Back in the days before engines and gears became so sophisticated and saddled with computerization and electronic controlling systems, gears really could take a beating. I remember being in a boatyard somewhere and seeing a gear come out of a boat that was blown all to pieces - the victim of an aggressive captain shifting from reverse to forward too quickly before the engine rpms could ramp down and allow for a smooth transition. Often, during a fish fight, the sudden thud and bang of the gear would send shivers through the crew, as they knew the gear could only take so many of those hits before coming apart.
As the horsepower-to-weight ratios increased in diesel engines and the amount of power produced by those engines climbed ever higher, transmission manufacturers started to ramp up their games as well. (For a little while, however, I think the power guys were just a little bit too far ahead because it seemed like a lot of gear failures were occurring.) Nowadays, without question, gear and transmission manufacturers produce far more robust products that can handle all the muscle you want to bolt them to.
With the inception of electronic controls and the use of computers to monitor both the engines' and transmissions' proper operating parameters, it's rare to see a gear failure due to aggressive boat handling. I think most gear problems today stem from a lack of maintenance and/or service. Transmissions are just another one of those overlooked items on the boat that some owners and less experienced crew just ignore until they stop working.
Proper Care
Manufacturers' recommendations on lubrication and service are very specific, and you should follow them religiously to avoid costly repairs. Companies like ZF and Twin Disc spend a lot of time and money to engineer and manufacture these products, and they spend many hours testing and creating specifications - ignore them at your peril. When you consider all the parts that move inside the gear box every time the engine makes a single revolution, you can begin to see the complexity of its internal operation. Imagine the wear and tear the gears experience when the engine is making 2,000 revolutions per minute. And when you take into account the series of bearings that supports the shafts, which hold the clutch plates, etc., you can understand the amount of moving parts inside a gear. Each of those parts offers an opportunity for failure, so preventive maintenance is essential to a healthy gear box.
You should check your gear oil regularly, inspecting for color, odor and level. I can't tell you how many boats I went on for a day's fishing or delivery and found the gear box low on fluid. Just because the boat sports a nice paint job doesn't mean the crew keeps everything in perfect working order down below. Without proper lubrication, metal parts start to eat into each other, changing the tolerances of the meshing gears and creating the wear that opens up the opportunity for failure.
Operators often don't consider this, but the oil in the transmission not only lubricates, but it also serves to cool. Engines have a separate cooling system; transmissions do not - they depend on the oil to disperse heat. A low oil level reduces the transmission's ability to cool itself. This is especially important with modern transmissions equipped with trolling valves. Trolling valves allow a permissible level of slip in the clutch packs by forcing oil through them. Long periods spent trolling can generate a serious amount of heat. So again, using the right amount of clean oil maximizes the life of your transmission.
My spiffy ZF gears on Brier Patch sport a screen filter designed to catch any particulates that may be suspended in the oil. These filters allow you to clean the screens via a built-in handle that scrapes the particles off the screen into a special reservoir. ZF recommends turning the handle daily every time you check the fluid. ZF also recommends using its own canister filters since generic ones may not filter out extra-fine particulates. Every six months or so, before and after each season, you should check the tightness of all connections, inspect the hoses and all external moving parts, and change the lubricant. You should also make sure that your actuators fully engage and disengage - if they do not, it will cause a problem for the gear over time.
