In recent years, boaters have been given more and more to choose from in the sterndrive arena. Various manufacturers have given us, new models, drives, electronic fuel injection (EFI) applications, twin-prop designs, and other refinements. How do they stack up against each other? To find out, we periodically conduct comparison tests. For instance, in March, we ran a story comparing three similar 4.3-liter V-6s with conventional single-prop sterndrives offered by Yamaha, MerCruiser, and Volvo Penta ("V-6 Sterndrive Shootout", March 1994). This time, however, we're comparing three equally powered V-8s. But, unlike the V-6 shootout that compared nearly identical units, the I/Os in this test, although having equal horsepower, feature diverse applications.

Specifically, our test pitted a MerCruiser 5.7 Bravo Three, a MerCruiser 350 Magnum, and a Volvo Penta 5.7 Duoprop against each other. All three engines are based on a 350-cubic-inch Chevrolet block, and all three are rated at 250 propshaft horsepower. The Bravo Three and Duoprop feature sophisticated twin-prop drives, while the 350 Magnum bears a conventional single-prop Alpha One drive. A major difference is that the Volvo Penta unit is fitted with a throttlebody EFI system and rated at 4600 rpm, while both MerCruisers are fitted with Weber four-barrel carburetors, roller lifters (for a more durable valvetrain at high rpm), and are rated for a maximum of 4800 rpm. (Interestingly, MerCruiser says a 5.7-liter EFI Bravo Three will be available soon.)

TEST PARAMETERS
To ensure that each unit was tested on an equal basis, Stingray Powerboats president Al Fink allowed us to use a section of his Hartsville, South Carolina, manufacturing facility to swap engines in a 719zp sport cruiser. (See accompanying story for details on the boat.) Installing each engine and drive in the same boat required more work and time than if we had put them in three different boats of the same model, but the test is more accurate this way.

Each engine was given the same amount of break-in time, which was conducted at Stingray's test site on Lake Robinson by a manufacturer's representative. During this break-in period, each rep was allowed to do his own prop testing to determine which prop he felt delivered the optimum performance.

SINGLE-PROP VS. TWIN-PROP
For the first test, the boat was powered with the MerCruiser 350 Magnum using the Alpha One single-prop drive. Theoretically, we expected this single-prop configuration to be the fastest and most efficient, because it has less lower unit and prop in the water to produce drag. And, sure enough, the performance charts show this to be the case. From the charts, we can see that this package was fastest, with a top speed of 50.6 mph, although the difference was slight, with only 0.4 seconds separating first from last. The charts also show it to be most fuel efficient, giving us 4.2 mpg at 3500 rpm and 36.0 mph. In our measured data, the only place the Alpha One drive finished less than tops was in acceleration from 0 to 30 mph where, as expected, it finished last. However, we were surprised that the Alpha's 7.4-second acceleration time was only 0.l-second longer than the second-place Volvo Penta Duoprop. The Alpha also had a slower planing time than both twin-prop units.

With the Alpha drive, the Stingray had a light and free-running feeling, crisp predictable cornering, and as is common with many vee-bottom hulls, a moderate tendency to wander from side to side at displacement speeds. Docking and handling maneuvers were easily controllable, and about as expected for a boat of this size. Overall, the 350 Magnum with an Alpha One drive is a very satisfactory package for the Stingray 719 zp.

Both the Duoprop and the Bravo Three twin-prop units exhibited the same handling characteristics. One attribute of twin-prop drives is that they provide a lot of stern lift, which helps the boat come on plane with much less tendency for the bow to rise. This also allows the hull to maintain a planing attitude at a lower speed. Once on plane, the trim system is less effective at bringing the bow up. Consequently, the boat has a much more glued-to-the-water feeling. Nonetheless, as our figures show, this has little effect on top speed. Other characteristics are that, with counter-rotating props, there is no tendency for the hull to list from side to side as trim angle is changed and there is a secure feel to the handling that just isn't there with single-prop drives. Cornering is another area where a twin-prop drive excels. While the Stingray 719 zp corners quite well with the single-prop Alpha One drive, when equipped with a twinprop drive, the boat maintains a tenacious grip on the water and it is nearly impossible to get the props to ventilate or make the boat slide sideways or spin out. Furthermore, you can take your hands off the steering wheel in a turn (although we do not recommend this) and it will still maintain its turning radius; the counter rotation of the props eliminates steering-wheel torque. So,even though the Alpha One was a shade faster, for overall performance we'd opt for a twin-prop drive.

