February 12, 2013
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During the past five years, Bill Johnson (Engineering Concepts LLC) has explored numerous causes and solutions to boat vibrations. In this preview to his IBEX 2013 seminar, Bill offers a few ideas about what type of mount might be better suited for a particular application.
Bill Johnson’s website opens with the offer to give your boat an EKG. What’s shaking? Johnson got into the marine industry from as his work in manufacturing became more and more focused. As a sailor and a powerboat power, he has a lot of sympathy for the owners and what they are dealing with in terms of vibration.
The greatest culprit turns out to be engine mounts, typically 80 percent of the time. After that, the issue depends on whether it’s a pedestal or conventional style mount. The pedestal style does not allow as fine of an adjustment, but a conventional mount properly adjusted can effective eliminate vibration completely by totally restricting engine movement.
Still, says Johnson, the pedestal mount is stronger and adds more support so it will take more thrust. A typical installation will take only four pedestal mounts instead of six conventional mounts. It comes down to installation costs. Also, on the pedestal mounts, there is no way to keep the lower adjusting nut from backing away. The conventional mount usually has two half nuts that can be torqued into position.
What happens when a nut backs off is that you can lose a prop shaft to engine alignment.
Both types of mount will work, but you do need to keep the other concerns in mind.
We asked, “How do you troubleshoot a boat for vibration?”
Johnson says first he takes off his shoes and walks thorough barefoot or in his stocking feet. Then he looks around to see what’s moving. “And then I go from there before I break out any equipment.”
That, of course, if only for the engine and the engine mounts.
Another topic in Johnson’s world is engine harmonics. “If you have three or six times the RPMs, that’s normal RPMs,” he says, “As you go up in speed, you’ll see that go up or drop. You can tell by what is happening whether or not there is a problem. “
What’s the tool this time? “In this care, I’ll place my hand on the engine. If it’s properly functioning, I can feel it. If I’ve done it properly, I can pick it up with my hand. I didn’t think it was possible, but it is. Put your hand on the mount, put your hand on the stringer. If you feel the same thing, you’ve got a bad mount. Your hand will give you a cursory check that will tell you whether or not you need more testing.”
When the vibration persists to the point where you can put a glass of water on top of the engine and see water spilling out of the top of the glass, then you have an issue. Minor ripples? You’re good to go.”
What is the main concern with engine harmonics?
The harmonics is basically an identifier. They will only tell you there is an issue; and they you know there is something that needs to be changed; for example, the mount loading. There’s no dial that you can turn to change the harmonics, but you can change other things. And then the next step is to make sure you have proper fuel flow, water cooling, and so on.
Is this all common sense? “Yes,” says Johnson, “it is. If you’re aware of it, you’re ahead of the game.”
Johnson attended TrawlerFest this years. “Good show! I talked about using vibration as a tool and using it as a plan of action. In other words, do we need to fix the prop or the engine mount? Do we need to haul the boat? It’s not always the problem.”