Pod vs shaft – duel of the drives

Mark Rothfied | VOLUME 25, ISSUE 3

Billed as the technology tussle of the modern era, we rate the two contenders as they duke it out for the hearts and minds of boat builders and buyers.

Ladies and gentlemen … in the red corner, weighing in with tradition and convention on its side, is the long-running defender, Shaft. Its opponent in the blue corner, looking lean and fit, is the young and up-and-comer, Pod.

Who will win the world cruiser-weight title for the heart and hard-earned bucks of Joe Average Boat buyer? It’s the rumble under the gunwale, the donnybrook of the donks, the battle of the bilge …

The two combatants couldn’t be more different in technique or technicality.

Shaft has struts, rudders, exhausts and often thrusters and trim tabs as its faithful cohorts – together they can pack a powerful punch, while being versatile and tireless workers; certainly hard to beat in the long haul.

Pod, otherwise known as Volvo IPS (Inboard Performance System) and Cummins Zeus, appears the complete package, with unparalleled manoeuvrability and lightning-fast reflexes. It jabs with computer-controlled precision and efficiency.

IPS entered the ring in 2006 and has rapidly built a fan base among buyers and builders alike. Cummins spent a little longer developing Zeus and, thus, was slower to hit the circuit.

Where IPS’s forward-facing props pull through the water, Zeus has rear-facing props that push in the conventional manner.

Both could deliver a one-punch knock-out if this stoush was to be evaluated solely on performance. Judges only have to look at how former Shaft exponents have been reinvigorated by Pod.

The Sea Ray Sundancer 44 is a case in point. To suggest it was an average performer may sound harsh, but it wasn’t exactly a standout in the class of 2003. But Zeus transformed it into an absolute stunner in late 2007.


Ten tonnes of express cruiser suddenly had the sprite and agility of a runabout. With twin 500hp diesels, the V-drive shaft version was good for 29kt (53km/h), while twin 425s and Zeus drives achieved 33kt (61km/h), while using 30 per cent less fuel.

More recently, Pod has brought huge fuel savings to Riviera’s 51 flybridge convertible – at 24kt (45km/h) the triple IPS model burns 146lt combined, compared with 270lt consumed by MTU 1085s spinning shafts for the same speed.

Compared to the conventional Riviera 4700 SY, the performance of the new Zeus-powered 5000 SY equally defies imagination. The reworked hull has brilliant holeshot – there’s no planing transition, just pure, athletic acceleration – and exceptional turning ability.

For all that, Shaft maintains strong support among traditionalists, fishos and industry professionals. While acknowledging Pod’s high-speed prowess, they argue that he lacks grunt and stamina.

Maritimo General Manager, Martin Lewis says the company’s current models would go backwards in performance if Pod was simply plonked in, while Riviera design manager, Alan Dowd reckons it seems “more real” to have a keel and rudders working in unison with Shaft.

Commercial operators, according to Stebercraft’s Alan Steber, will continue favouring the long-term reliability and servicing benefits of Shaft.

There’s obviously more to the clash than meets the eye. When designing a boat, and when buying that boat, other factors must be considered. So ring the bell and come out fighting …


Pod lands the first blow, being virtually a plug and-play system. Boat builders don’t have to worry about steering systems, exhausts and so on, resulting in production cost savings that flow onto the price or profit margin.

The hard work is done in the design stage, with the assistance of in-house naval architects from Volvo and Cummins, respectively. Existing hulls generally must be designed from the ground up, or otherwise considerably modified.

Like many builders, Riviera took a cautious approach to the new technology, but is now charging full-steam ahead.

“We saw the advantages, but it wasn’t a straight-forward process of changing boats,” said Alan Dowd. “Our hulls have rocker in the keel line and solid keels, which meant they weren’t easily adaptable.

“We initially had some arguments with Volvo about power and performance predictions. I’m pretty conservative in the way I do weight estimates – I predict quite high weights because owners always add gear.

