Why is a gearbox fitted to a marine engine?

Answer 1

There are a couple of reasons for fitting a gearbox (transmission) to a marine engine. One is that a lot of marine engines, particularly ones on smaller vessels, turn a lot faster than is practical for spinning a screw (the "scientific" term for a propeller) in the water. It's fairly easy to spin a screw fast enough to cause it to cavitate, which is, in general, not a good thing. Certainly smaller craft propellers will cavitate to obtain high speeds, but the smart money is on maximizing performance across the board. By using a gear set to change the engine-to-screw drive ratio, something like, say, a big block American V-8 or the equivalent of a truck diesel engine can be used to drive a screw a foot or so across for good power and efficient performance. There are some other things about a gearbox that help. What if you need to spin the screw backward to slow or reverse your direction? The engine cannot be stopped to facilitate this, but a gear box can include gearing and a mechanism to provide exactly that feature. You'll have a "throttle" control, and a "direction" control with a "forward" and a "neutral" and a "reverse" position. Done deal. Oh, and don't forget the advantages of vibration isolation. Let's tack this on here. Rubberized shock mounts for the engine will help things work better, and the vibration can be more effectively isolated there with the use of the transmission or gearbox. A vibrating shaft will chew up packing and cause leaks. The effect of vibration on propeller performance will be a negative one. Cut this off at the pass with a gearbox and the problem basically goes away, or is dramatically minimized. The engine in a boat is generally situated as low as it can be in the hull to keep the center of gravity stomped down. You don't have to have an advanced degree in marine engineering to see why this is a good idea. But sometimes the engine cannot be set quite low enough to drive the shaft onto which the screw is affixed. And to simply mount the engine in the boat and "point" the engine at the transom and "hope for the best" as regards where the shaft emerges and at what angle, well, that's not the best idea in the west. Better to optimize the location of the power plant, include a gear set (with reverse capability) that lowers the drive line, and then set the shaft for the screw to obtain an optimal placement of the water. Frequently the shaft leaves the area of the engine and slopes down toward the aft end, and it's not generally a good idea to mount an engine on a slope. The gear set allows for a change of the line of the drive between the power plant and the shaft. And that's in addition to lowering the line of the drive. Between optimizing drive ratio, providing for reverse operation (and vibration damping) and allowing for a bit of flexibility in setting up a shaft line, the gearbox is worth its weight in gold. There ain't no other practical way to get so much flexibility in your drive train as with a gear box. A link is provided to just one of many possible examples of this piece of marine drive hardware.