 280 hp Volvo Penta 5.7L OSi with the Ocean Series XDP composite drive. Our testing took place on California√��Ǩ�Ѣs Lake Castaic.
We ran the speed and fuel figures with the stock F4 stainless steel prop set ($1249) and repeated the test with the XF4 composite prop set ($699).
Composite DuoProp sets are available from Volvo Penta dealers only, and some special tools are required to remove and install both the stainless and composite props. Since the XF composite props have replaceable blades ($39.95 each), we also tested XF5 and XF6 blades. Both have higher pitches than the XF4, and these pulled engine rpm down considerably, as you can see in the test results.
Our Volvo V-8 engine is rated to turn between 4600 to 5000 rpm. Only the F4 and XF4 prop sets hit this range at wide-open throttle (WOT). These props also resulted in the best top speeds and acceleration times. Twin-prop drives maintain a tenacious grip on the water, and I was unable to discern an undue slippage in hard turns with either the stainless or composite props.
ECONOMY vs. SPEED
Based on these test results, it is clear that propping has far less effect on fuel economy than it does on performance. For example, all of the props recorded their optimum cruising speeds at 3500 rpm. Over-propping did raise the optimum mph, but not the mpg √��Ǩ��� it remained the same, no matter which prop we ran.
Even when approaching the upper end of the speed range (38 to 40 mph), the XF5 and XF6 prop sets were allowing the engine to turn 4200 and 4400 rpm, respectively, at WOT. While burning fewer gallons per hour, the 2.1 mpg remained relatively constant, regardless of the prop used.
Above 40 mph and 4500 rpm, fuel consumption rises dramatically. Performance costs money. What our figures show is that you can prop for maximum performance, but it will cost you only when you get heavy-handed with the throttle.
What conclusions can we draw from this test? For optimum performance, select a prop or prop set that allows the engine to turn in its recommended rpm range at WOT. Over propping does little to enhance economy, it hurts acceleration and it places additional strain on the engine.
There is a big difference in price between a stainless steel DuoProp set and a composite DuoProp prop set. Yet, our testing disclosed that the performance difference is minor. The stainless props have an edge in top speed, but the composite props offer slightly quicker acceleration.
In salt water and brackish areas where corrosion is a problem, composite props will reduce maintenance. For those who consistently fish shallow water or boat in debris-laden waterways, the replaceable blades of the composite prop will reduce the cost of repairs and make it financially possible to carry the parts necessary for a complete prop rebuild. With the proper parts, a composite DuoProp allows the owner to rebuild his own prop at the dock or on the beach, saving a trip to the prop shop and possibly salvaging a weekend of boating.
TEST PLATFORM SPECIFICATIONS
PURSUIT 2460 DENALI
(w/o trailer) $68,270
Price as Tested
(w/ trailer) $75,000
Length 24√��Ǩ�Ѣ 9√��Ǩ?
Beam 8√��Ǩ�Ѣ 6√��Ǩ?
Weight 4800 lbs.
Deadrise at Transom 21 degrees
Draft (drive down) 34√��Ǩ?
Fuel Capacity 100 gals
Base Power 280 hp
Max. Horsepower 300
Pursuit Boats, Dept. TBM,
3901 St. Lucie Blvd., Ft. Pierce, FL 34946;
ENGINE AS TESTED
VOLVO PENTA 5.7L OSi/XDP
No. of Cylinders V-8
Displacement 5.7L/350 cid
Weight 990 lbs.
Max RPM Range 4600-5000
Gear Ratio 1.96:1