“Also, we don’t have toe-in on our legs because that makes it awkward to build a boat. We had to resolve that. We may not have optimal low-speed manoeuvrability, but we have a boat that can be assembled in a more controlled fashion.”

Riv’s 3600 SY, formerly a shaft-drive boat, was modeled in 3D then reshaped, while the deck edge stayed the same. That allowed Riv to use the same moulds and internal tooling. Because engine weight was moved aft, the designers straightened the run to move the centre of buoyancy aft as well.

Maritimo’s Bill Barry-Cotter and Martin Lewis are adopting IPS for their upcoming 47 Cabriolet, but remain adamant that shafts are the way to go for craft over 50 feet.

“There’s no doubt in our minds that, with bigger boats, it’s not a relevant installation. Pods become more feasible with the smaller boats,” Lewis said.

“We know that pods won’t work on our current hulls. We modeled a 48 with some Zeus drives and it was a disaster – once we put tunnels in we had no buoyancy.

“With the 47 Cabriolet, we’ve given the whole project to Volvo and said ‘you tell us what we have to build and how much it has to weigh’.”

Mustang was one of the first Australian builders to fit IPS and offered Zeus as an alternative.

“From our 430 Coupe onwards we’re offering the choice,” former CEO Chris Heaton said. “Both systems have plusses and minuses, but installation-wise and in performance they’re basically the same.

“We just need to know prior to lamination which powerplants are specified because of different mouldings.”

US gameboat builder, Cabo provides an option of shaft or Zeus with its 40-footer.

“They left the engines in the engineroom and ran a kevlar shaft back to the pods under the cockpit sole,” importer Graham McCloy said. “With sportsfishers, you don’t want the engines under the cockpit floor.”

Pods simply aren’t suitable for many existing models. The Cabo 38 has insufficient room underfloor, while a newly-developed Cabo 44 Express retained shafts because Zeus is yet to reach suitable horsepower.


Shaft scores a vital blow for versatility as Pod is targeted at semi-displacement boats and planing hulls. Displacement boats aren’t well suited to pods, nor are single installations possible … only pairs, triples and quads.

Volvo and Cummins tailor the horsepower to suit the design application and have different propellers to cater for varying loads, but they still can’t match the horsepower spectrum covered by shafts.

With IPS, you’re limited to Volvo diesel powerplants ranging from 250hp to 660hp and V8 petrols of 375hp and 400hp, whereas Zeus can accept other engine brands. Current Cummins offerings are the QSB5.9 (330-480hp) and the QSC8.3 (490-600hp).

Yanmar National Sales Manager, Michael Blair says his company is content to partner with Zeus in the future, rather than developing a pod of its own.

“We’ve just launched our own sterndrive as we’re chasing the high-volume markets … and our sales are also coming from the repower market, which is shaftdrive-focused,” he said.

“We’re repowering everything from 130-foot fishing trawlers to 23-foot trailerboats. It’s not practical to slot a pod into an old Bertram, or a boat that has low-build numbers.”

Alan Steber believes pods aren’t the most economical option for commercial roles: “It’s like comparing a draft horse to a pony, with pods being the pony,” he said. “With shaft boats, you can put them in gear and chuff along all day … there’s very few moving parts to worry about.”

That said, a Secret Harbour 49 built by Australia Yachts recently cruised 2375 miles (3800km) from Sydney to Perth on one tank of fuel, propelled by twin 480hp Zeus pods and sitting mostly on six knots (11km/h) or less.


With pod drives and engines usually residing together beneath the cockpit, the area amidships is freed up for accommodation. That’s a huge advantage in a mid-sized vessel.

On craft like the Luhrs 37 Open, which was launched last year, it has afforded another bed and toilet/shower. You’ve got all the advantages of an express-style boat, with a layout that’s equivalent to a 37-foot flybridge convertible.

