Riviera 50By Capt. Bill Pike
East vs. West
We do a transcontinental wringout of two new pod-powered Riviera 50s, one sporting Volvo Penta IPS and the other a Cummins Zeus system.
Serendipitous developments generally start small, germinating as they often do from, say, an exceptionally plainspoken phone call or an in-depth gam dockside. But then, I guess, things just sorta progress, and progress some more, until finally a tidbit arises, such as, “Hey Bill, did you know Riviera’s got a couple of brand-new 50 Enclosed Flybridges comin’ into the country next month, one with IPS and the other with Zeus?”
“Huh,” I responded, my mind already sprouting questions. Would the two boats be equally, or near equally, powered? Where, geographically speaking, was each headed? And exactly how many airline reservations would be required to guarantee an apples-to-apples extravaganza?
“One’s gonna be on the East Coast—that’s what I heard,” my interlocutor added, apparently reading my mind, “and one’s gonna be on the West Coast.”
“East versus west,” I theorized, raising an eyebrow. As luck would have it, since the dawn of recreational-type pod propulsion in 2005, I’d never had a chance to fairly compare IPS and Zeus technologies, using virtually the same (or close to the same) test envelope. Was one technology better? Or were they just different? Here was a way to find out.
Details, Details, Details!
It turns out the East Coast boat—with two 725-metric-horsepower Volvo Penta D11 diesel inboards linked to IPSII pods—was delivered to a slip at Sailfish Marina in Stuart, Florida, not far from St. Lucie Inlet. And when I arrived there on a warm, sunny morning this past February, palm fronds were waving lazily overhead, temperatures were already in the mid-70s (but would soon hit 80 degrees), humidity was 73 percent, and the wind was light and variable.
Meteorological conditions were wilder and woolier when, roughly a month later, I stepped aboard the West Coast boat—with two 600-metric-horsepower Cummins QSC8.3 diesel inboards linked to Zeus pods—tied alongside at Chandler’s Cove Marina on Seattle’s Lake Union, within a stone’s throw of the offices of Emerald Pacific Yachts, Riviera’s West Coast dealer. Although the winds were again light and variable, temperatures were in the mid-50s and the humidity was a mere 49 percent. As if to counteract the edge such conditions would give our West Coast 50’s Cummins diesels, however, the wavelets lapping the pilings at Chandler’s were fresh, not salt. Fresh water’s lesser buoyancy would boost drag and cut our top speed once we got out on the lake, if ever so slightly.
Of course, although fluid levels and passenger loads were roughly equivalent boat-to-boat, there was a disparity in comparative oomph. With a total of 1,450 metric horses in her ER, the East Coast IPS boat had an obvious advantage over the Zeus-propelled West Coaster with just 1,200 total metric horses. Moreover, while the West Coast Zeus boat’s propsets were the largest available from Cummins, their 22-inch diameter made them considerably smaller than Volvo Penta’s midrange 25-inch IPSII propsets. This was going to be important most likely since our Rivieras weighed in at approximately 57,058 pounds (Zeus) and approximately 57,607 pounds (IPS)—considering such hefty displacements, the Zeus boat’s abbreviated blade area might again prove disadvantageous.
High Seas Performance
Let’s discuss the data I collected at this point, before moving on to driving and handling impressions. For starters, given that the East Coast IPS boat was hampered by some conditions yet invigorated by others, my read of the performance numbers shown here gives IPS the lead, at least in terms of speed. Beating Zeus’s top end by 7.3 knots is way more dramatic than my 27 years of boat-testing experience would lead me to expect, given IPS’s surfeit of just 250 horses. Had the difference in top end been something like 3 or 4 knots, I’d have called the contest a draw, all things considered. But 7.3 knots?
Fuel economies presented a more mixed picture. At lower, planing (and pre-planing) speeds, Zeus seemed to come out ahead. At higher cruise speeds IPS seemed to do better. And at wide-open throttle, the IPS boat turned in a number that was more efficient.
Then there are other parameters worth considering. First, the East Coaster with IPS came out of the hole at a much lower rpm although she assumed loftier running attitudes afterwards, perhaps due to the extra weight in her powerplants. And second, the Zeus boat was quieter at low rpm, a bit louder in the mid-range, and considerably noisier at top end.
