The internet is replete with guidance—some of it accurate—on the subject of auxiliary diesel engine maintenance and upkeep. There is, however, far less discussion on the subject of marine gears, otherwise known as transmissions.
Transmissions commonly run into problems too. For instance, the control cables. They’re the traditional connection between the shift lever in the cockpit and the gear down in the engine compartment. This system is a telegraph of sorts. Pushing it forward or aft mimics the same movement on the gear’s own shift lever.
The cable, which is made up of a steel core and a steel wire jacket covered in plastic, is fairly reliable unless it gets wet. Then it can rust and seize. The jacket can also melt if it comes into contact with a dry exhaust component. More likely failures involve various ways that the cable jacket may be immobilized and how the core is attached at each end.
The jacket clamp is critical. If it’s a gate style, then it should be safety-wired into place. If it’s a saddle, then it should be secured with self-locking or double nuts. If it loosens and the jacket is allowed to move, then shift control will be lost (and possibly stuck in gear).
The connection of the core to the shift lever must rely on proprietary parts from the cable manufacturer, including proper jaws, clevis and split pins. Don’t use common nuts and bolts. Check the interfacing parts for wear, a task that requires disassembly. Ensure that cable-end locknuts are tight. Make certain the cable is adjusted properly so that when it is in gear, the lever is fully engaged. Also be sure you know how to engage the lever manually if necessary.
For the coolers, be aware that there are two types of marine gears: mechanical and hydraulic. Up to about 100 hp, most engines are mated to a mechanical gear. Larger engines often utilize hydraulic gears.
Many, but not all, of the mechanical gears use an oil cooler that is bolted to the gear housing. It removes heat through the case wall without ever coming into contact with the oil.
Hydraulic transmissions utilize a traditional heat exchanger with oil on one side and seawater on the other side.
Coolers used on both types of gears are prone to corrosion and leaking. Hydraulic gear coolers, because the pressure is much higher, will leak oil into cooling water initially. Coolers used on hydraulic gears can become clogged because they rely on small-diameter tube bundles.
Damper plates, also called torsional couplings, connect the engine’s flywheel, or output, to the gear’s input shaft. This system uses springs or a flexible, rubberlike insert to absorb shock when shifting. If the springs break, or if the flexible material cracks or disintegrates, you might lose all propulsion, or it simply might get very noisy. (Some plates are fail-safe; they will maintain contact but lose all damping ability.)
If you notice a change in noise when shifting, or if you notice rubber or metal fragments under the after end of the engine, it might be an indication of a failing damper plate. Some bell housings (the cover that surrounds the damper plate) have ports that will allow for limited inspection without disassembly.
Finally, check your marine gear’s oil. This lubricant isn’t subject to the same sort of combustion contamination and heat degradation as engine crankcase oil, but it does wear out or shear, losing viscosity. It can also become contaminated with metal or moisture.
Hydraulic transmissions usually call for replacement every 500 to 1,000 hours, which is an eternity for most sailing vessels. It’s either the number of hours or every six to 12 months, whichever comes first.
Make certain you use the correct lubricant for your gear. Some systems require straight weight (not multiweight) motor oil, while others use automatic-transmission fluid. Also check the level correctly. Most hydraulic gears call for checking the level while running warm at idle and in neutral. By contrast, the mechanical gear oil level is checked at rest. Follow your manufacturer’s instructions.
Because the sump capacity is usually comparatively small, the cost of changing this oil annually is also small and offers a good return on investment. Ideally, have the oil analyzed as well. If your gear is hydraulic, it may have a filter (internal or external), and it probably has a suction screen. Remember to change or clean these too.
Steve D’Antonio offers services for boat owners and buyers through Steve D’Antonio Marine Consulting.
