Things Do Change…
by John Curnow, Global Editor, Powerboat.World 21 May 15:00 PDT
Superbad jumping off to light speed - 2023 Offshore Superboat Championship © Australian Offshore Powerboat Club
It's quite some time ago now. Three decades for sure, and into its fourth, quite possibly. It was one of those unwritten laws. An adage, if you will.
It went along the lines of, 'For larger, high-speed craft up to 30 knots it was all about inboard power (inline and V), the 30 to 50 knots bracket was far more of a mix and match between waterjet and surface drive, and then after 50 knots it was all the domain of the sashimi makers (surface drive).'
Yes, the stern drive (AKA inboard/outboard and Z-Drive) was in there, especially Bravo Three and then M6, even M8 from Mercury Racing, as well as say, Konrad Marine, which were purposely designed to handle the gear crushing torque from the hefty Diesel brutes in front of them.
Like a lot of these things, there were holes in the theorem, should you go looking, but it held enough truth to be a valid discussion starter. So, it was perhaps a bit more like al dente spaghetti, and much less reflecting the form of a steel girder.
None the less, it definitely still served for some initial framework. Flick through any set of ads for pre-loved big, quick craft, and the point is driven home beautifully (and yes that pun is intended)...
What a different world it is now. (And how!)
What might have once seemed a large boat may now need to be next to your rubber ducky in the bathtub to feel at home. The centre console has overtaken the go-fast boat as the preferred weapon of choice. Outboards can be seen on craft up to 75 feet LOA, and the pod arrived around 20 years ago with its supreme efficiency and impressive manoeuvrability just part of the song and dance routine to accompany the sex and sizzle.
As a counterpoint to stonking Diesels, a paragraph needs to be used to say massive petrol outboards, and whizzing turbines. Like wow.
The former, however, runs out of puff, no matter how many you strap on to the transom, so is best reserved for light displacement duties, and nearby bowsers. There's one of GE's gas rotisseries from an F16 in the centre of a superyacht's engine room that pumps four tonnes of water a second out the non-directional jet in her transom. It is so impressive that she needs a hydrofoil underneath in order to avoid sucking herself under.
Now this all serves to highlight the many nuances involved, and the different propulsion packages all have pros and cons, as well as vastly differing ride and handling characteristics, to say nothing of manoeuvring behaviours. We could get into that, but it would be a whole Editorial on its own, and as you can see, this one is all about drive.
Should you want said item, however, please write in and let us know. As always, we'd love your thoughts...
So, there was a time when Riviera was Volvo Penta's largest client for IPS. Could well be they were THE early adopter, and other manufacturers caught on along the way. A maximum of 1000hp was always going to offer some limitations, and Volvo offered triple and quad installations, which the inline six was suited to, but Riviera's owners weren't in the mind for that. So their big stuff retained shaft drive, some of which is encased.
Today, you would have to think that 50% to 70% of Riviera's product is still IPS. In the meantime, here is our real world, offshore, 400nm account of no less than four IPS powered Rivs. It is as valid today as it was then. Possibly even more so...
Play that funky music, white boy
So clearly this particular ditty is to be all about pods because of their inherent efficiency in direction of thrust.
Most specifically, this is about the Mother of them all, ZF's POD 4600, which is the only variant currently on offer. This is all in deference to Cool it. Cool it. Cool it!
You see, if you're going to be green, then you had better do it from CFD to construction methodology, piston to screw, lest anything left behind looks a little bit like the male dog's undercarriage. Would not want to stand out now...
Drag. Walk along in the shallows and you'll notice that it is way harder than walking on land. Same goes for boats, only they are not so good on land. Haul a shaft, skeg, and spade rudder around, and there is some amount of gear tied up in all of that. There is cavitation around an open shaft too, and that means vibration.
Then you have maintenance items like cutlass bearings and stern glands to ponder, as well as Captain Corrosion on all those fiddly bits. Even with tunnel drive you never really get to zero degrees angle, either. Low, for sure, but there is always an element of the screw trying to launch the boat out of the hole.
Then there's the pod. Been on liners and tugs for ages, where dollars saved, efficacy and enhanced manoeuvrability speak rock concert sized volumes. As for ZF's POD 4600, the magic number is up to 20% gain in efficiency over conventional shaft drive.
