Outboards: Are Two Props Better than One?
THE SUZUKI DF350A MADE big headlines when it was introduced, not only because this V6 powerhouse is the biggest Suzuki outboard ever built, but also because it has not one but two propellers spinning behind the lower unit.
Then this past spring, Seven Marine, the builder of high-end uber-powerful outboards which was recently acquired by Volvo Penta, introduced a new gear case which also spins twin props and is adaptable to three different models.
Anyone who’s ever run stern drive boats is probably familiar with the Volvo Penta DuoProp and the Mercruiser Bravo III. Both incorporate twin propellers spinning on the same single shaft.
This has a number of advantages.
It increases blade area and reduces prop slip, allowing an engine to push a heavier load up onto plane faster and more efficiently. It greatly enhances dockside maneuverability, especially in reverse, and it provides more lift at the stern. This prevents excessive bow rise while running. These same advantages can be felt at the helm of an outboard-powered boat with twin propellers, too.
One detail to note: although many people refer to these rigs as having “counter rotating” propellers, they’re properly called “contra rotating.” This is because the props are on a single shaft. Counter rotating props are on two different shafts and spin in different directions. Thus, a boat with twins, one spinning clockwise and the other spinning counter-clockwise, is properly called counter rotating, while these single engine rigs are not.
Why split hairs? That’s because these contra-rotating rigs gain you many of the same advantages—but with the single engine. This is an important difference because it means less initial cost, lower operating expenses, and less maintenance cost in the future.
So, what about those advantages? First, consider the ability to push heavy loads. Many mid-sized center console boats up into the 22- to 26-foot range perform just fine when they leave the showroom floor with a single engine.
Now let’s say the weather looks good, and you want to run far offshore in search of big game. When you load that same boat up with full fuel and water, 100-plus pounds of ice, full gear, and five friends, there’s a good chance it will struggle to get on plane
Also, as you climb the backs of large offshore waves, the boat may well bog down and lose speed. If the outboard has contra rotating propellers, however, you’ll notice much less of a performance difference—if any at all—between the loaded and unloaded boat.
Then look at maneuverability. With all that extra bite, the boat responds to commands at the wheel faster. When docking or maneuvering in close quarters, your turning radius will be tighter, and your ability to fight the wind will be enhanced.
In reverse this difference is even more noticeable, because all propellers lose some bite as the through-hub exhaust gasses pass over the blades and cause slip. On top of those traits, the contra-rotating props counteract steering torque. The engine won’t pull to one side nor the other, without the need for an added trim tab as is usually the case.
Finally, consider running angle. When a single engine pushes a heavy load, bow rise is often a problem. The boat may plow with its nose high in the air even when trimmed down.
However, the twin propellers provide a lot more lift at the transom, which helps you keep the boat running on an even keel. Put this factor together with the ability to jump on plane faster when loaded, and bay boat or flats boat anglers will be able to probe slightly shallower areas.
Because the engine doesn’t suck the stern down when you nail the throttle, you’ll have fewer issues trying to get out of the hole in the shallows. In some places, a boat that squats may rub bottom and be unable to jump on plane. Yet the boat with twin props may well have no issues popping up and running.
Everything on boats is a trade-off, so what’s the down-side to spinning twins on one shaft? More blade area does equal more drag. In some cases (such as running with a light load) a boat equipped with a single prop outboard of equivalent horsepower will have a slight top-end advantage. This amounts to one to three mph and possibly one or two at cruise.
These contra rotating rigs also require a bit more effort in the design department as well as the extra propeller, which is bound to push pricing up a hair. Your choices are quite limited in this department. As of right now, Suzuki and Seven Marine are the only manufacturers offering them.
We should point out that putting a contra rotating drive on an outboard is not exactly a new idea. Yamaha tried it years ago (they called it TRP, for Twin Rotating Propellers), but it didn’t survive in the marketplace. For whatever reason, today the advantages seem to outweigh any hesitation among yesterday’s boat buyers.
This is a good thing, but don’t take our word for it. Get behind the helm of a boat rigged with one of these motors, and go for a sea trial. We’d bet dollars to doughnuts that the difference you feel is so substantial, you quickly become a fan. Yes, it does often seem that two props are better than one.
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Email Lenny Rudow at ContactUs@fishgame.com