Which is better: Cummins Mercruiser Diesel’s Zeus system or Volvo Penta’s IPS?
The answer might surprise you.
One of the first things they teach you in Magazine Writing 101 is to pique a reader’s attention by making the title of your article a question. Humans, being naturally curious even if the topic isn’t strictly relevant, are likely to read your piece just to find out the answer.
The next thing they teach you is never answer that question until the end of the article. Make the reader read—or at least scan—the entire thing. However I’m going to violate that journalistic canon in the next paragraph, but if you stop reading there, you’ll miss out on a punch line or two.
IPS or Zeus? Well, regarding one criterion—which is more efficient?—it’s impossible to say definitively given the need to test two identical boats with the same load under the same conditions. But even if we had them, any difference would be so small as to be, if not unmeasurable, insignificant. Besides, the performance of any drive system depends largely on what boat it’s installed in. So IPS could perform better in Boat X but not in Boat Y.
Another criterion is whether forward—or aft-facing—props are better. I’ve tested both systems and have been unable to detect any difference I could absolutely attribute to the drives. Some say forward-facing props foul easier, although I’ve yet to hear of a specific case, while others claim a shorter turning radius or more responsive helm for one or the other. But such characteristics could be due to the drive system’s configuration and/or the hull design as much as to its design. However, because Zeus drives are in tunnels, their turning range is a bit less than IPS’s, though this would be noticeable only at slow speed and be rendered moot by a joystick maneuvering system.
This illustration shows the water flow of the aft-facing Zeus props.
Zeus drives sit vertically in molded-in tunnels while IPSs are angled according to the deadrise. This seems to be a factor only in high-deadrise boats that heel more in tight, high-speed turns, and the effect is often masked by the amount of steering range programmed into the IPS’s electronic steering system. Zeus uses hydraulic steering actuation, which it touts as more responsive and durable in a damp marine environment, claims I cannot verify. I do consider it an advantage that IPS’s electronic steering system can be modified in terms of response and range. Speaking of lower units, many wonder what happens when a pod hits an object. With IPS the entire unit outside the hull breaks away to maintain the boat’s watertight integrity. There are varying reports on how well this works. Zeus relies on just a breakaway skeg below the prop, a much less expensive proposition yet perhaps not sufficient when hitting something big. By the way, changing drive lubricant requires a haul-out for IPS but not for Zeus, although one would expect that job would be combined with other maintenance like bottom painting and thus not be an incremental expense.
Zeus also has an exhaust bypass, which IPS does not (exhaust exits only at the rear of the drive), which is supposed to make it quieter, especially at low speed. Again it’s difficult to attribute any difference to the drive as opposed to the boat’s acoustical regimen. However, I’ve noticed that at idle when the boat is not moving, IPS seems to generate a bit more exhaust-induced vibration than Zeus.
Not much difference, eh? Maybe what’s connected to the pods is important to you, like engines. IPS uses only Volvo Penta diesels ranging from 260 to 900 horsepower. Zeus uses the Cummins diesel line that ranges from 355 to 715 horsepower, plus the 575-horsepower Caterpillar C9 ACERT.
Running gear on a Volvo IPS boat is reduced to just the forward-facing drive units.
How about equipment? The joystick and integral GPS (for station holding) are standard on Zeus and optional on IPS, as is an autopilot. (IPS offers third-party autopilots.) Whether you’d see this difference in the price of the boat, and if so, how much, ultimately depends on the boatbuilder’s markup. In truth, only the boatbuilders know what each system costs, and they’re not telling.
One other Zeus standard missing on IPS is trim tabs integrated into the pod housings. In my opinion, it’s a worthwhile feature, though Volvo’s transom-mounted QL trim tabs are effective and produce little hydrodynamic drag. Zeus and IPS both offer a trolling or slow-speed mode. Having it in the joystick is convenient; IPS’s slow-speed function is in the binnacle. IPS also offers a Sportfishing Mode that enhances response when maneuvering and backing down. Zeus has no similar system.
Finally, there’s instrumentation, the important human-mechanical interface. I think the Zeus SmartCraft system has the edge here, with menus and pages that are more intuitive to manage. Your opinion may, of course, vary.
In one way the question posed by this article is moot, as few boaters can choose which system to have in their next boat—most builders offer one or the other. If you’re obsessed with pods, you could pick your next boat based on which drive the builder offers, but remember that the performance of either system greatly depends on the hull it’s in. In that sense, the whole question is something of a crapshoot, isn’t it?
Maybe, but at least I got you to read the whole piece.
This article originally appeared in the February 2013 issue of Power & Motoryacht magazine.