Bad Vibrations

By Tom Neale

Vibrations can come from OK causes, but usually they come from problems ranging from small to huge to potentially catastrophic. Here are a few.

Those of you who remember the song "Good Vibrations" by the Beach Boys may have some warm and fuzzies. But with boats it's different. Most vibrations are bad ... at least the ones that are new or that don't have good explanations. We've had a lot of Ask the Experts questions about vibrations. Almost all of them have been from boaters who felt that something wasn't right and who have tried to figure it out and fix it. This is a very good thing. Often these boaters saved a lot of money and hassle by simply paying attention to what the boat was telling them.

Photo of a worn cutlass bearing    Worn cutlass bearing.

A cutlass bearing resides in the strut. The shaft passes through it. You may have a second one residing in the outside of the shaft log where the shaft passes through the hull. Their purpose is to prevent the shaft from wearing and abrading on damaging material (such as fiberglass or bronze) and to help it to turn true and with support. But this bearing is sacrificial. Usually it is a rubber or synthetic material that's encased in a bronze housing that's inserted into the strut. The shaft turns in this and the "rubber" is, of course, softer than the shaft. Grooves run up and down the "rubber" to allow water to wash through to help with lubrication and cooling. But these wear with time and then the shaft has play within the cutlass bearing. It can cause excessive leaking at the packing gland, damage to the transmission and other things. Often you first notice this by a vibration. It's fairly easy to confirm the problem. When the boat's hauled you ask a knowledgeable yard worker to grip the shaft and try to move it in the bearing. An experienced diver can also check this. If the yard has the right tools (and ask if it has a cutlass bearing puller) this isn't a big deal to replace. If you don't fix this, it can be a very big deal.

Another vibration can come from a bent or worn shaft. If you've experienced a grounding (and who hasn't) there may be some impact to your shaft and it may be out of true. Just slightly out of true is all it takes to do future damage, and you really can't see this with your naked eye. A really good shop may be able to straighten a shaft that's bent, but it must be an exceptionally good shop and, considering the cost of straightening it, many prefer getting a new shaft. I do this occasionally as a part of regular maintenance. This is even more sensible when one considers that shafts wear. They wear a little in the cutlass bearing and sometimes much more so within the stuffing gland. So with a bend, with wear and with the ever present issue of metal fatigue after so many years, a new shaft may be the best way to go.

Photo of a new cutlass bearing    New cutlass bearing.

That shaft needs a fair run from that engine to the prop, and one of the things that will cause vibration is a misalignment of the engine. If you take the couplings loose, you may be able to see that they don't line up. But if you can "see" this you really have a problem because usually the tolerances are so small that it takes a feeler gauge to tell. Typically, once you back off the shaft with its coupling, the weight of the propeller at the other end of the shaft may cause the end in the boat to come up a little. This may make you think that you're much more out of line than you are. An alignment is something that you definitely need when you hit something and are experiencing some uneven running or a new vibration. It's also something that ought to be done as routine maintenance every so often. It's not unusual, for example, to develop an alignment issue when the boat has been hauled. The stresses on a larger boat out of the water and supported by jacks are very different from those while it's in the water which is its natural element. Some people have the alignment checked out whenever they're re-launched. Others are simply careful to notice anything new after a re-launching. But beware that this may be a source of new vibrations.

Another type of vibration can come from a distorted prop. The symptoms of this can range from bad thumping to "just something different" depending on how much of a problem your prop has and how you're running your boat. Anytime we hit something in the water (even a crab pot float) we need to be sensitive to this issue. "Swinging your blades" every so many years, depending on use, is also often helpful. The propeller(s) is removed and sent to a prop shop to get the blades balanced and fine-tuned. It can make a great difference.

Now let's talk about vibration coming from the engine. Actually, let's not. I say this because usually that's a far more complicated issue. Causes could range from bad fuel/air mixture, bad timing, even something really bad like bearings. It seldom comes from something external and easy to fix. But any engine is going to vibrate some as a part of running. Some will vibrate more than others. I had a series of Perkins diesels that vibrated with great enthusiasm, quite naturally. Now I have a Yanmar diesel that runs very smoothly. I had a 2 stroke Yamaha that vibrated all the time. Now my 4 stroke Yamaha, like the new diesel, is so smooth I hardly know it's running at low RPM. The point here is that you need to be familiar with your engine, including laying your hands on it (in safe places and in safe circumstances ... .not on the header tank for example) and know about its normal operating conditions including vibration, sound and other things. When this changes, look for a reason and if you can't find one, get a good mechanic.

Boats talk to us in many different ways, including romantically. But boat talk is seldom "sweet nothings." When they talk, we need to listen. 

Tom Neale, a technical and lifestyle writer and liveaboard cruiser. Read more of Tom Neale's articles here.

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— Published: September 2016


Tom's Tips About Good and Bad Vibrations

  • There are certain places on your boat that may be better than others for feeling vibrations.
  • Sometimes these are dangerous to be in, so be cautious and use good sense.
  • If you have an inboard engine, you might lift up the floor boards that cover the strut. If you see the strut bolts moving more than usual, there may be a problem with shaft or prop.
  • If you feel the struts vibrating you may have a problem.
  • If you feel thumping or hull movement in that vicinity there may be a problem with shaft or prop.
  • You may be able to lay the blade of a light flat slot screw driver against a shaft. If it jumps about you may have a problem. This is one example where you must be very careful to not get any part of your body, clothing etc. caught in any moving parts.
  • All of these tips contemplate that you establish what the norm is. You must do this with constant attention or perhaps by asking others. For example, any shaft will vibrate a small amount.

See www.tomneale.com

 

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