Straightaway handling at speed pointed up a few similarities. The ability to track with a modicum of helm input was equally evident in both boats, due in part I’d say to the existence of a significant keel (with a maximum depth of 1 foot 6 inches), the first Riviera’s put under a pod-propelled hullform. Sightlines were excellent from the bridges of both boats as well, all the way ’round, thanks to slightly dissimilar but still modest running attitudes, the result of sweetly-balanced, fairly comparable engine and drive-unit placements. And finally, the steering felt roughly the same to me, although I preferred the Zeus’s hydraulics (which produce a more intimate, synced-in response, I’d say) to the IPS’s fly-by-wire electrics.
But it was handling in hard-over turns that brought out the real open-water differences. The Zeus boat (with her aft-facing propsets and vertical drive legs ensconced in protective tunnels) carved corners in much the same way a straight-shaft-inboard might have, and evinced a very solid, positive prop bite in the process. The IPS boat, on the other hand, felt looser, with somewhat more inboard heel, undoubtedly because of its splayed, deadrise-mounted, forward-facing propsets.
The explanation for this is a little complicated. Unlike Zeus’s underwater units which push a vessel’s mass through the water, forward-facing IPS units pull or tractor. So, in the midst of a hardover turn to port, let’s say, the port propset (because of its tractoring nature as well as its steering orientation and its splayed stance) actually pulls the port side of the hull down as well as ahead while the starboard propset (for the same aforementioned reasons) pulls the starboard side up and ahead. The extra inboard heel all this engenders, particularly on lofty flybridge-type boats, can be reduced via steering software tweaks, but I found it still quite noticeable on our West Coast test boat.
Not long ago, interestingly enough, Volvo Penta introduced a new trim system that in part addresses this IPS-related characteristic. Called the Interceptor System (IS), it offers an “A” (for Auto) mode, whereby interceptor-type trim tabs automatically deploy in turns to dampen or remove heel. Although the system was too new to have been installed on our East Coast 50, IPS-outfitted Rivieras of the future may well have it.
After finishing with the open-water portion of our test on Lake Washington (a more seat-trial-friendly spot than Seattle’s Lake Union), I returned our Zeus boat to Chandler’s and docked alongside, with little current or wind in the offing. The maneuver went smoothly, thanks to the trolling valves in our Zeus transmissions and their ability to slip their clutches to reduce gear clunking during shifts. The Zeus joystick (larger and more pleasingly tactile than the IPS’s in my opinion) worked both authoritatively and intuitively. The behavior of the boat it produced felt straightforward and quite straight-shaft-inboard-like.
A month earlier, I’d had to park the IPS boat stern-first in a cramped, difficult spot after our sea trial. Our slip at Sailfish was at the end of a narrow fairway and getting into it ultimately required carefully turning the boat within her own length and then backing down while simultaneously walking her to starboard.
In spite of the complexity of the maneuver, it went quite smoothly as well. The larger IPS propsets (with comparatively more blade area than the Zeus propsets) seemed to transfer horsepower to the water with noteworthy efficiency and exceptional refinement and control, although I noted a good bit of around-the-hull turbulence and a slight rocking motion whenever I overdid the joystick, perhaps due to the special characteristics of IPS I’ve already alluded to. In addition, I did not experience the authoritative, straight-shaft-inboard-like, back-down steadiness I’d later enjoy in the Pacific Northwest.
So, To Wind It Up …
East versus west? I think my original take on this cross-continental comparison of our two, identical Rivieras still makes sense. Western waters, after all, tend to be generally rougher, more debris-strewn, and less forgiving than the waterways of the East Coast, especially those around the Sunshine State. “We don’t need to be going all that fast out here,” suggested Ron Scott of Riviera dealer Emerald Pacific Yachts once we’d finished the wringout of our West Coast Zeus boat. “So we don’t need the extra speed you saw with IPS back in Florida.”
Scott agreed with me that IPS technology sometimes engenders handling idiosyncrasies, especially during hardover turns. He added, though, that Volvo Penta’s new IS system sweetly addresses the issue.
“I just tried it out in Sweden,” he enthused, “and it simply does away with the phenomenon we’re talking about—on the dashboard monitor you actually see the tabs working in the turns to control the heel.”
If Scott is right, then the denouement here is fairly clear. Our IPS boat was faster and, with refined usage of the joystick, a tad more precise in terms of dockside maneuvering. Our Zeus boat was generally quieter and more fuel-efficient at the lower end of the rpm register and evinced handling characteristics, at speed and dockside, that felt more comfortably conventional, almost straight-shaft-inboard-like.
Big-time differences? Perhaps not, but worth considering all the same.
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This article originally appeared in the August 2014 issue of Power & Motoryacht magazine.