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Part 2: The speed and other performance enhancements I recorded during my comparison test speak for themselves.
By Capt. Bill Pike — February 2005
Fuel savings numbers followed suit. The widest spread in mpg occurred at a cruising speed of 3250 rpm, where IPS averaged 1.14 mpg and the V-drives averaged 0.96 mpg. Yet again, the increase was approximately 16 percent, a little more than half the 30 percent reported by Volvo Penta.
As for driving impressions, cornering at speed was tight but comfortably secure, thanks mostly, I’d say, to the articulation of underwater units governed by a reduced-travel feature at high speeds to prevent overly sharp turns. Cornering at low and maneuvering speeds was even tighter. Vibration was virtually eliminated, thanks to the O-ring isolation of the IPS’s underwater units from the hull and the soft-mount installation of the engines this isolation facilitates. And sound levels were reduced, too, especially on the lower end of the rpm range. Running angles during planing were steeper with IPS but lower after the boat was on plane. And acceleration (see curves in performance-data comparison, this story) showed improvement.
My docking impressions? While IPS offers close-quarters power and agility in spades, I hasten to add an admonition based on the extra IPS maneuvering time I now have under my belt. Whether you approach the system with inboard technique or stern-drive technique, you’re looking at a learning curve.
Consider the first option. While the inboard approach is by far the easiest of the two to grasp and master, it’s still slightly out of the ordinary for one reason. Thanks to fundamental design and installation variations, a boat’s propellers and pivot point when going astern are closer together with IPS than they are with most inboards or, for that matter, stern drives. This alters turning moments as well as boat-handling sensations in subtle but palpable ways. And furthermore, the prop walk that facilitates inboard-style docking is absent with IPS—twin counter-rotating props back straight, with neither a port nor starboard bias.
Now for the second option. I think IPS’s easy-spinning, variable-speed electronic steering and double-prop oomph may contribute to the tendency to oversteer, particularly when backing down. Indeed, I discovered that one engine works best when docking this way, as it engenders more control by slowing down the process. And then there’s the diminished distance between props and pivot point mentioned above—turning moments and boat-handling sensations change when maneuvering with the wheel just as they do when maneuvering with the clutches.
All this tempers my take on IPS only slightly, though. (For more, see “Talkin’ ’Bout a Revolution,” this story.) The speed and other performance enhancements I recorded during my comparison test speak for themselves. And while I suspect the dockside subtleties of IPS may prove initially problematic to boaters, some or most may ultimately capitalize on them. Has Volvo Penta got a winner here? Time will tell.
This article originally appeared in the January 2005 issue of Power & Motoryacht magazine.