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.)
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
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
Between the two twin-prop drives, we pick the Volvo Penta as the winner.
Not because of any measured performance advantage - as mentioned, the
differences are just not that great - but because of its smoothness in
operation. Someone is going to say, "What did you expect? The Volvo
Penta unit has EFI." True. However, throttle-body (singleport) EFI
doesn't do a whole lot to improve smoothness the way multi-port EFI does.
The smoothness we are talking about here is drivetrain smoothness, and
the MerCruiser Bravo Three has a tinge of vibration that is not present
in the Volvo Penta Duoprop. This is not the first time we have mentioned
this, and others have commented on it too. We don't consider the vibration
to be bad, and it certainly is not a reason to avoid the drive. But it is
there. And it is most noticeable in a direct comparison. I am guessing
that the props are the culprit and that the problem will be ironed out
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.
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