anchoring issues - broken motor mounts

[Barry33]

I have been plagued with some anchoring issues. I recently found that I had sheared off the bolts holding the rear motor mount flanges to the block of my new Yanmar 3YM30. The yard has suggested that I back too hard on the anchor when setting the hook...additionally, prior to re-powering, I had damaged the reversed gear on my old Volvo, twice.

I had not noticed any vibration issues (with either engine) suggesting mis-alignment.

Any thoughts? Could I really be breaking motor mounts with 29 HP??

Thanks

[Boasun]

Never heard of backing to hard and parting mounting bolts. This suggests that either or all of the Transmission, Bolts and mounting flanges were undersized for the job at hand.
You should be able to back down hard without breaking any of the above. Every time you have to.

[Valiente]

This doesn't make sense to me unless there is some kind of flaw in the mounting setup itself, as Boasun suggests. While there's plenty of vibration and torque down the shaft, there aren't typically the shearing forces that would break mounts, unless you are backing down at full RPMs with 50 knots on the side and the boat is rolling 35 degrees side to side.

But that should only cause lubrication issues...

The fact that you've busted stuff back there before does indeed suggest a misalignment, but one that would be obvious by wear in the stern tube or the cutlass bearing, as with a bent shaft. You sound like a candidate for a thrust bearing and a universal coupler, along with vibration-dampening soft mounts, which take alignment out of the equation for the most part, and, while expensive, are cheaper than new transmissions and engine hauls to redo sheared mounts.

[timebandit]

Bad bolts??

[SEMIJim]

I'm no expert, so, FWIW: Agree with Boasun and Valiente on this one. I'm hard-put to understand how going in reverse while anchored should break things in the power-train.

I don't know about reverse, but I've heard of people doing cruising-level full-power tests in forward while tied-up in their slips. Providing you're properly tied-up, I've read this is safe. Can't imagine why reverse would be any different.

Jim

[Barry33]

One other note on this, the yard that installed the motor used conventional motor mounts and not the Yanmar supplied.

[Valiente]

Conventional for a smaller engine, maybe?

[Barry33]

Mystery solved !! Both rear mounting flanges have 2 bolts that secure the motor mount to the block...the port side bolts we GONE altogether, and starboard flange had one loose and one broken bolt. The sheared head is what I found in my bilge that originally started this pursuit. The excessive stress on the one remaining bolt (the broken one) is clearly the source of the issue.

Thanks to all !!

[Boasun]

Two words: Quality Control!

A lot of people just look down through the engine room hatch and say it looks good to me.
Reality you have to go into that space and inspect every bolt, screw and mountings. Especially when you have had other people work in that space. And that type of inspection also includes the rest of the boat from the top of the mast down to the keel bolts and rudder fittings

[ABJ501]

I have never heard about motor mount bolts shearing off with the type of load that you described, however I am big on using the proper motor mounts that are recommended by the manufacturer. I had my motor mounts custom fabricated from a higher grade of aluminum and steel and then I painted them with several coats of epoxy paint so they would last longer, resist corrosion better, and withstand the rigors of unexpected loads while underway. Broken motor mounts are one of those disabling casualties that can impair your vessel greatly especially when you are miles from shore and in a situation where raising your sails may not be an option. So do not cut corners on the motor mounts. Inspect them regularly (I inspect them before any trip) and if you install them use what the manufactuer recommmends.