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Magical Mystery Tour


The way the average marine transmission accomplishes the mission is "Way more complicated."

Okay, my education concerning things mechanical has not been formal. Indeed, you might say I've extracted the whole shebang from the School of Hard Knocks—meaning I've typically learned about things by having to either maintain or fix them myself.

Not that I'm complaining—you play the hand life deals ya, right? But still, there's at least one glaring drawback to the figure-it-out-after-it-bites-you pedagogy. Seriously reliable, easily maintained pieces of machinery tend to linger in obscurity, primarily because you never have to tear them apart or perform lengthy, up-to-your-eyeballs-in-grease procedures.

The best example of this phenomenon is the marine transmission, also called the marine gear. Truthfully, I've relied upon this wondrous, oft-forgotten device for more than three decades of rough-and-ready seafaring, and I can't remember a single instance where one's blown up, burned up, or otherwise tossed its cookies. So, given the dearth of difficulties that transmissions have visited upon me over the years, I suppose I shouldn't have been that surprised to discover how little I knew about their inner workings prior to researching this story.

I hit up some seasoned and salty friends for info, and they were just as uninformed as I was for much the same reason, a development that got me wondering if there might be a few PMY readers in the same boat. So I dialed up major transmission manufacturers, and here's the gist of what I learned.

What marine transmissions do is significant but fairly simple. First, they take one-way rotational power of the engine crankshaft and allow it to be converted into the three modes necessary for propulsion: forward, reverse, and neutral. Second, they cut the number of crankshaft rotations per minute by a fraction


Major Components of a Planetary Gear Case

If this stuff seems a little cryptic to you, there is more. The way the average marine transmission accomplishes the two-fold mission I've just described is, in the words of Velvet Drive product manager John Bolton, "way more complicated than most people think." Let's return to my Velvet Drive for a moment. Its hydraulically actuated, planetary-type innards are still popular and continue to pervade the marine scene today.

Planetary systems like the one illustrated above freewheel in neutral, of course. When shifted into forward-propulsion mode, however, a hydraulically actuated clutch mechanically links the propeller shaft to a cup-shape ring gear assembly (with a separate pinion and spur-gear assembly freewheeling conjointly), all within the forward half of the gear case. Simultaneously, in the after half of the case, a reduction box imparts the aforementioned ratio to rotation via a set of precisely configured gears. Wow! Put all this into play, and the propeller rotates in a direction and at a speed that moves the boat ahead.

This article originally appeared in the January 2008 issue of Power & Motoryacht magazine.