A recent question on engine mounts prompts me to repeat a post that I made several years ago. Engine mount have several vital functions in the driveline in addition to supporting the engine. They must be soft enough in the vertical plane to absorb vibration and rigid enough in the horizontal plane to handle the propeller thrust. They must keep the engine in place when the boat is on its beam ends, even if the mount fails. They must permit adjustment of engine position during alignment, yet not permit the engine to move out of place as a function of normal operation. They are buried in the bilge but must not deteriorate because of oil or heat. Despite their importance, they are usually the last items a boater considers in an engine installation. In short, engine mounts don't get no respect. --------------- Making your propulsion engine purr like a pussycat is a lot more complicated than just slapping in a few flexible mounts and a flex shaft coupling. In fact, by selecting the wrong mounts, you can actually make the vibration far worse than it would have been had you just bolted the engine to the bearers. Think of an engine as a block of iron suspended on a Slinky like spring. If you pull the engine down slightly and let it rebound, it will bounce up and down slowly. The rate that it moves up and down is called the NATURAL FREQUENCY of the system and is dependent on the engine weight and the strength or restoring force of the spring. The heavier the engine and the weaker the spring, the slower the engine moves and the lower the natural frequency. If the engine is light and the spring is stiff, the natural frequency is high. Now, mount the engine in a boat on flexible mounts. These mounts can be considered as small springs that deflect or compress slightly by the engine weight. If the engine is heavy and/or the mounts are soft, the engine will compress the mounts a good deal and the natural frequency of the system will be low. With stiff mounts and/or a light engine, the frequency will be higher. Bolting the engine directly to the bearers is the equivalent of using very stiff mounts and the natural frequency will be high. The natural frequency of an engine mounted on flex mounts can be estimated by measuring the deflection of the mounts and working through the following equation: Fn = 3.13 x sq. rt. (1/ Ds) where: Fn = natural frequency in cycles/second Ds = static deflection of engine mounts in inches Let's put numbers to this. Assume that we have a Perkins 4-108 engine mounted on four flex mounts. The deflection of the mounts is .05 inch. The natural frequency of the system would be 14 cps or roughly equivalent to 840 oscillations per minute. If the mounts were softer and deflected more, Fn would be lower. Less deflection and Fn would be higher. Now engines donbt sit quietly in a boat. They are forced into vibration by the explosions within the cylinders which produce the power. Pistons move up and down, the crank rotates, the valves open and close. It is the imbalance of the moving masses within the engine which produces this forced vibration. The number of oscillations per second is called the FORCING FREQUENCY. This forcing frequency (Fd) is largely a function of engine design. A one cylinder engine has all its masses unbalanced and exhibits a strong vibration at every revol ution. A two cylinder engine can be configured to balance most large motions of the pistons (i.e. one moves down while the other moves up), but since the pistons are displaced slightly from each other, secondary rocking forces appear. A four cylinder engine such as the Perkins 4-108 can balance these forces to a great extent but not completely. The cylinder firing sequence also adds its bit to the vibration. It takes at least a six cylinder engine to fully balance all the internal forces. There is no simple way of calculating the forcing frequency without knowing a good bit about the engine design. A four cylinder engine usually has most of its vibration at twice the crankshaft rpm and directed in a vertical direction. A two cylinder engine has a vertical vibration component at twice the rpm and a rocking component equal to the rpm. A one cylinder engine vibrates vertically and horizontally at the crankshaft rpm. If the forcing frequency is higher than the natural frequency of the engine on its mounts then the vibration will not be fully transferred to the boatbs structure and vibration isolation will occur. That, of course, is what we want. Assume that our Perkins is running at 1000 rpm. Most, but not all, of the forcing vibration will be at 2000 oscillations per minute. This is more than twice as high as the calculated natural frequency of 840 oscillations per minute and vibration isolation occurs. In this case the isolation is about 80% and only 20% of the engine vibration is transmitted to the structure as compared with an engine bolted firmly in place. Theoretically isolation begins when Fd = 1.4 x Fn. For a system with a natural frequency (Fn) of 840 oscillations per minute, isolation would start at a forcing frequency (Fd) of 1187 oscillations per minute. If the forcing frequency is closer to the natural frequency, vibration AMPLIFICATION occurs and there will be more vibration transmitted to the boat than with a solidly bolted engine. When the forcing frequency and the natural frequency coincide (Fd = Fn), resonance occurs and the engine will literally shake itself out of the boat. If boat engines ran at only one speed, there would be little problem. You would simply run the engine at a speed where the forcing frequency was at twice as high as the natural frequency and have at least 90% isolation at all times. Unfortunately, engines start from rest, idle, and go slowly. At those times, the forcing frequency may well drive the system into the amplification mode and excess vibration will be transmitted to the hull. There are a couple of ways to minimize the problem. Part 2 will discuss the selection of engine mounts.