The efficiency of pods allows fuel tanks to be reduced in size, but as more space is devoted to the cabin and less to the engineroom, designers are being compromised in the placement of batteries, air-conditioning units etc.

Shaft boats are inherently better at facilitating transom garages, though pod designers still get around it.

Multiple installations can get ‘squeezy’, even when access is achieved by lifting the entire cockpit floor, and personally I’m not a fan. Graham McCloy concurs: “Fishermen don’t want triple motors – that’s triple trouble.” So does Martin Lewis: “Triple and quadruple installations are complete nonsense in our view.”


Both contenders have strengths and weaknesses and neither is immune to a severe grounding or propeller impact.

IPS’s forward-facing props may concern conservative buyers, however real-life instances have shown that the legs can reasonably handle the rough stuff.

Out of hundreds sold locally, only one IPS leg has been sheared – that occurred when the vessel struck a breaching whale in Western Australia. The hull’s integrity remained intact.

A diesel mechanic I spoke to, Joel, reports some cases where IPS props have picked up ropes and damaged the clutch, but overall he reckons they’ve been relatively trouble-free.

“You’d have the same problem with a shaft. With IPS, it stops and they’ve got a sacrificial shear, whereas I’ve seen shafts that have picked up a chain and ripped the strut out of the bottom of the hull,” Joel said.

The Zeus skeg takes the brunt of a debris strike before the props and also shears off in a severe collision. Further, the pod resides in a hull tunnel, reducing draft and exposure.

Both pod systems rely on a microprocessor and fly-by wire wizardry for throttle and steering, which is a worry, but the designers have provided redundancies. IPS wiring systems have an earth, a positive, and two digital signal wires – should one signal wire die, the back-up comes into play. A failsafe feature puts the steering back in central position and the skipper can turn manually.

Zeus also reverts to manual control and can operate on one engine, if necessary, while a backup pump will cover for hydraulic steering failure.

Of course, shafts and bow thrusters aren’t immune to failure, either.

Martin Lewis fears that grave danger lies in a two-compartment boat. Maritimo’s shaft boats have three compartments – a lazarette with a sealed bulkhead, then an engineroom with a sealed bulkhead, then the accommodation space that has a crash bulkhead.

“When you put IPS in and pull the accommodation bulkhead back, it leaves two compartments. You then think: ‘If this thing sinks, what’s going to happen?’,” he said.

Typically, with a two-compartment boat, the stern sinks, the bow rises, and the hull turns turtle. People can get trapped inside.

“In our modeling, we added two buoyancy tanks in the back of the boat, fairly high and outboard of the engines … in a still-water situation, you wouldn’t even get water into the saloon,” Lewis said.

“In a real-life scenario, with waves, you could comfortably get people out of the forward accommodation. Lives would be saved.”


Ross Gray is a Holden dealer from Bundaberg, Queensland, and the owner of a Riviera 3600 SY with IPS. His former boat was a Riviera 3000 Offshore, with Volvo 285hp diesels driving shafts.

“The 3000 was a great little boat, but I waited to get pods and I think they’re fantastic,” Gray said.

“The joystick berthing is great. We get the sou’easterly winds and strong currents so it’s always choppy, but berthing is an easy process. My 14-year-old boy and my wife can get the 3600 in an out.”

Ross has noticed reduced sound and vibration, but most of all appreciates the extra room that turned the 3600 from a four-berth into a fiveberth.

Sportsyacht buyers seem to be the prime candidates for pods. For them, it’s a lifestyle choice and they’re not fussed about machinery, whereas flybridge customers generally know how to drive and they like to demonstrate their prowess.

It’s ironic that novices have been the first to seize on this whiz-bang technology. But even old stagers will use the joystick if an ill wind is blowing and the berth looks increasingly narrow.


“You get better bang for your buck out of the pods. There’s no argument there – we’ve seen it on every boat.” So says Tony Poole of Bluewater Power Yachts, the Australian importer of Luhrs gameboats.

