WHAT causes vibration, and what's excessive? There are ways to measure and categorize unpleasant vibrations. Essentially, every vibration has a frequency, and each frequency relates to the speed of a part of the rotating equipment- shafts, props, engines... A proper vibration analysis by a specialist with the right equipment can identify and categorize what people subjectively perceive as excessive vibration. Any small structure housing a powerful engine and moving at speed over the surface of the water will vibrate- but not uncomfortably, and not to the point where damage is done, or where the shaft is seen to be whipping around.

    HOW do you correct a vibration problem? If it's at all possible, get a vibration analysis. The analysis will identify the problem, and provide a benchmark. It's not unusual to see a vibration problem fixed because of areas other than the drive train, such as underwater exhausts, resonant structures, engine repairs, fuel system repairs, when the natural assumption was that alignment, shafts, or props were involved.

    LET'S GO with a simpler scenario- a boat that has run perfectly until a grounding. It's a medium size yacht, twin-screw, diesel, with rubber cutless bearings mounted in forward and aft struts, close-coupled marine gears, and flexible and adjustable engine mounts. The boat is hauled, and the yard manager says the mechanics had a hard time pulling the port shaft, the side which showed more extensive prop damage.

    IN A SITUATION like this, where the boat had a history of running perfectly, it is correct to proceed on the premise that fixing the damage will restore the boat's performance to its original state. The important thing is to identify all the components that need repair. The props are visibly damaged, so they go to the prop shop.

    NEXT, the yard should dial indicate the shafts to check and report on runout. The shafts should be checked at the small and big ends of the prop tapers, approximately midway between the struts, and close to the hull entry. Shafts showing excessive runout should be drawn and sent to the machine shop, with their couplings. As a side note to marine shaft repair: it is important that the shafts and couplings be treated as an assembly. After the shafts are straightened, or with new shafts, the coupling must be lapped and fit to the shaft. The coupling should be faced on the shaft, not just chucked up in the lathe and faced. Facing the coupling on the shaft ensures that the face of the coupling is truly perfectly perpendicular and coaxial with the shaft.

    NEXT, inspect the cutless bearings. The yard said they had trouble pulling the port shaft, and the question is “are the bearings distorted, or has a strut been tweaked?” Measure the bearings, or use a sizing plug. If there is any sign of wear or distortion, the bearings should be renewed. If the strut barrel has been damaged enough to distort the bearing, the strut may have to be repaired or replaced.

    NOW, with the shafts out and new bearings installed, it is time to check alignment of the bearings and engines. While there are different ways to do this, we'll stick with the boat we're using as an example, and we'll do an optical alignment check.

    OPTICAL ALIGNMENT is an industrial standard for aligning rotating equipment. An optical alignment telescope is a durable and accurate piece of equipment, and is used to align mills, rollers, shafts, and other equipment. It is used extensively in ship and aircraft building, but still takes people by surprise in a yacht repair yard. Why? The equipment is expensive, and the benefits of precision alignment equipment do not really show up until you get into shaft sizes of 2 ½” and greater. Rubber cutless bearings and flexible engine mounts are tolerant of slight misalignment, and target wires and feeler gauges are generally accurate on the smaller shafts.

    BUT, in the case we're dealing with, an optical alignment check can quickly provide the yard with a total picture of the drive train, including strut and bearing positions, and engine alignment. Optical alignment sets up faster than a target wire, is more accurate, and can do something a target wire can't: show the angular position of the marine gear output coupling relative to the shaft line.

    READ ON to Page 2.

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