Boom. Mic drop moment right there.
OK. So the ZF POD 4600 with its traditional, rear facing props is basically 2000mm tall (screw tip to oil filter atop the gearbox), 2600mm long (nose cone to spinner), and the baseplate is some 1100mm wide, which means it is not exactly petite. It is 1540kg dry, without screws, takes 90l of oil (fully serviceable from the top end), all the while noting that the three-bladed NiBrAl fore screw can be up to 130kg on its own, and also that the four-bladed aft screw is still hefty.
Let's call it two metric tonnes and be done with it, which really means the ZF POD 4600 is not exactly dainty, either. For what it is worth, we have not even accounted for the jack shafts yet, either.
And remember, you need at least two of them, which kind of gives you an idea as to just how crucial the drag beast really is when you have up to a 20% gain on offer.
If one needs an exclamation mark, then consider this. The fore screw nut is tensioned up all the way to 900nm of torque before you hear that special click. Hang five guys off the end of a ten-foot pole if you like, or preferably use ZF's special five to one tool, and do it on your own, but you might still break a sweat.
Why so serious? Well, there are 11 sets of standard screws to be matched to the application. The largest fore prop is 978mm (38.5"), and largest aft prop is 876mm (34.5"). Q.E.D. Remember. Big boats. Even bigger gear!
Now if you are starting to join the dots, then you'll know that a vessel running these (and you can go for a triple installation if you want), will need to have a fair bit of buoyancy out aft. Good beam, and low deadrise being the key determinants. The original offering (the ZF POD 4600 is rated to 1700hp) was mated exclusively to the Azimut Grande 26m and MAN V12s in the recreational boating market.
Now it is about to become open slather in the large, swift yacht space. Consider then what it might be like to add 300 more ponies to each, thereby ensuring there is plenty of giddy up!
ZF do have other parts in the catalogue that may work into making an even higher rated POD. You'll need a different internal kinematic chain, so think beefed up bearings, gears, shafts, clutch, and so forth. These would be essential too, especially when paired with the continuously rated mtu 12V2000M96X that spins out to 2450RPM and delivers not only 2002hp, but a mind melting 6866nm of torque, all day, every day.
And if such a product was around and it teamed up with CAT's 32B, then it has 7119nm to absorb, but with the latter it is only in bursts. Still. The point is made. Nothing is made from cheese when it comes to ZF, and they have the parts around in other products, so it is not so much a stretch, as a gather round...
Now as it turns out, coming soon is the new ZF POD 4900, purposely designed to be mated with the high boost, 2000hp, 12-cylinder behemoths.
Lay it on me
Ultimately this all plays out in the 30 to 40 knot bracket, and it meets the requirements for a lot of this very rarefied space; all but say the top end of the Italian Hoonwagons, and the express Marlin Chasers the Americans build. If you accept that, there is a chance that you could be looking at smaller engines to attain the same or nearly the same top end. Less weight, less fuel, equals more booze. Your choice, I guess.
Time for some bullet points to get through a few key elements efficiently (yes, pun intended):
- Simply must be said that a lot of the componentry is already trialled and tested inside other ZF products, including their amazing 2000 series of gearboxes. ZF HQ in Friedrichshafen Germany, as well as Padova Italy (boxes) and Kaohsiung Taiwan (screws) are all involved in this product line.
- Note also, that with the box now atop the leg, there will be more space in the engine room, so the design parameters for boatbuilders are open and provide for great opportunities.
- Jack Shaft typically one to two metres, but there is no maximum. Three degrees angle is the maximum allowed from flywheel to input, however.
- 2.434 reduction helps run the big screws, which delivers even more efficiency. On the same vessel type a MAN 1900hp shaft driven boat was slower by three knots (and change) and burned 22% more fuel than MAN 1650hp ZF POD 4600 installation.
- One lower leg ratio, and all applications are individually assessed in the ZF configurator, utilising full CFD analysis on naval architecture and mass calculations, with hull resistance a key variable. This is critical to final top speed projections. There is also an application questionnaire to go with it all, which further helps to set the parameters.
- Joystick control for docking and position hold, as well as also a traditional helm with clutches/throttles. Deploys ZF TotalCommand, utilising CANBUS comms that is CE, RINA, ABYC, and BV approved.
