I Know it's True. I witnessed it firsthand On the Waters Off Palma de Mallorca, SpainEight demonstrator boats bobbed on the brilliant blue water the Mediterranean is so famous for.
Among the flotilla were two Cranchi 41 Mediterranee: one rigged with twin Volvo inboards and its partner with the new Volvo Improved Propulsion System (IPS).
Accelerating from dead in the water to wide-open throttle the IPS boat leaped onto plane so rapidly it made the conventional inboard look like it was lost in time. IPS zoomed from zero to 25 knots full two seconds faster. From the cliffs I could plainly see differences in each boat's running attitude. The conventional Cranchi ran true to form, whereas the IPS Cranchi forefoot rode noticeably higher and drier, like a flying fish trying to escape the water. With about the same horsepower as its inboard brother the IPS Cranchi ran seven knots faster. Cruising at 30 knots its mileage showed 30 percent better. To get the same performance out of a conventional inboard you'd have needed an additional 150 hp per engine.
Where the Volvo IPS-powered Cranchi spoke so softly you could hardly hear its muted mutterings the conventional inboard rumbled loudly, well, like an inboard. At 30 knots IPS whispered its message a full seven decibels quieter. Because dBs are calculated logarithmically, seven dBs means fully half the noise at the helm.
The difference in maneuverability was dramatically illustrated by the Cranchi to Cranchi match up. No matter what the maneuver, docking, running wide open or loafing at cruising speeds, the IPS ran in tight little circles inside the inboard boat. It was no contest.
Besides the Cranchi, Rodman Polyships, a Spanish boat builder with US distribution, also demo-ed a 41 footer. One of its demo boats was fitted with twin inboard Volvo TAMD75s rated 480 hp each, its amigo with twin 370 IPS propulsion. Even with 220 hp less (560 versus 740), top speed remained equal. But IPS accelerated much stronger. Rodman 41 time to plane was slashed from 15 seconds to just five seconds. Depending on cruising speed, mileage improved anywhere from 25 to 36 percent, effectively extending the range from 70 to 100 miles. As for maneuvering, at 35 knots with the wheel cranked hard to starboard, IPS spun the 41 footer within 2.5 boat lengths, while the conventional inboard required five boat lengths to make the same turn.
In short, what we have here is a faster, more fuel efficient and easier to handle package that breathes new life into inboard performance. By now you're probably wondering how they managed this impressive feat.
Engineers stuck double universal joints and a short drive shaft between a common rail diesel (high efficiency - low emissions) and a highly modified Duo Prop gearcase. In this its first incarnation two models are offered. The IPS 400 features D-6 310 horsepower motors, while the IPS 500 features a D-6 rated 370 hp. Only twin engine installations are available.
IPS 400 is a direct replacement for conventional twin 400 hp inboard diesels, with the IPS 500 aimed squarely at 500 hp inboards. Volvo 5.5 liter six-cylinder engines build great gobs of torque from low rpm to wide-open throttle thanks. Both are intercooled and supercharged, with the more powerful D-370 also benefiting from a supercharger for even stronger low end performance.
As for the drive unit proper, its bronze gearcase protrudes through a specially shaped hole molded into the bottom of the hull positioned slightly forward of the transom. Gigantic O-rings seal the edges of the hole where IPS pokes through. The drive axis is splayed at the same angle as the hull's dead rise so propellers run parallel to the hull. Conventional inboards point shafts downward at an angle severe enough to sacrifice efficiency. IPS props running parallel optimize efficiency. Volvo Penta IPS spins two, counter-rotating propellers on a single propeller shaft.
Yet another unique twist, the twin, counter-rotating propellers face forward. Instead of pushing like a DuoProp or even a conventional inboard, the blades pull like a turboprop airplane. Because front-facing props grab clean water undisturbed by the hull or gear case, the blades spin in clean water, more efficiently converting horsepower into forward motion. Just like the Duo Prop stern drive, IPS twin propellers counter-rotate. One wheel spins clockwise, its partner counter-clockwise. This tried and true configuration nearly doubles surface area of the blades, once again, converting more hp into thrust.
Each gearcase articulates 13 degrees, to port and to starboard, steering the boat with prop thrust. When the propulsion units articulate, the full flow of propeller backwash works to turn the boat. Similar to cars, steering is speed sensitive making it easier to maneuver at slow speeds and more predictable at high speeds. At all speeds the hull runs straight and true. Handling is precise. The turning radius (four turns lock-to-lock) is extremely tight. Even with only one engine operational, 70 percent steer-ability remains. Should the electronics ever fail, a hand crank backup allows limping home. Shifting and throttle controls are also electronic.
Obviously the bronze castings are vulnerable to coral heads and submerged rocks, just like inboard shaft, struts and rudders. But if an IPS boat were to ever strike an object at speed its propulsion unit is designed to shear off cleanly. Similar to an outboard motor propeller shear pin, IPS sacrifices itself in order to save the hull. According to Volvo, repairing damage is no more complicated than replacing the fallen away drive unit with a new one. Cost is about the same or less than if you were repairing bent shafts, struts, wheels and hull repairs.
Theoretical outcome of a violent impact is one thing. Real world results are something else. In the interests of due diligence Volvo conducted a crash-dummy test, intentionally slamming one of its IPS boats into a submerged rock.I witnessed the 25 knot Kamikaze impact so powerful that it threw the driver out of the boat. The bronze drive unit deep-sixed itself and the bilges remained bone dry.
One particular tantalizing design characteristic is the way hot exhaust gas flows out the cylinder head ports, transitions through exhaust runners, travels down through the drive and exhausts at the trailing edge. Buried deep beneath the bottom of the boat, exhaust gas is literally sucked out. The entire body of water acts like a gigantic muffler. Not even the slightest whiff of smoke or smell makes it to the surface.
Besides substantial performance gains, IPS' compact installation takes up less space. On all eight demo boats substantial room was freed that boat builders could reallocate for either aft cabins or storage.
Finally, IPS is intended for planing boats ranging from 37 to 50 feet. Nickel aluminum bronze alloy propellers are available in nine different sizes to accommodate hull speeds from 25 to 45 knots. That's the thing about IPS. It's a complete package including engine, propulsion unit, propellers, steering wheel, controls and instrumentation. No wonder boat builders like Tiara, Cranchi, Rodman, Regal, Carver, Sealine, Fairline and a dozen others are lining up to outfit their boats with the thing since the stern drive.