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Marine Engine Mounts and System Alignment

February 14, 2012

The number one reason that drive systems go out of alignment is that the engine mounts are worn or have sagged. The engine sits lower and lower and moves around more so there is increased wear and vibration on the entire drive of the vessel.

Marine engine mounts can make the difference between a low vibration engine, mounted stable in your boat or an iron monster that shakes the hull, produces noise and may lead to damage. Broken, damaged or worn engine mounts are not always obvious when 100’s of pounds of static motor are sitting on the mounts. Excess vibration can be caused by many things, including; mounts that are too soft or hard, worn engine mounts or how the mounts are attached to the bed. Of course, there are other things that can cause vibration, including; misalignment of transmission to shaft, worn components (cutlass bearing, transmission) or damaged components (propeller, shaft, transmission).

The forces of a high revving, high horsepower modern marine engine are passed directly onto the engine mounts. Even small one cylinder diesels really pound the engine mounts. For all their apparent simplicity, engine mounts are subject to a number of forces:

  • Longitudinal – The forward / aft motion of the engine
  • Lateral – The side to side motion of the engine
  • Vertical – the up and down motion of the engine

Most of these forces on a motor mount act in a form of chaotic unison. Not only must the engine hold its own position based on motor and transmission weight, but it also must resist the shearing force of the propeller under thrust. What looks like a simple job for an engine mount gets complex, quickly when throttling up; the engine mounts on one side are ‘stretched’, one the other side they are compressed, they are also subjected to shear by the thrust of the prop. Now add to the equation of a boat throttling up in rolling seas, or depending on the vessel, being subjected to storm conditions or high-speed pounding. The simple combination of metal and rubber that makes up an engine mount sees real abuse in a harsh environment.

Figuring out what engine mount you need:

  • Number of mounts. Most marine engine/transmission units use 4 engine mounts, some smaller/older units use 3
  • Matching up the weight and horsepower to an engine mount
  • Match the Make Model of your engine

Once you know how many mounts you need and a data about the engine/transmission then nearly every modern marine engine can be found with The Engine Mount Cross-Reference Guide. In summary, should you feel that your system has gotten out of alignment, check your engine mounts first. It is the sagging engine that puts pressure on the cutlass and shaft seal and wears them to the point of needing replacement.

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