What to Look for in a Quality Marine Transmission

Learn some of the features to look for when selecting a marine “gear” for your boat.

What to Look for in a Quality Marine Transmission
ZF is a multinational concern that, besides its prominence in the marine market, is one of the world’s largest automotive suppliers, providing driveline technology, as well as active and passive safety systems.Courtesy ZF Marine/Martin Meissner

A marine transmission — or marine gear — is a wondrous thing. It couples all the torque a big diesel or gas engine can deliver to the shaft that spins the propeller, so it must be built like a tank. It also endures tens of thousands of shifts from neutral to in gear during its life, and reverses rotation as well, so it must be built with clockworklike precision. In order to interface with the latest electro-hydraulic computer-drive technology, a quality marine gear ranks as one the most sophisticated bits of gear a boater owns. Naturally, a marine transmission must thrive in the corrosive vapor bath of heat and salt that is an engine room.

How does such a thing so important to every inboard-powered boat, from diminutive launches to watersports boats to ocean-prowling sport-fishermen and cruisers, come together?

What to Look for in a Quality Marine Transmission
The main bearing supports the clutch system of a marine gearset and takes all the thrust of the propeller, which, of course, is carrying the boat.Courtesy ZF Marine/Martin Meissner

To find out, I packed my overalls and headed to Italy, headquarters of the premier builder of marine transmissions in the world: ZF Friedrichshafen AG, or “Zed-Eff,” as it is known among boaters, boatbuilders and marine-engine technicians. There I toured two plants and got to spend a day working on the line, following the construction of a marine transmission from its humble beginnings as raw material to a finished marine gearset, crated and ready to ship to a boatbuilder. Grab your safety glasses and learn what goes into a quality marine ­transmission.

What to Look for in a Quality Marine Transmission
The assembly follows a designated path through the plant.Courtesy ZF Marine/Martin Meissner

Purity The ZF Selvazzano Dentro plant lies outside the ancient city of Padua, Italy, the setting for Shakespeare's The Taming of the Shrew. Rather than study the art of writing, I'm here to write about the art of building a marine ­transmission.

Editor's Note: Capt. David Rathgeber, a print subscriber from Virginia, wrote in to remind us that filling the transmission with proper lubricant was left out of the story.

The ZF plant is a sprawling, bustling campus that, besides ­manufacturing, houses offices, materials storage, shipping and receiving, and I am thankful to have Federico Decio, director of ZF’s pleasure-craft product line, and Martin Meissner, North American marketing and communications manager, as my guides. I am apprenticed to them and the rest of the staff, all of whom graciously suffered my presence and eagerly shared so that I could convey what they know and what they do for your benefit.

To begin life, all parts of a marine transmission are alcohol-washed. This ensures there are no impurities. As you might imagine, and as I will soon see, precision equipment such as what we are building can tolerate no contamination if it is to be relied upon to shift confidently and constantly during a tough docking situation, for example. ZF backs up this good practice with practicality. Every gear is load-tested, as I will also see, and crated for shipment warm from the test bed.

What to Look for in a Quality Marine Transmission
ZF cuts gears in-house.Courtesy ZF Marine/Martin Meissner

The assembly follows a designated path through the plant, stopping at a series of workstations, each manned by a craftsman skilled in the task at hand. Each procedure must be completed and then signed off on by the technician. ZF can track every step of every build of every gear. At one of the more interesting of these ­workstations, I installed the main thrust bearing. In addition to my safety glasses, I was given a wooden mallet. The parts required were delivered to me by a robot — a motorized cart with a brain. This just-in-time delivery of parts to the various workstations helps improve overall efficiency in the production process throughout the factory. I asked if they had a nickname for this robotic cart and found it interesting that they had not graced the machine with a name — at least, not that they were admitting to. The robot is just one way technicians and technology meld to make a transmission at ZF.

The main bearing supports the clutch system of a marine gearset and takes all the thrust of the propeller, which, of course, is carrying the boat. I seat the part by tapping it in using a large wooden mallet to strike the shaft, which is initially set by eye, feel and experience. (I’m coached by the guy who really does this job.) Then a computer scans the work to make sure the tolerances of the fit are correct. I sign off on the part — yep, I installed it, so my name is on the paperwork that will follow this marine transmission forever — and then I move to ­another station.

