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The propulsion revolution

Pod drives are changing the way boats are powered, offering better handling and efficiency

The propulsion revolution

Pod drives are changing the way boats are powered, offering better handling and efficiency

This might sound like a shameless plug, but I’ve been updating my book, “Sorensen’s Guide to Powerboats” (due out in a second edition in October), and of the 15 chapters in Part One, the chapter on propulsion systems took by far the most work to update.

That’s for a very good reason: A lot more has been happening in the propulsion world than anywhere else in this industry, and it’s all good news for boaters. Over the next couple of months I’ll take an in-depth look at what’s been going on in the world of marine power over the last few years, and consider what it all means for you. I’ll focus on the three types of propulsion systems that have changed the most — pod propulsion, outboards and sterndrives — and also take a look at changes in diesel engines. This month let’s look at pod power.

Far and away the big propulsion news for boats 35 feet and larger is pod power. It’s called pod power because there’s a pod below the boat, forward of the transom, that multitasks with the best of them. The pod holds the counter-rotating propellers, turns to both sides to control maneuvering, and contains the plumbing for cooling water inlet and exhaust discharge. The pod is very streamlined, like a sterndrive or outboard lower unit, so resistance is minimized, reducing drag and increasing speed and fuel economy.

So what makes pod power so great? Up to 30-percent better fuel economy and 15-percent greater speed, for starters. Pods use counter-rotating props, which take the twist out of the discharge race, resulting in more forward thrust per drop of fuel. And since there are two props, blade loading is reduced, so the system gets more “traction” around the dock and on acceleration.

As is the case with sterndrives, there are only two players in this field, and they happen to be basically the same ones: Volvo Penta with its Inboard Performance System and Cummins MerCruiser Diesel with its Project Zeus drive. With either, there’s only one vendor to deal with for the entire propulsion system, so one phone call covers all of the engine and drivetrain components.

Pods create a lot less drag with their slippery profiles than conventional running gear, and the difference between the two increases with speed. IPS’s forward-facing props operate in clean, undisturbed water and pull the boat, which makes them even more efficient. Zeus has pusher props mounted abaft the pod, so even though they are working in a disturbed water flow, their discharge race doesn’t impinge on the pod like Volvo’s does. It’s probably a wash (so to speak) efficiency-wise between the two units.

Rather than being angled downward like an inclined shaft, the pod’s thrust is parallel to the boat’s keel, which is more efficient than conventional inboards with their upward thrust component. (“Inclined shaft” is how naval architects refer to conventional inboards with prop shafts connected to the engine inside the boat and exiting the bottom through a shaft seal at typical angle of 11 to 14 degrees.) The pods are electronically controlled by a joystick for effortless dockside maneuvering. With the exhaust exiting well below the waterline, they are much quieter and virtually fume- and smoke-free.

Since the engines are soft-mounted — the pods take up the propeller thrust, not the engines — much lower vibration levels are felt inside the boat. The engines are connected to the pods by a short jackshaft, so they take up much less space on board. This means that compared to a conventional inboard the builder can move the engine room/aft cabin bulkhead farther aft — anywhere from 4 to 8 feet — making more room for the people compartment. A pair of pod drives can be installed in a half-day or so, compared to most of a week or more for conventional inboard gear.

Volvo was the first to offer a pod system, and as of this writing it is still the only manufacturer actually selling them to boatbuilders. Volvo has been producing a lineup of diesel pod drives for the last three years and just launched a number of new units. A new 4-cylinder D4 diesel powers the IPS 350, while 310-, 370- and 435-hp versions of its 6-cylinder D6 diesel power the IPS 400, 500 and 600, respectively.

These are all basically the same underwater drive units but with different engines powering them. All are named according to the horsepower that an inclined-shaft inboard would require to produce the same speed. For example, the IPS 600, powered by a 435-hp diesel, does the work of a 600-hp diesel driving a conventional inclined shaft. That also gives an idea of the efficiencies produced by IPS (and similarly by Zeus): more than 25 percent less horsepower and 25 percent less fuel to produce the same result.

Volvo also has introduced an expanded IPS lineup that includes, most notably, a bigger and more powerful pair of IPS units equivalent to 750 hp and 850 hp in a conventional inclined-shaft setup. They are powered, respectively, by Volvo’s 575-hp D9 diesel and a new 670-hp D11 diesel made of compact graphite that’s stronger and lighter than the iron used in other engines. And Volvo has introduced a gas-powered IPS 500 that uses a 375-hp 8.1-liter big-block engine as the prime mover. It uses the same pod unit as the diesel IPS 500, only with a deeper gear ratio suited to the higher-revving gas engine.

This gas IPS unit should be very popular with moderate- to high-volume production boatbuilders producing cruisers and maybe even sportfishing boats in the 35- to 45-foot range. The gas engine costs a lot less than a diesel, making the gas IPS a good choice for those who want the maneuverability IPS offers at less expense.

