Motor Boat and Yachting reports on what it is terming the single-most important invention in a generation, Volvo Penta's new Inboard Performance System, or IPS. Tony Jones and David March report.

Volvo have just rewritten the rules with regard motorboat propulsion systems. In a stroke they may have consigned stern-drives and shaft drives to history, and in the process revolutionised the way motorboats are designed and built.

Inboard Performance System, or IPS, is an innocuous title for the most important development in propulsion since the invention of the outdrive, and something that may render the conventional shaftdrive obsolete.

IPS is a combined engine and propulsion system, sold as a package in the same way that an outdrive can be. You can’t buy the propulsion unit on its own, and currently IPS is only approved for twin installation. There are two models, both employing the same propulsion/transmission unit. Combining this unit with the 310hp D6-310 gives the IPS-400, attach the 370hp D6-370 and you have the IPS-500. The engines are the same as used in sterndrive and conventional shaftdrive applications.

Lets start by listing the main benefits compared with conventional shaftdrive, go on to see how those benefits are delivered, and then look at the implications of this system for recreational craft of the future. For implications there certainly are, especially for manufacturers of diesels in the 350hp to 500hp bracket. The longer term depends on whether Volvo intend to develop the concept to accept greater power outputs. And I think we can all guess the answer to that.

Higher efficiency

Significantly increased propulsive efficiency is one of the key benefits of IPS, but that increase has been made possible only by a fundamental rethink of propulsion principles. About the only thing IPS has in common with conventional shaftdrive is that it uses fully submerged propellers rather than jets or surface-piercing propellers.

IPS uses two forward-facing contra-rotating propellers per unit. The superior efficiency of twin contra-rotating propellers over a single larger diameter one was appreciated many years ago, and Volvo’s Duoprop outdrive is the established manifestation of that theory. Because they operate in clear water, propellers work better in tractor (pulling) form than in pusher mode but shaftdrive installation naturally ensured that the pusher principle has remained dominant in the marine world.

Forty-plus knots

Another advantage of the IPS propulsion unit is the ability to break through the 40-knot barrier that the inefficiency of conventional propellers imposes. This is significant because propulsion for 40 knots plus boats has traditionally been the preserve of relatively expensive surface drives like the Arneson or Buzzi drive. We’re not yet in a position to directly compare the handling and performance of all the different drive systems, but the IPS does seem to provide the potential for a new generation of higher speed boats that inhabit the mainstream rather than the esoteric.

Improved handling

Great news for devotees of outdrive legs, the handling of the IPS will delight you. And excellent news for shaft drive junkies, the handling of the IPS will charm you too. The impossible dream? It would appear not. Nowhere was this more evident than driving the Sealine S42 sportscruiser. This is the boat we have tested more than any other, and in all its forms; with outdrive legs, with Buzzi surface drives, with shaft drives, and with Volvo, Yanmar and Mercruiser engines.

Low speed manoeuvrability was excellent. In practice, IPS provides all the (outdrive) advantages of vectored thrust so you can push or pull the stern around and turn extremely tightly using just one engine, or both if you need more haste. But unlike most outdrive setups, IPS also works okay when you leave the wheel centred and use ahead and astern commands as you do with shaft drives. The response is slower but still distinctly shaft-like.

The drive units also have plenty of keel area so they provide remarkably good directional stability. Pottering out of the marina on just one engine required only the slightest deflection of the wheel, and like shafts and rudders, side winds have less effect than on outdrive leg boats. The great thing is that you can pick and choose depending on what you are trying to achieve. Centre the IPS and turn in your own length. Or vector the IPS, dab the bowthruster, and see yourself pulling smartly sideways and backwards (or forwards) out of a tricky cross-tide marina berth. Master the IPS fully and you will be giving those implausibly manoeuvrable twin jet-drive boats a run for their money.

At high speed, the good directional stability remains. Rudders work well at medium to high speed but IPS permits a tighter turning radius and better speed through the turns because, like outdrive legs, there is nothing blocking the prop wash.

Reduced noise and vibration

Again, the Sealine S42 provided the perfect yardstick. It was not possible to measure noise accurately (too many excited journalists onboard gabbling away) but subjectively there was no doubt whatsoever; the IPS S42 was significantly quieter than its counterparts.

