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Your oft-overlooked engine mounts need regular inspections just like every other system aboard your boat. Here’s how.
A typical, good engine mount. Photo, Chris Diggs
Engine mounts just don’t get the respect they deserve – until problems arise. Even after that odd noise or excessive engine vibration is noted, chances are they still won’t get any attention until catastrophic failure occurs. As it never pays to use equipment failure as an indicator that required maintenance is past due, here’s how to inspect your engine mounts for possible issues.
What They Do
The purpose of engine mounts is to isolate the vibration generated by the engine, as well as isolate and protect the engine itself from externally generated shock loads or vibration during vessel operation. Properly functioning engine mounts are crucial, both to prevent engine damage and to improve the comfort of those aboard, who would otherwise be subjected to constant engine-generated vibration during operation.
Construction
A generic description of an engine mount would be a mounting stud (sometimes two) embedded in a block of elastomer (typically natural rubber, neoprene, EPDM, or silicone) that’s bolted between the engine and engine mounting bed via metal plates. This design and combination of materials allows limited movement between the engine and bed, while absorbing vibration and minimizing shocks that could cause damage.
Engine mounts must be designed to handle everyday issues such as chafe and corrosion while possessing high fracture-toughness to protect against unnatural stress, which could result in them becoming bent and misaligned.
What’s The Service Life?
Manufacturers are all over the board on this one, with service life expectations ranging from 1,000 engine hours, 4 years, 7 years, 10 years, or even multiple decades. As with many onboard maintenance items, factors such as how often you operate the boat, the conditions you operate in, and how hard you drive the boat (particularly in rough seas) will all affect the lifespan of your engine mounts.
The bottom line is that you’ll want to follow not only the manufacturer’s guidelines but your own observations and inspections as well. Regardless of projected lifespan, one thing that’s certain is failure to replace damaged engine mounts can result in not only engine damage and reduction gear failure (due to misalignment issues), but also shaft damage due to twisting. Damage to the vessel itself can also occur if the engine is not securely mounted, as may be the case if a mount fails.
Warning Signs
Signs of a failed (or failing) engine mount include the following:
1. Unusual or excessive movement when starting or stopping the engine. A worn or damaged engine mount may seem OK during engine operation (due to the stabilization effect caused by constant torque), however, excessive engine movement when starting or shutting off your engine is a possible indicator of engine mount issues.
2. Bad vibrations. Increased vibration or unusual noises when the engine is started or while at idle are common signs of a faulty engine mount. The antivibration properties of a damaged mount are reduced, resulting in more noise and vibration.
3. Visual cues. Physical problems such as cracks, separation, corrosion, permanent deformation, and outright failure (e.g., a broken stud or frame) are all signs of engine mount failure.
The cracked rubber shown in this damaged engine mount is unfortunately not always so obvious. Photo, Chris Diggs
The “clean” area to the right of the engine mount washer indicates movement of the mount. Photo, Chris Diggs
This damaged engine mount stud prevents proper alignment of the engine and must be replaced. Phot, Frank Lanier
4. Excessive movement during vessel operation, particularly in rough weather. A damaged engine mount that gives little indication of failure in general may show its true condition during engine acceleration and while cruising at higher rpms. Excessive noise, increased vibration, and even impact sounds or damage (where the engine strikes something around it due to excess movement) are all telltale signs of one or more damaged engine mounts. In such cases you may discover that gravity is the main force keeping the engine in place.
Inspections
The first step in checking your engine mounts is simply looking at them. Does the engine mount look serviceable, or is there corrosion or obviously damaged components? Does the engine bracket sag or rest too far down on the mount’s stud (a typical indication that the elastomer material has lost its elasticity)?
“Engine mount inspection is mainly visual,” notes Chris Diggs, owner of Diggs Diesel, LLC, located in Hayes, Virginia. “Look for corrosion where the rubber meets the aluminum or steel in the mount. Also look for any cracking in the rubber, which will diminish the amount of weight a mount can hold. This will cause the engine to drop slightly and then become out of alignment with the running gear. The jacking nuts should also be free of corrosion and lubed for easy movement during an alignment. If the nuts will not move freely, the mount should be replaced. You do not want the mount in a torsional bind when aligning the engine, which should sit down straight on the mount.”
Next, check mounting bolts for looseness. One trick to spot loose or broken mounting hardware is checking if the grime or residue on the engine bed around the base of the engine mount has been disturbed due to movement of the mount. Depending on the installation, you may even see fiberglass dust, a result of mount movement along a fiberglass engine bed.
Is the elastomeric material cracked, cut, or showing signs of delamination or separation where bonded to metal? Is the elastomer material swollen, “sticky,” or showing other signs of deterioration due to age or possible contamination from engine fluids (oil, diesel, coolant) leaking onto the mount? To inspect for elastomer-to-metal bond failure, gently insert a thin feeler gauge between the two and note the measurement. If you can insert the gauge greater than 3 to 4 millimeters, the mount should be replaced.
What does this rusty mount tell you? In this case, it indicates that the raw water pump located above it is leaking, allowing saltwater to leak onto the mount. Photo, Frank Lanier
A damaged engine mount that gives little indication of failure in general may show its true condition during engine acceleration and while cruising at higher RPMs
While you’re at it, check the metal components of the mount for signs of fatigue or mechanical failure, such as cracks, gouges, loose bolts, or plate deformation.
Another option (depending on installation and engine size) might be to use a pry bar to lift the engine slightly while checking for separated or broken mounts. “This method works on gas and the smaller diesel engines installed in sailboats,” says Diggs. “But for larger diesel engines, it’s hard to get enough leverage in most engine rooms to really move it effectively.”
Fortunately, you can get similar results by observing the mounts during vessel operation. “One running test I always do is a backdown test,” says Diggs. “This will torque the engine in the opposite direction from forward, where most engines run at speed. When you put the vessel in reverse and rev the engine to 1200 to 1500 rpm, the mounts should hold the engine tight with minimal movement.”
Chris says that one of the most difficult things to spot during a visual inspection is a broken engine mount stud: “Conducting a backdown test and watching engine movement will help spot them. This is the best way to stress test engine mounts and fully inspect their condition.”