- Underwater exhaust behind the screws, and this reminds me very much of the original ports on the very first OMC Sterndrives. There is an over the side option (different baseplate configuration, but same footprint), but once you have had the former, there is no going back to the latter.
- Hydraulic-electric steering rams going through plus and minus 30 degrees for exceptional manoeuvring.
- PTO (Power Take Off) of up to 130kW say for thrusters, and Power Take In, or PTI (max 150kW) for optional hybrid integration.
- Elephant in the room. Safety first, and always. The pod propulsion design has been carried out in order to prevent water intrusion in case of impacts with underwater objects. The leg has a safe release system that not only avoids massive deceleration, but also reduces the likelihood of further impact after separation. If you have ever seen a bent shaft go up and carve a hole in a hull, then you will know exactly what this means.
- ZF POD 4600 pod has been given the green light for triple and even quad installation, which could certainly appeal to some military applications. However, there could be some Continental boatbuilders that would look at potential offset configurations, given you can have the jack shafts run between its sibling's blocks. Hello! Whooshka.
- Alfetta GTV, Porsche 928, and the Prancing Horse's 550 Maranello. Yes. Ponder the transaxle for a second. Typically the ZF proposal accounts for 1-2m of jack shaft, but ZF also point out that 5m is not outside of the working parameters. This is interesting, for whilst it means that yes a V-Drive set up can be reversed very easily, it also brings the inline system into the mix, as the driveline could run all the way back to the lazarette, and therefore, the overall engineering would not have to be altered that radically. Gyros, gensets and AC plants move forward, main engines slightly aft, and as there is no gearbox there anymore, the engine room could be smaller, or better yet, get the big donks in and gain 3-5 knots, or more. Yes please!
- Not a bar with way too much glitter and make up. No. This drag show is the hydrodynamic one. Interestingly, both Volvo Penta IPS and ZF POD 4600 have roughly the same submerged surface areas. Triple IPS 1350s makes for 3000hp, and twin ZF POD 4600 can deliver you up to 3400hp. Upshot? Less pods to start with, a 33% reduction on drag, and potentially up to 400 more ponies if you so choose. What's not to like?
- Now take another step. 4000hp is often the benchmark. Presently, that means four IPS1350s. So, in the case of the soon-to-be-with-us ZF POD 4900, then you would have a 50% gain on offer immediately, simply by virtue of the number of underwater bits. That's cool. Undoubtedly also make manoeuvring a bit tidier, as well...
- Helicopter view. Yes. This one is always important, and in our discussion of pods, there is an even more crucial aspect to it all. Consider that a chopper has a huge shaft running up from the engine to spin the main rotor, and then a tail rotor out the back to stop the fuselage spinning around said shaft. The leg of a pod has a massive drive shaft to deliver the goodies to the pod below, and thence the screws. Take a very close look at the leading edge of an IPS leg and you'll see a small drag plate on one side of the casting (consider it a permanent vertical interceptor), which is there to keep the whole pod in the desired direction (remember the fuselage).
In all honesty, I could write nothing better than ZF's own reply to this point, which simply said, "ZF POD 4600 mechanical design has been carried out in order to compensate lower unit drive shaft torque inside the steering mechanism and related hydraulic cylinders. In other words, steering cylinders are strong enough to withstand shaft torque. This solution optimizes lower unit drag and avoids further appendages and related drag."
Q.E.D, and there goes the mic on its way to the floor once more...
Ultimately, this is all a fantastic step on the way forward to the new way of boating. As we have said, boating will not look like it does now in ten years' time. Many things will be different.
The ZF POD 4600 offers real benefits to large, high-speed craft, and this is crucial to making sure that relativity remains paramount. Seeing superyachts vandalised by wayward greenies is just not cool, and so factually incorrect as to be absurd. We need the top end of town to be there so that companies like ZF deliver the products of the future.
The two-speed directional box in Mercury's famed 600hp V12 is made by ZF, after all, and the shifting between the two gears on that unit is just sublime, even under full throttle acceleration.
OK. Today you will find that the website has an abundance of material from right across the globe, and if you cannot find something, just try the search box right up the top of the landing page, above our logo. If you cannot find what you want or wish to want to add to that, then please make contact with us via email.
Finally. Please look after yourselves,
John Curnow
Global Editor, Powerboat.World