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"NoVibes Are Good Vibes"
How to eliminate excessive engine and drivetrain vibration.
By Capt. Alan Ross Hugenot


August 2002 Sea

The Beach Boys celebrated "Good Vibrations" in song. But for power boaters, there's no such thing as "good" vibration.

Not only is excessive vibration irritating (nobody likes the experience of uncontrollable chattering teeth), but it also indicates problems with your propulsion system or running gear. Getting rid of this unwanted shaking is so important to boaters, many companies have sprung up that specialize in anti-vibration systems - and sell nothing else.

The first thiing you need to do to eliminatevibration is to determine exactly what is causing it. We can do all kinds of things inside the hull to quiet down an engine and shaft. We can add resilient mountings, constant velocity joints, flexible feet and couplings - but if the "knock" is coming from a wrong-sized propeller, we will not get rid of those vibration problems until we go outside the boat and change the wheel.

Figuring out what is causing the vibration involves a simple process of elimination. Vibration is almost always caused by one of four things: the engine, the shaft, the shaft supports or the propeller. In my experience, I have found that most boats' vibration problems stem from the propeller - but it is easier to look at the other areas first.

Shake and Shimmy No More

Getting to the root of a vibration problem often requires some detective work. You may have checked your prop and your engines - but did you look UNDER them?

Old or worn engine mounts just might be your problem. New flexible engine mounts (available from Vetus, Aquadrive, R & D and other manufacturers) are designed to be rigid enough to absorb forward and reverse propeller thrust, yet are soft enough to isolate drivetrain and engine vibration from the hull.

Next, take a look past the propeller, at the prop shaft. Flexible shaft couplings, such as those made by R & D, fit between the transmission output flange and the existing shaft coupling, helping to eliminate vibration. A flexible coupling disc creates a barrier that absorbs vibration and compensates for temporary misalignment of the shaft and engine.

Constant velocity (CV) joint axles, such as those used in the Aquadrive anti-vibration system, can also be used to dampen engine vibration. They also eliminate the need for absolutely precise alignment. Aquadrive uses a thrust unit on the load-bearing hull section to reduce stress on transmission and engine mounts, so that the propeller's action pushes the boat instead of the engine.

Manufacturers report that these tactics can produce some remarkable results. N.A. D'Arcy Co., which sells the Aquadrive anti-vibration system, claims that most installations result in a 50 percent or greater reduction in cabin or cockpit noise and vibration.

Engine Vibration

Is this where your problem is? Here's how to find out. First, run a dockside test. With securely tied spring lines, after the engine is warmed up under a partial load, reduce the speed and shift into neutral. Then, run the engine up to maximun rpm in neutral and check for vibnition.

By now, you will have found out whether your engine is misfiring. If it is misfiring, that means it needs a tune-up (it it is a gasoline engine) or it needs to have its injectors cleaned (if it is a diesel).

The motor mounts are the next places to look. Just one loose motor mount can cause engine vibration, so you must make sure they are all tightened. Unless they are loose, motor mounts are not usually where the vibration problem lies.

If the engine vibrated when it was under load and did not vibrate when it was in neutral, the vibration problem is not coming from the engine. There are only three other things that could be causing it: engine misalignment, the propeller shaft or the propeller.

Engine and Shaft Alignment

To test your engine alignment, run the engine at idle and shift it in and out of gear. While doing so, observe the rotating shaft coupling.

What you should see is smooth turning, with no "wobble." If it wobbles when you place it in gear with any kind of "out-of-round" movement, you need professional engine and shaft alignment.

The alignment is usually to within .003 inch. If your boat has "settled" over the years, you may need flexible couplings, a constant velocity joint or even a thrust bearing installation, to overcome your boat's new shape.

For a further check, loosen the coupling bolts. If the shaft coupling flange does not appear parallel to the transmission coupling flange, then it is definitely out of alignment.

Bent Shafts And Bad Bearings

With the engine off, turn the shaft by hand. If you see more than a 1/4 inch wobble, the shaft is bent.

With modern short shaft installations, most boats no longer have thrust bearings. If you DO have thrust bearings, here's how to find out if they're still good: If the shaft has more than 1/16 inch of "play" movement, it is time for a new bearing.

In my experience, if the bearing is worn, it is usually because the shaft is bent. I usually measure for a bent shaft with a flashlight. When the shaft is turning under load, I watch to see how much wobble there is in it.

If the bearing is absorbing more than a 1/16 inch movement, the shaft is bent - and the bearing probably needs to be replaced, as well. If a bent shaft is indicated, remove the coupling bolts and hack off the coupling from the transmission -just enough to insert a feeler gauge. Rotate the shaft and measure the clearance with the feeler gauge at every 90 degrees.

Be sure to recheck the original location, once you get all the way around, to verify that you did not back the shaft out farther during the rotation. If you found any of the test locations to be tighter or looser, you have a bent shaft.

Cutlass bearings can be checked while the boat is in or out of the water. Simply push up on the shaft and see how much play it has. If it moves more than 1/16 inch, your cutlass bearing will need to be replaced.

Worn cutlass bearings result in vibration, which can cause damage to the shaft itself, the shaft coupling and the keyway.

Propeller Vibration

The prop is where 80 percent of all vibration problems occur. Vibration at the prop can be caused by one of several different problems. The propeller can be out of balance, have a worn hub, have inadequate blade area or have insufficient tip clearance.

Out-of-balanec propellers and props with worn hubs wobble on the shaft as they turn. The easiest way to observe this is under water - with a dive mask and fins - while the boat is in relatively clear water. Be careful to stay away from that propeller while it is turning.

Another good test is to remove the propeller, then run the engine up to its full rpm. If the vibration disappears, the problem is definitely with your propeller.

Take your poor-performing prop to a professional propeller shop, where it can he balanced and the huh can he rebuilt to tlie original specs. If you have a feathering propeller, the only way to inspect it is to remove it and take it to a propeller shop.

Cavitation is another common cause of vibration. The most overlooked area of propeller design is adequate blade area. If there is too little area, the propeller blades become overloaded - causing cavitation, blade erosion and vibration.

Propeller tip noise is also common - as well as shaft noise from bearings, and poor alignment.

Tip noise comes mostly from having insufficient blade tip clearance. A propeller actually works just like a sail, creating lift on the leading edge, which "pulls" the propeller forward. The blade tip has a tip vortice just like any airfoil or hydrofoil - and this excess energy is swept away by the passing water. However, if the tip is too close to the hull, the energy will "slap" the hull and make a real "knock."

The distance from the tip of the blade to the hull should be 0.15 to 0.2 times the blade diameter. If it is less than that, you will almost always experience knock and vibration. Have your teeth stopped chattering now? Good.

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