Our Next Big Project

Marathon, Florida
24 42.40 081 N 05.68 W


Regular readers know of our persistent problem with vibrations.   We've had the problem since buying the boat nearly 7 years ago.  We had it when the surveyor inspected the boat.  We had it both before and after installing a new engine.   We thought it was normal and something to live with until it chewed up two sets of motor mounts.  We posted the question to this blog and got back a dizzying array of helpful comments.   Here is a list of things we tried or considered.

1. We realigned the engine with the propeller shaft (5 times!)
2. We replaced the motor mounts (2 times)
3. We sent the propeller shaft to a machine stop to check straightness.
4. We installed a new flexible shaft coupler.
5. We put in a new cutlass bearing.
6. We removed, aligned and re-bedded the shaft log. (We don't have a shaft tube, but rather a bronze shaft log.)
7. We hired diesel mechanics (4 times)
8. We considered the thrust bearing.
9. We considered changing to a stiffer or less stiff transmission damper plate.
10. We considered insufficient clearance between the propeller blades and the hull.
11. We considered the propeller pitch. (Too much pitch runs the engine at too low RPM.  It would be like driving a car up hill in 5th gear.  The engine lugs in too high a gear.)
12. We considered getting bigger tougher motor mounts.
13. We considered modifying the engine pan that the motor mounts bolt to to make it stiffer and more stiffly bonded to the hull.  That would be a fiberglass modification.

None of the above changed anything and none of the things only considered seemed likely to lead to a cure according to my expert advisers.


Missing from the above list is having the propeller balanced.  We have a Maxprop.  It is a so-called feathering prop.  The blades reverse the engine is in reverse and the blades feather when we sail without power.  The prop has many moving parts.   We sent it to a prop shop as a box full of loose parts, but there wasn't much they could do with that.  


We asked the Beta engine manufacturer for advice.  He had us run a test with the engine and shaft decoupled.   The result was no vibrations.  The test seemed ridiculous to me because we reported that there were no vibrations with the engine in neutral.  Their conclusion: nothing wrong with their scope of supply; align it yet again.


All that had me pulling out my hair.  I visualized a future spending thousands of dollars every year trying and retrying various solutions and testing numerous theories; with no results.


Finally, almost by accident, I stumbled across one diagnostic test never tried before.  I ran it full speed in reverse.  WOW, no vibrations!   Nobody thought before to try that.  What difference should it make in forward or reverse?   Besides.  backing up a sailboat with tiller steering at 6 knots is difficult and dangerous.  It is like backing up in your car at 40 mph. In normal circumstances, one would never attempt it.  None of the four mechanics, nor Beta, nor our Westsail expert Bud Taplin, thought to run a test in reverse.


Note that all of the possible causes listed above would produce the same vibrations in forward or reverse.  This simple diagnostic test eliminated all of them.


With that additional bit of diagnostic information, it took only a few minutes and one phone call to home in on what I think is the real explanation.   Some of the internal gear teeth in the Maxprop propeller are worn out.  99% of the time when the engine is in forward, all the load wears on the same teeth.  Different teeth are engaged when in reverse or when feathered.  Of course there is no significant load anyhow in reverse or feathered. So only the forward teeth wore.  Worn teeth allow play in the blade position and they shake.


Now we've ordered a new fixed propeller. It has no moving parts.  It is just a solid block of passive bronze.  As soon as it comes, we'll go up on the hard and install it.  No more complicated feathering prop for us.  I've decided that owning and maintaining such a device is above my pay grade.  Let the KISS principle rein.  


Wish us luck.