“With IPS 350s (250hp), we’re getting equivalent performance to 350-370 horsepower engines, with much better fuel economy. From the efficiency point of view, they’re a definite goer.”

Graham McCloy compared a 40 Cabo Express with 800 MANs and a 40 Cabo Flybridge with 600 Cummins Zeus. While the 800hp motors had a 1.5kt speed advantage at the top end, he said the Zeus boat planed quicker and accelerated much faster.

Pods provide direct thrust and minimal drag. Engines tend to be smaller and lighter to achieve the same top-end speeds as their shaft counterparts, though torque is not a strong suit.

“With the 5800 Sports Yacht, we’re only using 1200 horsepower,” Riviera’s Alan Dowd said. “We’re getting good performance from small diameter propellers, but it doesn’t give you as much latitude to be overweight or have additional growth on the hull.

“Overload it and the top-end performance drops off very quickly.”

A large diameter propeller turning slowly, with an even and smooth impulse, is the ideal configuration. The last thing you want with pods, however, is to hang monstrous propellers under the boat. There’s no choice but to be small and nippy.

Even with dual counter-rotating props, the pod efficiency curve has a small peak, where a shaft-drive boat typically has a wider peak.


Savings are reported to be in the order of 15 to 30 per cent with pod drives, which would make it appear a no-contest.

But Martin Lewis isn’t so sure: “Under load, pods become less fuel efficient than shafts, particularly if you install the shafts with low shaft angles and no tunnels; just a nice clean bottom.

“When we compared our (Maritimo) 48 with a 50-footer that runs triple IPS, we found that at 22kt (40km/h) we’re using 39 per cent less fuel, and at full speed we are nearly 20 per cent better on fuel. They’re huge numbers – it saves thousands of dollars.”

It needs to be said, though, that Maritimo is definitely going against the flow with its fuel claims.


When you think about a conventional configuration, everything is straight – the strakes, the tunnels, the props – and you have two tiny rudders trying to deflect the water. It’s a struggle.

But articulating drives point the thrust in the desired direction, like a clean punch.

The joystick makes berthing as logical as playing a computer game. Port, starboard, forward, aft, spin … all with one hand, not the three you sometimes need to operate throttles and thrusters.

As it happens, Twin Disc and Cummins have developed integrated joystick systems for shaftdrive craft (see The best of both worlds overleaf) with thrusters, and ZF is also said to be ready to launch its system. Early indications are that these systems offer excellent close-quarters manoeuvrability, but the jury is still out on how well they compare with the manoeuvring abilities of pod systems. In fact, according to some, the IPS Sportsfish mode is the ultimate in boat manoeuvrability and has no serious rivals on the horizon.

“You don’t have to drive like a maniac, but it allows you to use your throttles to manoeuvre on a marlin without any effort whatsoever. You don’t have to try hard to be a gun driver,” said Tony Poole.


Shaft requires some straightforward TLC once a year. Pod can be a more demanding beast, depending on whom you believe.

“The problem we’re having is getting to the bottom of what’s required for IPS servicing. We’re getting 10 different stories from every man and his dog,” Tony Poole said.

According to Volvo sales manager Andrew Nicholson, the maintenance agreement dictates a first service at 50 hours and another service at the end of the first year to carry on the second-year warranty.

That most likely means two services in the first year.

While Nicholson insisted that oil can be extracted internally, mechanics say the boat really must be lifted to drain the leg completely.

“The drain plug is up through the exhaust – you take the props off and bob’s your uncle,” said Joel.

IPS uses liquid gold for lubrication –14lt per leg of a synthetic oil that costs around $30 a litre!

Our Riv owner, Ross, wasn’t aware of servicing costs and admitted to being shocked when told he was up for almost $900 in oil alone “For the size of the boat, the ease of handling, the fuel consumption, I’m still in front,” he reasoned.