Zeus models include the 3500, powered by a Cummins QSB5.9 diesel rated between 330 hp and 480 hp, and the 3800, powered by a Cummins QSC8.3 rated between 500 hp and 550 hp. CMD combines Cummins’ high-tech, clean-running diesels, Mercury’s expertise with underwater drives, and Mototron’s expertise with SmartCraft electronic control and networking systems. (Like Mercury, Mototron is a division of Brunswick.) It’s the electronics that makes it all possible for both Volvo and CMD. Without the electronic control modules, or ECMs, you’d have no joystick. And the steering (with responsiveness varying according to hull speed) and engine controls are digitally controlled, fly-by-wire as well. Word has it that Zeus will be in full production this fall, with a number of boatbuilders offering new models with the system.

Now, let’s put pod power into perspective. Many builders have designed new boats entirely around IPS and Zeus propulsion. Tiara, for instance, has basically designed its entire new Sovran cruiser series around IPS. Building boats that can’t be powered with conventional inboards is a big risk for any builder. But with pod drives’ tremendous advantages in maneuverability, economy and range — in addition to added accommodations space, quietness and virtually zero-emissions — it seems like a sure bet.

Builders also have to make an efficiency-related decision: Do they keep the same size fuel tanks to increase range by 20 to 30 percent, decrease fuel capacity to lighten the boat — further increasing speed and efficiency and increasing accommodation space — or some mix of the two, perhaps slightly smaller tanks with slightly more room and incremental speed and efficiency improvement. It’s a nice decision to have to make, and any way you go you have a better boat.

My belief, in fact, is that boatbuilders that don’t get on the pod bandwagon in a big way will no longer be competitive. Pods simply make boats more appealing and, therefore, more competitive. I believe that conventional inclined-shaft inboards will represent 20 percent or less of the 35- to 90-foot inboard pleasure boat market in five to 10 years.

All the other advantages aside, for many owners the incredible maneuverability pods offer will be the biggest attraction. Both Zeus and IPS have low-speed joystick maneuvering control that allows a boat to be walked sideways or obliquely or twisted in its own length, or any combination of these maneuvers, by merely moving a little stick in the direction or rotation you want to go. After a few hours of practice, a reasonably adept novice can control a 60-footer as well as a seasoned captain can with conventional inboards. After a bit more practice, that novice will be driving circles around the professional captain in a non-pod inboard.

Think of the lifestyle benefits. This amazing control in close quarters reduces the stress for the operator, so everyone on board is happier. No more skipper yelling at the mate because he screwed up the approach in front of 100 thoroughly entertained spectators. It also removes the major impediment (besides being too poor) to moving up to a larger boat: fear of driving a big boat dockside (even though bigger boats often are more predictable and, therefore, easier to handle, when you get right down to it). The confidence that pod power appropriately instills in owners means builders will be selling bigger boats and more of them — that’s ka-ching ka-ching for the builder and happy cruising for relaxed owners. Think of pod propulsion as putting the pleasure back in pleasure boating.

I believe the concern some people have about pod power being more susceptible to grounding damage is largely unfounded. Unless you’re in a single-engine, full-keel lobster boat (or a waterjet boat), the props are going to be the first thing to hit bottom in any boat. Even if you hit so hard the pod shears off, which is what both IPS and Zeus are designed to do, the units are designed to maintain a watertight seal.

And chances are very good that you’ll have a new pod installed and be on your way to the next island long before the guy in the next repair yard has even gotten his new props, struts, rudders, shafts and shaft seals, let alone repaired the engine beds where the power plants were torn loose from their mounts and shifted a foot aft, and put everything back together again. I’d take the pod boat any day. As is the case with sterndrives and outboards, Zeus also benefits from its props being protected in certain scenarios from object strike damage, although the pods won’t pop up on hard impact like the other units will.

Volvo is in full swing with a second generation of IPS pod systems, and CMD will reportedly be in full swing with eight different boat brands this fall. I’ve run more than a dozen IPS boats from 37 to 75 feet, and I ran the 44-foot Sea Ray Sundancer Zeus test boat; all handle superbly and work as advertised. I also got a chance to play with the Zeus Skyhook, which uses GPS to lock the boat into position. Just push a button and the boat maintains its heading within 5 or 10 degrees and its position within a few yards.

On my test ride, Skyhook worked very well indeed off Government Cut in Miami in a 3- to 4-knot current and 20 knots of wind, locking us in position some 30 yards from a channel buoy with the engines autonomously speeding and slowing, and the pods changing directing on their own to keep us locked in position. IPS now has a similar system called GPS anchor.

Volvo offers its IPS units in double, triple and quad installations. The biggest boat so far is the Lazzara 75 LSX Quad (as in four IPS 600s), which does well better than 30 knots on a little more than half the fuel it would take with conventional power. Lose an engine on the Lazzara, and the boat will continue on its merry way, maxing out at more than 25 knots and cruising at well better than 20 knots all day long on three engines. Lose an engine with conventional twin inboards, and you’re going home at displacement speed (below 10 knots), and good luck docking the thing when you get there.

I see no reason quad IPS 850s couldn’t be installed on a 90- or 100-footer with the same results. When Zeus is in full swing, I’ll revisit the pod issue with an update and go into more detail on both systems.

Eric Sorensen was the founding director of the J.D. Power and Associates marine practice and is the author of “Sorensen’s Guide to Powerboats: How to Evaluate Design, Construction and Performance.” A longtime licensed captain, he can be reached, and his book purchased, at .