IPS makes things far quieter and noticeably smoother largely because the propulsion unit feeds thrust loads directly into the hull, allowing the use of much softer engine mounts. This is the same principle used by the highly effective Aquadrive coupling. Vibration damping is further enhanced because thrust is transmitted via two large-diameter rubber O-rings, which also provide the watertight seal between the IPS unit and the hull.

Reduced maintenance

Worries about long-term maintenance is something that shaftdrive devotees often cite for avoiding outdrive legs. Fortunately, the maintenance requirements of IPS should be minimal. Certainly far less than an outdrive and possible even cheaper than a conventional shaft drive set-up due to the elimination of the sterngland and P-bracket bearing. The steering motor is maintenance free and there are no tilt, trim or steering joints to worry about. Accessibility is outstanding which makes keeping an eye on the condition things like the rubber exhaust hoses dead simple; outdrive owners have permission to eat their hearts out. The mechanical parts of the IPS transmission that come into contact with the water are made almost entirely of stainless steel. The leg is made from bronze, rather than aluminium ,and the propellers from a special nibral (nickel aluminium bronze) instead of stainless steel. Volvo hope this will negate the corrosion problems that so plague aluminium outdrives in certain circumstances. The only anode is a small cast-iron cylinder inside the exhaust outlet.

Increased safety

We now come to what looks like the IPS’s Achilles heel: what happens if you hit an underwater obstruction. As you might expect, Volvo have given this aspect a great deal of thought. There are two entirely different scenarios: low speed contact with the bottom and a high-speed encounter with a submerged object.

Touching the bottom at low speed shouldn’t be a problem as the skeg on the bottom extends below the propellers. The subsequent vertical force is transmitted into the robust GRP mounting ring inside the hull and assuming the engines are put into neutral promptly, no damage should result.

In the high-speed scenario, the leg and pod will shear off flush with the hull. The lower bearing carrier has been engineered specially to break at a point below the O-ring seal between the fixed and steerable parts of the drive. As the leg bends backwards, the vertical drive shaft simply pulls out of its spline. The inside of the transmission is now open to the ocean but no water can escape into the boat and the vital mounting joint between transmission and hull will remain not only intact but undamaged thanks to the energy absorption of the big twin O-rings.

More room on board

Existing flybridge boats could take advantage of IPS by installing the engines in what was the lazarette and using the old engine room under the saloon floor for storage. But the full benefits of IPS will only be realised by entirely new interior layouts – some of which will undoubtedly feature an extra cabin or more. The rearward weight change of IPS also provides the opportunity for transverse fuel tanks located at the centre of gravity, meaning no trim change in relation to fuel contents.

What the future holds

At present IPS consists of just one transmission unit and a choice of two engines, 310hp and 370hp. Bearing in mind the 35% or so efficiency gains over shaftdrives, that suggests a take-up on boats designed for around 350hp to 500hp. IPS blends seamlessly with Volvo Penta’s existing sterndrive packages of between 130hp and 350hp, so the company now have the 26-46ft (8-14m) market covered with the exception of shaftdrive flybridge cruisers around 33ft and single engine designs. But there doesn’t seem any technical reason why the leg shouldn’t be coupled to the recently uprated 260hp D4 for smaller boats where outdrive legs would be unsuitable. And if IPS settles down in service without any major teething problems, it will probably be certified for use with higher horsepower engines within about 18 months.

For power outputs above 500hp an entirely new transmission would probably be needed, but there are no technical impediments to producing an ‘IPS 750’ capable of absorbing the D12’s 715hp. In fact, I’d be surprised if such a unit wasn’t already on the drawing board. That could make IPS the power system of choice for virtually all recreational craft up to about 65ft – which is the vast majority in terms of units built.

IPS represents a multi-million pound investment and five-year R&D project for Volvo Penta, so it’s unlikely that we will be seeing a competitive product emerging any time soon. But perhaps the most important aspect is that IPS puts Volvo Penta in the unique position of being not just a supplier of engines for inboard boats but a provider of complete inboard propulsion, steering and instrumentation packages. I hope they are tooled up for the rush.


For more technical details and on-water impressions, see the February issue of Motor Boat & Yachting, on sale on 6 January.