CARBURETED TWIN-PROP VS. EFI TWIN-PROP
Comparing the Bravo Three with the Duoprop in terms of overall performance reveals a close match. The Duoprop was only 0.1-mph faster than the Bravo Three, and that is really trying to split hairs. On the other hand, the Bravo Three trounced the Duoprop (and the Alpha One) in 0-to-30-mph acceleration. But the Duoprop had a slight edge in fuel economy. So, from the performance figures alone, it would be easy to call this match a draw. However, there are other factors that come into play.

I mentioned earlier about Volvo Penta's EFI and fuel economy. Many people expect EFI to dramatically improve fuel economy, but compared with a properly set up carburetor that is not subject to smog controls, it simply is not the case. Generally, with the exception of idle speeds, where EFI can shine, fuel-consumption figures are very close, and the carbureted MerCruiser Bravo Three might very well equal or exceed the Volvo Penta if the MerCruiser were propped to turn a maximum of 4500 rpm instead of the 4800 rpm the engine did turn. (Independent of our test, MerCruiser later conducted further prop evaluations and subsequently recommends a different set of props - specifically a 26-inch set - for optimum overall performance than what we tested.) The MerCruiser Alpha One, also carbureted, did exceed the EFI fuel figures. It is interesting to note that both twin-prop drives got their best economy at a slower speed than the Alpha drive. This is due to the twin-prop's added stern lift and its ability to hold a slower planing speed.

By the same token, we might expect the Volvo Penta Duoprop to come closer to MerCruiser's Bravo Three acceleration figures if the Duoprop were propped to reach 4800 rpm at wide-open throttle instead of being held to 4550 rpm by a higher-pitched prop. Keep in mind that, with its roller cam, the MerCruiser is going to be more comfortable at 4800 rpm than the Volvo Penta. It is also interesting to note that the Bravo Three props are noticeably larger in diameter than the Duoprop's, and this may be responsible for the excellent acceleration figures, since there is more blade area to couple to the water. MerCruiser also uses stainless-steel props as standard equipment, while Volvo Penta offers stainless props as an option. Nonetheless, Volvo Penta doesn't consider this a disadvantage. To the contrary, it maintains that the performance of its aluminum props is just as good as stainless in all areas except impact resistance. Hence, Volvo Penta used aluminum prop sets for this test.

We did notice some prop burns on the Volvo Penta props. Prop burns are caused by vacuum bubbles that come off the lower unit and collapse on the surface of the propeller. In very mild form, it simply removes some paint from the prop. In more moderate cases, aluminum will be eroded from the prop. And in severe cases, loss of thrust (blowout) or excessive ventilation will occur, and the props can actually have a hole eroded in the blades. The problem is due to the shape of the lower unit. The faster you go, the harder it is to control. Volvo Penta lower units often start giving trouble in the upper 40mph range. While the problem was not severe at the speeds we were traveling, the unmistakable signs were there. Volvo Penta has known about this problem for years and has a cure in the newer DPX drive. If Volvo is serious about the role of Duoprops in the high-performance market, it should seriously consider offering the DPX drive (at least as an option) on any drive package capable of pushing a boat faster than 50 mph.

SUMMARY
If we were to announce a winner based solely on top speed, a look at the charts would tell you that the MerCruiser 350 Magnum Alpha One is top dog. The charts also reveal that the 350 Magnum to be most fuel efficient. As mentioned, we expected as much. What we didn't expect is how close the twin-prop drives came to matching the performance figures of the Alpha One. However, there are important factors that the charts don't tell you.

When you consider the superior handling, maneuverability, planing ability, and other such factors offered by a twinprop drive, we have to say the Duoprop and Bravo Three come out ahead. And, between those two, based on performance and perceived smoothness, we have to call the Volvo Penta Duoprop the overall winner.

If money is no object, we stand by our choice. There is, however, still more to consider. At the retail level, a twin-prop drive is going to cost approximately $500 more than a comparable singleprop drive. In our opinion, from a performance standpoint, this is a bargain.

There is a down side to twin-prop drives. While they are mechanically simple, they still have more parts to maintain; and with counter-rotating propshafts, you face a more difficult lower unit sealing situation. Twin props on the same axis are going to be subject to hitting the same rock, and if your boating waters have many unmarked hazards, the expense of prop repairs and replacement should be considered. The single-prop drive is the least expensive to purchase and repair, yet provides a very satisfactory level of performance, particularly in the area of top speed, which most of us consider to be an important performance objective.

Trailer Boats Magazine
May 1994