Mounting Satisfaction: Engine Mounts

Are boats objects of romance? Sure, they are. Just ask the characters played by Leonardo DiCaprio and Kate Winslet in the movie Titanic, as they stand on the bow of the great, doomed ship, as Kate’s character makes like a bird in flight while Leonardo’s character embraces her from behind. In fact, since Leonardo’s character will


be dead by the end of the movie, that’s probably the best time to ask them.

But that doesn’t mean all the parts of every boat are filled with romance. Some parts are too grittily utilitarian to make any pretense of romance, as anyone who has ever rebuilt a marine head will tell you. And other parts, taken thoroughly for granted in their quiet, passive simplicity, fail to attract even the minimum level of attention required from their owners for their continued effectiveness.

Today, I discuss one of the most critical parts on any boat with an inboard engine: the humble, rarely considered engine mount.

Yes, that’s right. An engine mount is exactly what it sounds like. There’s no romance here.

Applied physics

The primary purpose of a boat’s inboard engine is to impart rotation to a propeller shaft to which it is connected through an intervening gearbox. At its specified maximum speed of 3,400 RPM, Meander’s twenty-five-year-old Yanmar diesel engine will turn our prop shaft through about 1,600 revolutions per minute, or about 26 times per second.

The shaft in turn will impart its rotation to the propeller at its aft end. The propeller in turn will exert a force on the water surrounding it, and the water will exert an equal and opposite force on the propeller. And those forces will push the boat through the water.

This image of how an engine moves a boat with respect to water is one with which most of us are familiar. But we are not, I suspect, usually conscious of a rather obvious assumption we’ve necessarily smuggled into that image: that the engine itself is not moving with respect to the boat.

It is here at this point in our happy oblivion that the engine mounts sit, sublimely (and subliminally) completing the picture. It is through these mounts that the forces generated by the propeller’s rotation are transferred to the boat, coaxing it finally into motion.

Two of our boat’s new engine mounts in situ.

Two of Meander’s new engine mounts in situ.

Really, it just makes me want to burst into song.

Oh, the Prop is connected to the. . . Prop Shaft! And the Prop Shaft connected to the. . . Gearbox! And the Gearbox connected to the. . . Engine! And the Engine connected to the. . . Engine Mounts!

And the Engine Mounts are connected, finally, to the hull of the boat.

Okay, it’s not as catchy as “the Legbone connected to the Kneebone.”  But the physics behind it is incontrovertible.

Engine mount anatomy and function

Meander has four engine mounts. Each one consists of a threaded stud welded to a specially formed metal top plate, which in turn is bonded (either by advanced adhesives or by arcane magic—our manual did not say) to isolators of rubber-like elastomer material, which in turn are bonded to a specially formed bottom plate into whose ends slotted holes have been machined.


Sort of like this.

This design allows Meander’s mounts to provide three essential services.

  • Their bolted connections to the hull, as noted above, provide the means through which the motive forces developed at the propeller are applied to the rest of the boat.
  • Their slotted holes and threaded studs allow mechanics who know what they’re doing to align the engine, gearbox and prop shaft correctly with the stern tube through which the shaft exits the boat.
  • Their elastomeric components dampen the engine’s vibrations and keep them from being passed through to the hull.

With respect to this last item, engine mounts are not all the same. Rather, the amount of dampening and isolation a mount will deliver depends on how stiff its rubber isolators are. On Meander’s mounts, the isolators’ stiffness ratings are indicated by a number molded into their sides. Higher numbers indicate greater stiffness.

And the engines these mounts dampen and isolate are not all the same, either. Some engines, like Meander’s, require four mounts of equal stiffness. Others, however, may require two isolators with one stiffness rating on one side of the engine, and two with another stiffness rating on the other side.

So having the right mounts is important. Mounts that are too soft will not dampen the engine’s vibration sufficiently, while those that are too stiff will allow too much vibration into the hull.

But there’s no reason for you to make that mistake on your boat, because the engine mount spec that is just right for its engine can easily be obtained if you just know where to look.

I myself have no idea where that is.

For my part, I found all this stuff out from Mack, our knowledgeable ABYC-certified Master Marine Technician here at the Deltaville Yachting Center. Mack observed that although Meander’s specs called for mounts rated “100,” the mounts on which her engine was actually sitting were rated “200.” (I have no idea what these numbers specifically mean to engineers. But I’m sure they’re not just making this stuff up). Then Mac had to explain to me the significance of what he observed.

From my experience, I infer that your own knowledgeable marine technician would be a good source of this kind of information.

For the moment, however, just be aware that if you someday decide to replace your boat’s mounts without having paid sufficient attention just now, you might end up with isolators that are either too soft or too stiff for your boat’s requirements.

Engine mount replacement

Meanwhile, over at the Ocean Navigator website, author Harry Hungate  challenges us in his engine mount replacement article to fight that happy oblivion creeping over our own engine mount awareness by changing them every five to six years, whether we think they need it or not.

Simple as these parts are, they yet transmit large forces and absorb constant vibration in order to do what they do. As a result, their studs and plates are subject to fatigue, robbing them of their strength, and their rubber isolators can delaminate. This kind of damage is not readily visible. Mounts that look fine externally may in fact be in trouble, and the failure that is coming upon them will likely be catastrophic rather than progressive.

Other signs that it’s time for a change, however, are easier to spot. Hungate advises replacing mounts contaminated by seawater, engine oil and coolant, noting that the first substance will rust metal parts and that the other two will cause deterioration and delamination of the elastomeric blocks. If you see rust or peeling paint, corrosion is already underway.

