It has been said that feet are the most important part of a runner’s body. Our feet transfer the energy of our muscles into the ground. Weak feet, poor alignment, and wear and tear can do a runner in.
Your boat’s engine runs on a similar principle: The power the engine creates must be transferred to the boat. In most cases, this transfer takes place through the engine mounts. These mounts must be strong enough to withstand the weight of the engine and transmission, as well as the shear loads created as the propeller pushes against the shaft and engine.
On the other hand, the mounts must be soft enough to minimize the transfer of sound and vibration to the hull and accommodations areas. The softer the rubber (lower durometer reading), the lower the transmission of sound and vibration. The harder the rubber (higher durometer reading), the better the resistance to side loading. It’s a balancing act.
THRUST OR NO THRUST
Selecting the proper engine mount begins with a discussion about how the drivetrain transfers thrust to the hull. On most cruising boats, the shaft pushes against the transmission; the transmission pushes against the engine; and the engine pushes against the engine beds (or stringers). In this configuration, the mounts must be firm enough to withstand the loads as the engine is pushed forward on the beds while the mounts hold it in place.
An alternative method transfers the thrust to the hull without pushing against the engine. Several systems, including Seatorque, Aquadrive and Evolution, provide varying ways to decouple the engine from the thrust of the shaft by transferring the load to the hull prior to the transmission. These systems offer several advantages, but here we will focus on one in particular: the lack of thrust absorbed by the engine.
The absence of thrust means that the engine mounts no longer must withstand the side loading. Without the shear loads, the mounts can be made with softer rubber, reducing the transmission of sound and vibration into the boat’s structure.
MATCHING THE MOUNT
Three factors must be considered when choosing a mount: thrust or no thrust, engine weight, and horsepower. In addition, some engine manufacturers specify a different mount for the port side of the engine versus the starboard, or for forward or aft mounts. To simplify production, some boatbuilders and engine manufacturers opt for an engine mount design that covers a range of engine models. In those cases, you might find that the mounts that came with the boat are not optimized for your engine.
The engine manufacturer or the engine mount producer can help you identify the correct mount for your application.Whatever you do, don’t guess. Choosing the wrong mount can result in excessive noise and vibration, and premature failure.
Finally, best practice calls for replacing all mounts at the same time.
The mounts should be installed so that they are centered on the engine stringers and parallel to the centerline. If the engine, when properly aligned, is not parallel to the engine stringers, then the mounts must be shimmed to create the proper angle. They also should be perpendicular to the engine brackets. In both cases, the mounts should not vary from the guidelines by more than a few degrees.
If any of the conditions described above have not been met, then the mounts will need shims. The shims should be made from noncompressible materials such as steel or aluminum. Wood and synthetic material (such as Starboard) must be avoided.
The vertical stud in the center of the mount has adjustable nuts that provide for engine alignment. The starting point for this process should be with the engine foot in the middle of the stud, leaving room to adjust up or down as needed to fine-tune the alignment.
When inspecting installations, if all four adjustments have been set to the maximum amount of travel, one has to wonder: Did it work out that the engine reached proper alignment just when all four mounts ran out of adjustment, or did the installer reach the limit of adjustment and stop before properly aligning the engine? The latter of these two possibilities seems far more likely.
The bolts (known as foundation bolts) that hold the mounts to the stringers must be grade 8.8 carbon steel and should be threaded into a metal plate laminated into the engine stringers. Lag screws going into wood will not hold up to the side loads and vibration. Each bolt must have a washer—and some manufacturers specify a washer thickness for a given mount size. Torque settings are available for these fasteners.
The mounts will settle slightly after installation. When the final in-water engine alignment has been completed, place a witness mark on each stud and nut so that any loosening will be easily detectable.
Mounts require almost no maintenance, provided you protect them from water, fuel and oil. Minor leaks, such as from a weeping water pump, will result in rusted threads and difficult, if not impossible, adjustments. Fuel or oil leaks will attack the rubber components, resulting in premature failure of the mounts. For cleanup, stick with natural-based soaps.
Inspect the mounts for any signs of skewing as described above under installation. The stud should be perpendicular to the mount base. Eventually, the rubber will compress, requiring replacement. On larger mounts, some manufacturers provide information about how to check for excessive compression.
Service life varies based on the load factors and specifications of the mounts. Some brands and models have an expected life span of 10 to 12 years, while others call for replacement every five to six years. Mounts kept free of exposure to liquids, and installed at the proper angles, will achieve the longest service life.
Steve Zimmerman is the president of Zimmerman Marine, which operates five boatyards in Maryland, Virginia, North Carolina and South Carolina. Zimmerman has been building and repairing boats for more than four decades.