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Your marine transmission may be dull and mysterious, but your boat wouldn’t get far without it.

Of all the myriad mechanisms and devices that combine to push your boat through the water, none is more ignored and misunderstood than the marine transmission. That’s probably becausee it is so boringly mundane and steadfastly reliable that it doesn’t really seem to warrant attention. Think of it: When is the last time you heard a fellow boater either brag or complain about his marine transmission? 

Marine Transmissionand Coupling

Yet because this simple but durable device is so critical to getting you where you want to go safely and efficiently, you really ought to have some idea of what it does and how it does it. And the first step in understanding the marine transmission is realizing that despite the identical nomenclature, it has virtually nothing in common with the transmission in your car. The function of that device is to change the ratio of engine rpm to wheel rpm a number of times so that your car can accelerate from a dead stop to a desirable cruising speed in a relatively short time and run at that speed with a high degree of efficiency. A key part of the automotive transmission’s function is providing initial slip between your car’s engine and wheels so that your engine doesn’t stall as it tries to get your car moving. It does this either via a foot-operated clutch (manual transmission) or a torque converter (automatic transmission). 

None of this applies to a marine transmission, which is one reason why most mechanics refer to it not as a transmission but as a marine gear. First of all, slip is built into the marine system; there is no positive physical link between your propellers and the water such as there is between your tires and the pavement. So when you hammer your throttles, your engines don’t stall or balk. The propeller automatically slips enough to get your boat underway. A second major difference between the two types of transmissions is the fact that the vast majority of marine gears have only one set of gears and therefore one ratio. (ZF makes a two-speed marine gear.)

So your boat’s marine gear has three functions that are completely different from your car’s transmission. One is to engage and disengage the engine from the propeller—in other words, to provide neutral. Another is to provide reverse rotation so that you can back your boat into your slip. These two functions are accomplished by a series of internal clutches, which when moved via the gear lever at the helm, engage different gear sets. Each marine transmission has an internal oil pump that generates hydraulic pressure, which forces these clutches together to provide engagement. Because the pump generates heat as it pressurizes the oil, every marine transmission also must have an oil cooler, which is typically mounted on the outside of the transmission housing and is identifiable by the water hoses leading into and out of it.

When the gear lever is at one position there is no engagement (neutral) among the gear sets; in another there is engagement between two gear sets (rotation in one direction) and in yet another there is engagement among three gear sets resulting in rotation in the opposite direction. Note that strictly speaking there is no such thing as forward and reverse, just clockwise and counterclockwise rotation. In some installations counter-rotation of the propellers is actually accomplished by simply swapping the linkage so that the opposite gear set is engaged when the lever is moved.

The other function of the marine transmission is to set the ratio between engine rpm and propeller rpm. A typical diesel rotates at around 2000 rpm at cruise setting. A propeller turning this fast would be highly inefficient, so it is the job of the gears inside the transmission to slow down the propeller rotation to roughly half that of the engine, which is why this is always referred to as the reduction ratio. Note that this ratio is fixed; the only way to change the ratio between engine and propeller rpm is by changing marine transmissions or changing the gear sets within the transmissions. That’s why boaters spend so much time playing with different propeller configurations: It’s the only practical way to fine-tune the propeller-to-engine ratio.

The marine transmission performs one other vital function: It is the most convenient location from which to drive an auxiliary device, such as a hydraulic pump. Most marine transmissions have a power take-off point, which if unused, is usually covered by a plate. Remove this plate and you can see the transmission gear, which engages the gear of the auxiliary device.

Pretty exciting stuff, huh? Okay, maybe not, but you should still be impressed. For a comparatively simple device, the marine transmission does a lot of work. And if you didn’t have one that’s as durable and reliable as the one in your boat, your time on the water would be a lot less enjoyable and a lot more worrisome.

This article originally appeared in the December 2012 issue of Power & Motoryacht magazine.