Back in 1998, Volvo Penta invited a group of writers to its Chesapeake, Virginia, headquarters to introduce us to its new diesel Duoprop sterndrive system. They had rigged a Carolina Classic 28 with twin 260-hp KAD 44 diesel engines, coupled to these new outdrives that featured two counterrotating propellers on each drive.
Twin 260-hp diesels sounds like insufficient power for a big 28-footer like the Carolina Classic, but the Duoprops provided astonishing performance across the rpm range. The boat handled like an outboard, with strong acceleration, excellent maneuverability and a top speed approaching 50 mph. Volvo Penta claimed an increase in efficiency of about 25 percent over straight inboards, and we all came away impressed.
Fast-forward to last fall. Volvo Penta once again upped the ante with the introduction of the Inboard Performance System, a radical new variation on the Duoprop theme. The IPS drives are positioned on the bottom of the boat, not on the transom, so they eliminate all the hardware hanging off the stern of the boat that can snag a fishing line. And while sterndrives are typically built with aluminum casings - creating corrosion issues for anyone who needs to keep their boat in the water full time and complicating the use of antifouling paint - the Volvo makes the IPS casing from a special nickel-bronze-aluminum (Nibral) alloy that resists corrosion and takes paint easily.
The counterrotating propellers on the IPS face forward, like an airplane prop, so they spin in totally undisturbed water as they pull the boat forward. The front prop sports three blades, while the rear props have four.
More important, with this system the prop shafts run parallel to the boat's bottom, providing perfect horizontal thrust. Conventional inboards lose a great deal of efficiency because of their downward shaft angles. This gives the IPS a tremendous efficiency advantage - about 30 percent over straight inboards, according to Volvo's calculations. Propellers for the IPS come in a wide variety of pitches, from the T2 prop set, which has a theoretical top speed of 25 knots, to the T10 props, which should reach 45 knots.
The IPS drives swivel beneath the hull when you steer, pulling the stern of the boat to the side to turn. This greatly enhances maneuverability, as all the engine's thrust is applied in the desired direction. With a straight inboard system, a great deal of thrust is wasted in a turn since the rudders deflect only a portion of the thrust to the side - allowing a great deal of the thrust to continue straight aft.
The ability to steer the drives greatly simplifies docking and significantly reduces the turning radius of the boat while at speed. It also improves a boat's ability to back down on a fish. Imagine being able to truly steer the boat in full reverse. And with the thrust running parallel to the bottom, the transom is much less inclined to dig down into the water as the boat backs down. With the shaft angle on most straight inboards, the more throttle you apply, the more the props pull the transom down.
So far, Volvo Penta has coupled the IPS with only one engine, available in two different horsepower ratings. The D6 diesel, an inline six-cylinder engine, displaces 336 cubic inches and features common rail fuel injection for maximum efficiency. The IPS 400 uses a turbocharged and aftercooled D6 that's rated at 310 hp at the crankshaft. The IPS 500 adds a belt-driven compressor to that mix to boost the engine's crankshaft horsepower up to 370.
|The IPS drive attaches to a boat via a special mounting collar featuring inner and outer rubber gaskets. The collar must be designed into the boat from the beginning, and therein lies the system's main shortcoming. It will be very difficult, if not impossible, to retrofit existing inboard boats with an IPS drive.|
The drive units must be installed close to the transom, because putting them too far forward, where existing shaft tubes are found on most inboards, could create some potentially unpleasant steering issues while at speed. You can connect the IPS drives to the D6 engine with a jackshaft, however, so someone may figure out how to retrofit this system in time. For now, however, you'll just have to wait until a fishing boat that incorporates IPS appears on the market.
That day is coming sooner rather than later. Volvo Penta is presently working with several high-end sportfish builders, but they won't say which ones. However, the performance attained on the existing test boats should transfer easily when the IPS is finally installed on a fish boat. On a Tiara 3800, for example, the IPS 500 reached a top speed of 38 mph, burning only 38.2 gallons per hour. An identical 3800 with regular twin 480-hp Volvo Penta diesel inboards hit 37.5 mph wide open, but it had to burn 47.8 gph to do so.
Volvo Penta has installed the IPS in boats as large as 40 feet, with impressive results. This advance just might change everything because you can now power larger boats with much smaller engines than ever before, still reach acceptable cruising speeds and burn much less fuel while doing it. Who wouldn't like that? And because the IPS releases exhaust gases through the underwater drive unit, they will surface well behind the boat so people in the cockpit will never get a whiff of diesel.
With all these obvious advantages, does the IPS have a drawback? Skeptics ask what would happen if the drive unit struck an underwater object at cruise speeds. Would the drive be ripped from the hull and possibly take a large chunk of the hull with it? "This is the most-asked question about IPS," said Jens Bering, the U.S. product manager for IPS and for all large Volvo Penta diesels. "The IPS comes with a shear-off function designed into it. If you hit something hard enough to shear the lower unit off, I think that would be the best result you could have hoped for."
Bering could be right. Hitting something hard enough to shear off a drive unit would probably occur only in a catastrophic grounding situation. The break point engineered into the drive ensures that the drive will be sacrificed before the hull integrity becomes compromised. Can you say the same of a straight inboard? How many boats have been sunk or at least flooded when a strut got pushed up or torn off in a similar situation? And in a much more likely collision with a piece of floating debris, the IPS drive will likely only get bent - once again, always a possibility with straight inboards as well. And because the IPS will be offered only in twin-engine configurations, your chances of getting home remain quite good.
The IPS comes standard with electronic vessel control. The CAN-bus-based EVC system controls shift, throttle and steering, and it monitors all systems in both the engine and the drive. Electronic controls also bring other advantages - easier steering and shifting. "The 'rideability' of the IPS reduces fatigue substantially," says Bering, "because steering is almost effortless. And since the rubber suspension and seals absorb vibrations, you get a much smoother ride. You can stand right at the transom while underway and feel almost no vibration whatsoever."
It will be interesting to follow the progress of IPS as more builders begin to incorporate it into their sportfishing models. It offers undeniable advantages in efficiency, maneuverability, acceleration and overall performance. If it proves to be reliable, as other Volvo Penta products have, it could very well be the system of the future for inboard diesel boats under 40 feet. And the company is hinting about higher-horsepower IPS models currently being tested, so that envelope may get pushed even further. It could mean quicker acceleration, better maneuverability and less fuel consumption for a great many of us - all good things.