I have a Tayana 37 with a Perkins 4-108. How do I get rid of that terrible noise and vibration between the transmission and the prop?
Tom Wood responds:
Drive train vibrations are one of the most difficult of all problems to diagnose and repair. And it can be cheap or very, very expensive. Basically, the problem can stem from the engine, transmission, output flange, shaft, cutless bearing, strut, or prop—or a combination of any of these. Here's how I have attacked it in the past:
1. Run the boat at full rpm in open, smooth water. Your 4-108 was rated (I believe, but you must verify) to run at 3,200 rpm when wide open. If you are getting more than 100 rpm over or under Perkin's specification, you have the wrong prop. Prop cavitation from too large a prop diameter or too much pitch will cause all kinds of strange noises and vibrations, and also makes the engine work too hard causing other problems in time.
2. Disconnect the transmission output flange from the shaft coupling. If the two halves of the coupling "spring" apart and do not naturally line back up, the engine alignment is off by a considerable amount. If it has been misaligned for some time, damage may have been done to the shaft, cutless bearing, or strut (if the boat has one). Certainly, re-align the engine properly, but inspect the shaft, bearing, and strut as soon as possible, preferably out of the water.
3. If the alignment is good, rotate the shaft by hand while it is still disconnected and note the difficulty or any resistance—it should be easy to turn. If there is grinding, lopsided hard spots, or other anomalies, you may have a damaged shaft, a spun cutless bearing, or a loose or bent strut. Any of these require hauling the boat for repair. A mis-machined coupling can also act like a bent shaft and can be impossible to detect without taking the shaft and both coupling halves to a machine shop that can check the "run out" on all the parts to within thousandths of an inch.
4. If all that seems in good order start the engine while the shaft is still de-coupled, and run it at slow speed in both forward and reverse. If the noise and vibration are present with the shaft disconnected, you may have serious damage to the transmission. This requires expert attention and repair.
5. If all still checks out OK get a diver to pull the prop. If it has obviously hit something, you have your answer, but just looking at it isn't enough. Take it to a good prop shop, have it cleaned and checked for proper pitch, blade timing, and balance.
6. Prop shaft overhang is a little discussed and much abused standard. The cutless bearing is designed to be as close to the forward face of the prop as possible to support the prop's weight. The rule of thumb is that the distance from the aft end of the cutless bearing to the forward face of the prop hub should be equal to one shaft diameter. I have seen one-inch shafts, however, with as much as five inches of shaft overhang, and this allows the shaft to "whip" and bend, causing accelerated wear on both the shaft and cutless bearing.
7. Another standard that is commonly breached is prop diameter to aperture ratio. The prop must have enough clear water around its blade tips to prevent water starvation and cavitation. Cavitation can sound and feel like you've got multiple strings of tin cans tied to the shaft instead of a prop. The distance from the tip of the prop blade in its closest approach to the hull or rudder should be a minimum of 20 percent of the diameter of the prop. Thus a 20-inch diameter prop should clear both the top and bottom of its aperture by at least 4 inches.
Take it one step at a time. If you don't find just one big problem, you may find many little ones. We once met a boat in the Bahamas that had snagged a fish trap. It dinged the prop a little, but it also ever so slightly bent the shaft. This, in turn caused a minor engine misalignment and began to wear the cutless bearing unevenly. After a few weeks of additional cruising, the shaft became scored from the wear in the bearing and the whole system nearly seized up. Note that most powerboats carry spare shafts and props.