What to Look for in a Quality Marine Transmission
Every gear is load-tested, and crated for shipment warm from the test bed.Courtesy ZF Marine/Martin Meissner

I’m handed a tray of bolts and can of goop. Applying threadlocker to the bolts for the ­housing might seem mundane, but an even coating ensures both proper torque for these fasteners when they are installed as well as the serviceability of the gearset for years to come. The fasteners themselves? These are checked by centering them in a lathelike machine that ensures ­precision.

Perhaps one of the coolest jobs I was privileged to undertake was installing the race for the bearing in a transmission intended for use on a Seven Marine outboard. The race is, of course, the part that the bearing rolls or runs in. To fit this, I am handed a long-handled hook and required to don special gloves, then directed to a vat of liquid nitrogen and instructed to insert the race into the mist-shrouded miasma. Cooling the part to minus 200 degrees C shrinks it considerably, and at that point I am told to retrieve the race and install it in the ­transmission assembly, where it drops right in. Once it warms to room temperature, it will re-expand, most likely producing an as perfect fit as can be achieved. This process really struck home ZF’s commitment to quality and need for precision in producing a top-quality marine gear.

What to Look for in a Quality Marine Transmission
Installing the race for the bearing in a transmission requires cooling it to minus 200 degrees C.Courtesy ZF Marine/Martin Meissner

Of course, the heart of a marine gearset is the gears, and in order to deliver consistent performance and reliability, ZF cuts gears in-house, using a mill called the hobbing machine. In this part of the process, I feed hardened steel blanks into a beast that attacks it with multiple lathe heads. I monitor the process through the viewing port and am reminded of the way in which sculptors create, not by carving the piece itself, but by removing everything that is not the piece. The visual experience lies somewhere between watching a time lapse of ice melting and the final scene of The Terminator. After the cut gear emerges, it is checked for perfect tolerances in order to reduce noise and vibration in service aboard our boats.

What to Look for in a Quality Marine Transmission
Perfect tolerances are checked for throughout the process.Courtesy ZF Marine/Martin Meissner

Test, Test, Test! All parts and assemblies used in ZF marine gearsets undergo evaluation in the in-house Test Lab. Here, durometer hardness of materials is verified, and microscopes are used to inspect for micro-fractures and other invisible-to-the-naked-eye flaws that might leave a boater stranded — or stuck in gear while trying to dock in a crossing current.

What to Look for in a Quality Marine Transmission
Each completed gear and transmission are load-tested at a special bench.Courtesy ZF Marine/Martin Meissner

Each completed gear and transmission are load-tested at a special bench, the pieces wired and plumbed as though to replicate installation in a boat’s driveline. A computer monitor displays the gears’ workings in much more detail than even the large, modern multifunction displays that grace boat helms ­today can render.

What to Look for in a Quality Marine Transmission
My gearset passed all final tests with flying colors.Courtesy ZF Marine/Martin Meissner

My gearset passes all final tests with flying colors. I am given the controls for an overhead chain hoist. I hook on to the gearset I’ve helped build, and with one hand on the crane control dangling from the ceiling and the other steadying the chain, I swing it over to a pallet where a padded crate awaits with its top off. Director Decio is still by my side, and he shoots me a smile. As I lower the gearset in, I feel a surge of pride and no little amount of wonder. I was made part of a team — if only for one day — but tangibly felt the commitment to a common goal that permeates ZF’s manufacturing.

What to Look for in a Quality Marine Transmission
Marine gearsets differ in several ways from an automotive transmission.Courtesy ZF Marine/Martin Meissner

Gear Vs. Transmission Real motorheads — such as trained mechanics and boatbuilders — often use the terms "marine gear" or "gearset," which is more accurate than marine transmission. What's the difference? A transmission provides a variety of gears, for one. While ZF does make true multispeed marine transmissions for specialty applications, most marine gears offer just one forward ratio. Another difference is that an automotive transmission provides slip, to prevent stalling when accelerating, via a torque converter or manual clutch. A boat's drivetrain gets its slip from the propeller. So, call it a tranny or transmission if you like, but understand why others call it a gearset.

While ZF is a fairly well-known brand in the marine industry, it’s actually a much bigger global organization. The company was established over 100 years ago by Count Zeppelin to build transmissions for his airships. The two letters in the company name stand for Zahnradfabrik Friedrichshafen, which translates to “gear manufacturer of Friedrichshafen,” the town where the business was established and where its global headquarters still reside today. With over 150,000 people employed worldwide, ZF is one of the world’s largest automotive suppliers. ZF provides driveline technology as well as active and passive safety systems for the automotive industry, and also provides these technologies to construction, agriculture, rail, wind and aviation markets.