Tuesday, June 14, 2011

Engine Mount Epilogue

This is a follow-up post to Changing Engine Mounts.

It's been over a month since I first ran Donnybrook with new engine mounts under her Universal M25XP and, after some learning and alignment work on my part, the job is done. 

The engine was extremely quiet and smooth on the 6+ hour motor back to Belmont.  The next day I started the engine to show a friend what an improvement the mounts were, and it was--until I put it into gear.  There was some rumble and an occasional knock at low RPM.  It cleared up at higher RPMs.  How could this be so?  It was so smooth the day before!

The next week we went for a short sail and it still wasn't smooth at low RPMs.  Higher RPMs (above about 2000) were fine.  I adjusted the alignment at the coupler but still no luck smoothing the engine out.  I read everything I could find on engine alignment and drive system noise.

Finally, I found the problem:  The drive shaft wasn't centered in the shaft log.  On the hard I visually inspected the shaft at the cutlass bearing and where it enters the hull from the outside and it looked centered.  When I moved the shaft with the coupler disconnected, it clearly wasn't centered but was sitting low.  I raised the engine 1/8" to 1/4" and the engine is now smooth at low RPMs with some vibration over 2500 RPMs.

A few days later I went out again.  It was "okay" but not "kitten smooth" like I wanted.  I talked to a friend and trusted engine mechanic who talked me through alignment.  His advice:  Patience.  Start with the engine a little low to account for the thrust pushing the engine forward.  Take into account the torque of the engine.  Adjust it a little at a time and watch to see what the engine is doing.

It took half a dozen trips up and down the companionway to get the alignment right.  My steps were:
  1. Run the engine in gear and feel the vibration.
  2. Go below.  Loosen the coupler.  Feel the gap between the two coupler halves. 
  3. Adjust the engine up or down until the gap was minimal (use a feeler gauge or finger nail).
  4. Tighten the coupler halves together.
  5. Start the engine and run in gear.  Was the vibration better or worse?
  6. If worse, repeat 2-4 but move it down or up (opposite to what was done in #3).
  7. Repeat until engine runs smooth.
Thankfully my problem was vertical position.  The horizontal position was okay.  Otherwise, you should adjust horizontal (side to side) position first.

I also had a friend run the engine in gear while I observed engine movement.  After almost two hours I'm finally satisfied with the alignment.  She idles smooth at 800 RPMs (based on my tachometer).  There's minor vibration to 1500 RPMs, and smooth to 3000 RPMs (full throttle).  My friend's observation is that Donnybrook is smoother than his boat, which had a yard replace her engine mounts and align this spring.

During my research I found this excellent article called Drive System Alignment by David Pascoe at http://www.yachtsurvey.com/.  It helped me to understand what is happening with the drive train.  An excerpt from the article:

Lessons Learned
The Nature of Inboard Drive Systems It is a common belief that engines and shaft couplings have to be aligned to tolerances of a few thousandths. There is some misunderstanding about this. Yes, the shaft coupling to transmission coupling needs to fit within several thousandths, but what were talking here is the coupling fit, not the shaft alignment. This is an important distinction that is often confused. I'll explain why.
 A conventional shafting system is essentially a free-floating, semi self-aligning system. How's that? Well, because the engine is mounted on rubber mounts, and the shaft is mounted in rubber bearings. Of course rubber being soft, that means that both the engine and shaft can and do move. See my point here? Since neither the shaft nor the engine is held rigidly in place, there's not much point in attempting to perfectly align the shaft with the engine, is there? No, because if everything is approximately aligned, the shaft will tend to self-center as a result of centrifugal force. The fact is that conventional shafting systems will tolerate a great deal of intolerance because of these factors.
The truth is that it is virtually impossible to correctly align engine and shaft with the methods that are commonly used. Because of the rubber mounts, the engine will not be in the same position when running as it is when stopped, when the alignment is made. Propeller thrust and engine torque will cause the engine to change position. And since the weight of the shaft sitting on rubber cutless bearings causes the rubber to compress, the shaft is not in alignment with the bearings anyway. When the boat is running and the propeller spinning, the shaft will align itself (but not if the basic alignment is out).

  1. I made sure the shaft was centered at the cutless bearing and where it entered the hull.  I didn't check it at the stuffing box.  Next time I'll remove the stuffing box hose to verify the shaft is centered.
  2. The hull really flexes!  The boat ran smooth the day the boat went into the water but not the next day.
  3. It takes two people to do the job.  One to run the engine and the other to observe what the motor and shaft are doing down below.

No comments:

Post a Comment