The Unsung Hero

Posted: June 1, 2012

The engine mounts provide the glue that connects your boat’s ­powerplant and hull.

By: Deane Hislop

The unsung hero in your engine compartment is the motor mount. Think about it: Every foot-pound of thrust created by the propeller must be transferred to the hull through the motor mounts. The prop pushes the engine, and the engine drags the boat along with it, whether it’s a 40-foot cruiser or a 120-foot yacht, all thanks to these fasteners.

There are usually four motor mounts for each engine, and each motor mount’s saving grace is its simplicity. It typically has only two moving parts: the adjustment nut and the locknut, and these enable it to fine-tune engine alignment. The typical motor mount consists of a foot — cast or pressed aluminum or steel — affixed to the vessel’s hull or stringers. Within this base or foot is a rubber vibration-absorbing pad or ring. A threaded stud rests on top of or within the rubber, offering it a degree of isolation from the metal base and the stringer. Because the motor mount must keep the engine in place even in the roughest sea conditions, the stud is usually permanently affixed to the mount’s base independent of the shock-absorbing rubber insert.

Inspect the visible portions of the rubber shock-absorbing material on your mounts. If it shows signs of cracking or separating from the metal, it’s time — probably well past time — for its replacement.

In addition to keeping the engine firmly attached to the vessel, the motor mounts adjust to facilitate engine alignment. Rotating the adjustment nut on the mount stud enables a mechanic or savvy do-it-yourselfer to move any one of the engine’s four corners up or down to align the shaft coupling with the transmission output coupling. Once the alignment is set, it rarely has to be adjusted, but herein lies the problem: With lack of use, any mechanical component can become cranky. So, when the day comes to use them, you may find that the mounts are frozen solid with years of accumulated corrosion.

Inspect your mounts. If they’re not new, make sure they move freely. Since you’re there, you might as well check the alignment, too, and don’t forget to secure the mounts in place with the locknuts. When you’re done, spray the mounts liberally with a non-petroleum-based corrosion inhibitor, such as CRC’s 6-56, to keep rust at bay. Petroleum-based lubricants and rust preventatives may attack the mount’s rubber shock absorber.

Finally, check the bolts or screws and washers that attach the mount’s base to the vessel’s stringers. These fasteners are notorious for loosening, deforming, rusting or stripping their threads.

Also, be on the lookout for the possibility of wet, rotting or mushy stringer-core material, into which lag-type motor-mount bolts are often screwed. The bond between the engine and boat is only as solid as the material that makes up these key structural members.