Cummins sales and marketing manager, Paul Zeinert says Zeus servicing intervals are the same as that for the engines – every 12 months or 250 hours. It requires slightly less (and cheaper) oil.

The best of both worlds

While the two camps – those who push the pluses of pod drives and the traditionalists who swear by the shafts – continue to promote the virtues of their respective causes, a third camp has been quietly working behind the scenes to merge the outstanding attributes of both systems.

American marine transmission specialist, Twin Disc chose Sanctuary Cove to show off its new Express Joystick System (EJS), which seeks to offer the manoeuvrability of pod drives with the simplicity and proven performance of shafts.

Fitted to a Maritimo C60 Sports Cabriolet – the first production installation anywhere in the world – powered by otherwise conventional twin Caterpillar C12 diesel engines, journalists were invited to trial EJS at the show and I have to say that I came away very impressed by the system’s user-friendliness, performance and smoothness.

According to Twin Disc, EJS offers seamless docking and slow-speed handling on traditional shaft-driveboats by combining Twin Disc’s patented QuickShift gearbox technology, with hydraulic bow-only or bow and stern thruster systems.

The system on our trial craft used thrusters at both ends, with two joystick stations – one at the helm and the other in the cockpit. It was simply a matter of pushing a button on either station to move control of the craft to the respective joystick.

EJS works by electronically manipulating the shafts and thrusters in combination to move the boat in any direction. It translates minute movements of the joystick into mechanical actions on either the shafts or thrusters, resulting in forward, rearward, sideways or diagonal movements.

What impressed me most was the smoothness and seamlessness of the Twin Disc system. There were no clunks or sudden lurches as the joystick was moved from one direction to another and the boat was extremely easy to place in any position, making it ideal for docking for those who might be challenged by the more traditional system in which the skipper uses a combination of throttle, gear lever and thruster movements to manoeuvre the boat.

Australian Twin Disc MD, Glen Frettingham said the system can be retrofitted to most boats, depending on their current transmission set-ups, and is a more cost effective method to achieve precise joystick control than any of the current pod drive systems.

“With EJS, we don’t significantly alter the original design of the boat,” he said. “If an owner already has standard QuickShift transmissions and controls installed, an upgrade to EJS joystick control is relatively simple when compared to upgrading to pods.”

He said pods require a uniquely designed hull structure and positioning.

“Part of that structure sacrifices a keel on the boat that can compromise tracking,” he said. “With EJS, there is no sacrifice.”

We were told that the system added about $80,000 to the price of a typical production craft, such as the Maritimo.

– Chris Beattie


Cost-wise we’re talking $200,000 to $300,000 (approximately) worth of pod/engine units, but there’s not a lot of change if you were to buy larger horsepower shaft motors with electronic throttles, bow and stern thrusters, an autopilot and so on.

Martin Lewis and Alan Steber remain unconvinced of Pod’s merits.

“I personally don’t think they’re worth it,” Lewis said. “They will be a flash in the pan – someone will come up with a vee-drive installation that’s less maintenance-intensive and they’ll blow them out of the water.”

Our other interviewees begged to differ. Graham McCloy: “If I got people onto a Zeus boat and showed them what it could do, I reckon 90 per cent would be converted.”

Tony Poole: “Personally, I would go with the latest technology. I’ll tell you why – they appear to be proven and they’ve definitely done their homework as far as corrosion is concerned.”

Alan Dowd: “Riviera will pursue pods further because of fuel efficiency. Hydbrid drives with biodiesel and electricity are probably the blue-sky future.”

Joel: “I’ve been in the industry since 1988, seen a lot of development, and I think they’re on a winner with pods.”

Chris Heaton: “There are too many advantages with pods. You get the performance, you get the efficiency, you get the joystick and then you get all that extra room.”

So, it’s a comfortable points decision to Pod for modern, mid-sized performance cruisers. But by no means have we seen the last of Shaft. ¿