Reverse propulsion's even more complicated. When I shift my control lever aft, a separate reverse clutch stalls the ring gear in the tranny, a happening that forces the conjoining assembly of pinion and spur gears to stop freewheeling and rotate in a direction opposite to the one above. Again, this reverse movement is mechanically linked to the propeller shaft through the reduction box, and double wow! The propeller rotates in a direction that moves the boat astern.
Now since my transmssion's just a tad dated, let's take a fast look at the simpler, more modern "gear-driven" transmissions replacing the older planetaries these days, mostly because gear drive offers full-power reverse operation (the clutches of some planetaries are not strong enough to tangle with full-reverse torque) as well as accommodate high-horsepower applications.
Consider the illustration below for a moment. The torsional input coupling is connected more or less directly to the engine's flywheel. In neutral (with neither the reverse nor forward clutches engaged), it and virtually everything else simply freewheels. But energize the forward clutch via hydraulic pressure, and the forward pinion moves to engage the output shaft gear. Bingo! The prop rotates to move the boat ahead at an appropriate speed thanks to a gear reduction designed into the output and pinion gears themselves, not pulled from a separate reduction box.
Actuating the reverse clutch has the opposite effect. It meshes the reverse driving gear with the reverse driven gear, which in turn energizes the reverse pinion and thus rotates the output shaft gear (as well as the propeller shaft and propeller) to move the boat astern. And like before, thanks to the ratio designed into the gears themselves, no separate reduction box is needed.
So that's it. Just bear in mind that brief explanations of the two most common transmission systems extent in the world today are far from comprehensive. Hydraulic actuators, control valves, oil-cooling systems, splines, shafts, and thrust bearings, to say nothing of two-speed and V-drive applications, are also part of the picture.
And incidentally, for a more in-depth understanding of your very own transmission, read the manual written for it. Twin Disc offers a fits-all beauty that is particularly detailed and easy to understand. Even for those of us who typically don't read manuals.
A Word to the Wise
You've undoubtedly noticed that the dipsticks on your transmissions are marked with an admonishment to check hydraulic fluid levels only when the trannys are hot. Moreover, there may be a second admonishment to avoid overfilling. Important warnings!
Let's deal with the first one first. Most transmissions today have fluid coolers that are alongside or above them, an arrangement that gravity-feeds fluid back to the main sump once the engine's turned off. Crank the engine and warm it up, however, and the fluid travels throughout the transmission and cooler, leaving much less in the sump. Thus checking levels with the engine warm gives an accurate reading of the tranny in the operational mode. Checking cold gives an inaccurate, overfull reading.
Overfilling raises the fluid level in the sump, allowing moving parts to churn fluid into a milkshakey froth, thereby lubricating said parts with a mix of oil and air rather than oil alone. Overheating and mechanical damage result.
A quick tip. Once you correctly fill a hot transmission, shut the engine off, let the transmission oil cooler drain back into the tranny's sump, and pull the dipstick. Then make a cold-level mark—if there isn't one there already—with a small file. In the future you'll be able to check fluid levels without having to crank the engine.—B.P.
This article originally appeared in the January 2008 issue of Power & Motoryacht magazine.