If you own or plan on owning a large boat with engines up to 1200-hp, then you need to read this. We recently had a chance to conduct a full test of the ZF POD 4000 series drive system (with joystick) installed on a 76–foot motoryacht where it propelled our 106,000-lb. (48,181-kg.) with a nearly 22' (6.70 m) beam to an impressive top speed of 34.3 mph. Not only that, but the easy joystick maneuvering and docking that we have been experiencing for the last four years on 40-footers is now possible for big motoryachts and convertibles.
ZF Marine POD 4000 Captain's Report
The ZF 4000 POD series has a max horsepower rating of 1200-hp, which is the highest hp-rated pod we have tested to date. This is an important fete of engineering and construction because the torque and force created by 1200-hp is enormous. Not only must the metal inside the two 90-degree angled gear cases be able to withstand the power and heat generated, but the force must also be transmitted to the hull without damage or distortion.
For this reason, the team at ZF has designed the 4000 series to be mounted directly to the stringer system.
The ZF POD 4000 mounted to 8 stringer points.
Each ZF 4000 POD passes through the hull and mounts to the stringers. You can see this in the above image where the circular bushings have bolts going through them to the metal beds affixed to the stringers.
This system of pod mounting significantly reduces the stress load on the hull and from our inspection of the installation on our test boat it became clear that this was a masterful solution to a difficult engineering problem.
Another design feature of the ZF 4000 POD system is that it utilizes a traditional transmission rather than incorporating the transmission in the pod itself. This means that the output from the engine goes first to the standard transmission, allowing the gear ratio to be to be controlled prior to being sent to the ZF 4000 POD. The pod itself has max rpm rating of 2500.
With a variety of prop sizes and pitch angles, the system can be optimized for hull sizes from 50' to 75' (15.2 - 22.8 m) and even up to 120' (36.57 m), we are told by the team at ZF.
Rubber bushings connect the 4000 series pod frame with the metal bed that is attached to the boat's fiberglass longitudinal stringers. In the way the force created by the dual props on the pod are transmitted vertically directly to the stringers, which are the back bones of the hull.
This view illustrates the installation of the pod as connected to the drive shaft and transmission. All gear reduction takes place in the transmission.
The ZF POD 4000 Series with transmission connected allows boat builders to have all the benefits of pod drives while at the same time providing design versatility. Because of the prodigious weight of diesel engines and transmission in the 1200 horsepower range many motoryacht designs cannot handle all of the weight in the stern. Further, in convertibles, there is not enough room under the cockpit to mount the engines. In both of these cases, jack shafts are used so that the power system can be moved forward where it needs to be for optimum trim angle and CG.
This installation diagram demonstrates just how the versatile nature of the system can allow for optimum weight distribution.
Pod drives have proven to be more fuel-efficient than inboard-powered boats in the midrange in numerous tests conducted by boat builders and BoatTEST.com. The horizontal angle of attack, the dual props, and reduced drag seem to be the primary reasons that many boats achieve fuel consumption improvements of from 30% to 50% at cruising speeds with pod drives.
The ZF Controls
The ZF Smart command system was installed in our 76-foot test boat. For the operator at the helm this meant we had the ZF SC throttle controller and the ZF joystick. In a word, this system is "simple." The operation is intuitive and easy.
What we like most about it is the drive-by-wire set-up. All commands are sent to a processor via digital signal, and then actions are executed via electro mechanical devices. We should note that the steering for each ZF 4000 pod is accomplished the same way -- steer-by-wire -- the same as on modern passenger aircraft.. This means you have an electric motor connected via direct drive on each unit.
More Reliable Than Hydraulic Steering?
Is the steer-by-wire system more reliable than conventional hydraulic steering systems? The folks at ZF tell think they are, and they have some good reasons for that claim.
Theoretically, steer-by-wire should be more reliable than hydraulic steering given all of the fittings, hose clamps, and lengths of exposed rubber hose those conventional systems have.
The ZF JMS Joystick controller is small and can be easily mounted most anywhere.
The Glorious Joystick
The joystick could not be any simpler: it has the joystick and one button to activate. Not only can you move the joystick in any direction on the compass rose, but you can also rotated it by twisting the top (high-hat) to port or starboard. The stick is progressive which means that the more you push or the more you turn, the greater the speed of the boat will be in that direction.
The ZF Smart Command engine controller.
The ZF Smart Command processor is the brains behind the JMS and POD systems. It is a computer that receives input from the joystick and controller electronically, makes all the calculations, and then sends commands to the engines, transmission and pods.
Docking a Large Yacht
While the joystick has been embraced with enthusiasm by owners of small boats, most skippers of larger boats have confidence in their abilities. Yet we have never met a captain of any size boat that didn't like a joystick -- and they all use them if they are available. The reason is that as good as they are at maneuvering a large vessel, usually with just its props, current, wind, and the very tight nature of modern marinas where space is at a premium can make maneuvering problematical.
Being able to carefully and confidently control a 106,000.-lb. yacht in a tight harbor like the one in Cannes, France where we used the system is a good test. In the marinas there like most of those all over the world space is at a premium and therefore tight.
ZF Software is the Key
Because the pods rotate independently and the Smart Command computer is instantly and continually making all the control instruction calculations, the captain needs only to turn the joystick in the direction he desires to move the yacht. The computer determines which pod should face in what direction, controls each engine's rpm to apply the correct amount of thrust wanted, and changes from forward to reverse automatically.
Our Rating is "Excellent"
During our test of the new ZF 4000 POD system software performed exactly as advertised. It was sensitive when it needed to be and responsive at all times. We can only rate the system as "excellent."
We should point out that our test exposed advantages beyond the obvious maneuverability at the dock. Once under way, our 76-foot motoryacht handled more like a sport cruiser, and that is no exaggeration. There was a distinct difference in the turning characteristics over a conventional inboard drive that relies on rudders.
Here is a look at the test numbers. Powering our testing yacht were twin MAN 1200-hp V8’s. We had half load of fuel, half a tank of water and calm seas on an 87-degree day.
While the boat will go 29.8 knots at WOT, we would prefer to cruise her at a comfortable 19 knots and burn 67.3 gph which we think is terrific fuel consumption given the displacement and horsepower of this 76' boat.
Down in the engine room we noticed trolling valves, which are essential for the precise operation of the joystick operation. We also noticed, or noticed the lack of, transmission noise. In joystick mode, when the computer is moving the engines in and out of forward and reverse, we did not notice the shifting into and out of gear.
In one final show of the control available, to off load our test team and gear, the 76-foot motoryacht was backed towards the concrete pier in Cannes' inner harbor, the passerelle was extended, and our team off-loaded -- all while the 76-footer was held perfectly in position with the joystick.
We will have more testing and updates on this system in the near future.