Join Date: Sep 2010
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I have comments I want to make regarding motor mounts.
Motor mounts fall into the category of vibration isolators. Isolators are designed to dampen (attenuate) vibrations of particular frequencies or range of frequencies although the higher frequencies are easier to dampen. Low frequencies such as found on single cylinder engines are a real dog to dampen. Further, the isolators are designed for a weight range and also a thrust limit. It would really be nice if boat engines were protected from the thrust load (horizontal force vectors) with a thrust bearing on the prop shaft. I have not seen any on the size boats I have been on although I would believe larger boats use them.
Isolators also have their own resonant frequencies. If there is an engine caused vibration frequency that happens to also be an isolator's resonant frequency, that frequency will not be attenuated, it can in fact be made much worse.
One other comment that I want to make regards to so called "fit all" mount with an adjustable threaded bolt. The horizontal forces on the mount will use the mounting height as a lever. One can minimize this problem by trying to raise the height of the entire isolator (engine mount) and not just use the mounts adjustment. And of course we all understand that the prop is going to provide that horizontal force vector.
OK--- I understand that most folks are just going to order replacement mounts and install them or pay to have them installed. But what you get is what you get and just maybe there is an isolator better designed for the job. The real problem is finding a manufacturer who will share his product's data sheets with the load and frequency characteristic curves.
Years ago I purchased a 30 Hunter (1979) that used a single cylinder horizontal mounted piston. What a dog! The life lines on my boat vibrated at their natural frequencies when under engine power. They vibrated as much as 6or more inches!!! Everything in that boat seemed to move. I redesigned the engine mounting system with new mounts which I matched as best I could to frequency attenuation, loading weight and horizontal thrust. I was able to get rid of most of teh vibrations that calmed the life lines and door rattling. A few years later, I replaced the engine itself with a Kubota 2 cylinder.