Excessive engine vibration is also implicated in engine mount failure. Sometimes the vibration is a direct result of already failed mounts. At other times, vibrations emanating from another cause, such as a misaligned shaft, will accelerate the fatiguing of bolts and studs, bringing failure on sooner.

So stay out in front with your engine mount replacements. As with all your other maintenance items, choosing your own time and place for this work will be more convenient and less expensive than having a sudden failure choose them for you.

And having just watched as Meander’s engine mounts were replaced here in Deltaville, VA, I can tell you that it is not a difficult operation. All one needs is a set of new mounts, a set of combination wrenches readily available in any hardware store, and, to lift the engine off the old mounts, one of these handy-dandy cherry pickers.

Bigger than a breadbox.

Bigger than a breadbox.

If I could buy a used cherry picker, I would consider doing this job myself in the future. But I’m not sure I could get it stowed in our quarter berth.

Maybe if we cleared some other stuff out.

Maybe if we cleared some other stuff out.

When engine mounts go bad

Having established the role the mounts play in ensuring an engine is not moving with respect to the boat, let’s consider what could happen when they, for all practical purposes, go missing. And let’s do it with a true story Mack the Master Marine Technician shared with us.

The story began with a couple who were motoring several miles offshore one day in their sailboat when its propeller hit something that wedged and stuck, stopping it cold and simultaneously punching a small hole through their hull.

Now when a propeller is suddenly stopped, its engine, being a not-terribly-intelligent machine, will not notice. It will just keep working to send that spinning action down the prop shaft, trying to turn the prop. But since the stopped propeller cannot be turned, it will send an equal and opposite reaction back up the prop shaft, the result of which is to try to turn the engine.

You know, the one that is attached to the hull through the engine mounts.

In a smaller pleasure boat of fiberglass construction, the engine mounts are usually so attached by no more than eight bolts. And when a suddenly stopped propeller suddenly tries to turn the boat’s engine, it is around these eight bolts that all the engine’s mighty force is suddenly concentrated.

Now for all I know, there may well be fiberglass hulls in the world that can withstand this sudden concentration of force.

This couple’s hull was not one of them.

And so, Mack continued, this couple’s mounts were ripped from their hull, and their engine did a few revolutions of its own in the engine compartment.

In turn, those revolutions tore off the engine’s fuel line, flooding the compartment with diesel. They also tore off the wires between the alternator and the boat’s battery bank, disabling the electrical system and putting the automatic bilge pump, the one trying to contend with the water pouring in through the aforementioned small hole, out of operation.

And so the couple spent the next 36 hours issuing Mayday calls on their handheld VHF radio while working their manual bilge pump to keep the water from rising above waist level.

Mack’s story ended better than it might have. The couple were eventually located by a Coast Guard helicopter that dropped some self-powered bilge pumps. This allowed them to turn their attention from pumping out the boat to sailing it until they were close enough to shore to get towing assistance. In the end, they got out with no worse than mild hypothermia, a repair bill that probably exceeded what I’m currently looking at for Meander’s latest round of repairs by a factor of ten, and, if they are at all wired like me, a firm resolve to sell the damned boat and take up chess.

That couple, incidentally, cannot be faulted. Hitting something that can completely disable your boat in ten seconds is the dumbest of luck, and dumb luck can inflict itself on anyone.

But the story does put a point on the unquestionable importance of the lowly engine mount.

Mounting stress

“But Mike, boats being such objects of romance and all, whatever made you decide to write instead about the unquestionable importance of the lowly engine mount?” Glad you asked.

As it happens, Meander’s mounts had been on my mind ever since our marine survey, during which our surveyor, Frank, had noticed those visible symptoms of rust and peeling paint on the aft one to starboard. And our recent installation of a new cutless bearing here in Deltaville required us to pull and reinstall the prop shaft, which, in turn, required its realignment with the engine. And performing such an alignment over a questionable engine mount made no sense to me.

So, with Meander on the hard and getting related work done, all signs pointed to our getting that aft starboard mount replaced.

However, I’m a completist who likes all the members of any given set to be shiny and new and evenly matched all at the same time. Also, I’m made of money, as I’m sure we all are.

So, considering the price of $135 per mount excluding labor, and presenting the exhortations in Harry Hungate’s article to Pam, I argued, “Why stop at one?”

And we didn’t. We instead had all four replaced. And by the time all four were replaced, we were very glad we decided to replace all four.

Because when Mack and his workmate Tony were hooking up the chains that would lift Meander’s engine, the forward mount to starboard—not the horribly rusty aft mount previously identified, but rather the mildly rusty yet otherwise apparently quite solid forward mount—looked like this.

Yeah, you saw this before.

Yeah, you’ve seen this before.

And after they lifted the engine about an eighth of an inch (three millimeters), the mount looked like this.

But you didn’t see it like this. And at first, neither did we.

But you didn’t see it like this. And, at first, neither did we.

What we see here is a shear failure in the mount’s threaded stud. The years of constant vibration Harry Hungate had warned about had indeed parted the metal through about 95% of its sectional area, leaving the threaded stud hanging—shall I say it?—by a thread. (Sorry, but you had to know it was coming.)

Here at the end of its life, it had taken only the slight lifting of the engine to complete the break. And I, for one, am pleased as punch that the lifting was induced by a cherry picker on a clear day in a boatyard rather than by a fifteen-foot wave in a gale at sea.

Mounting satisfaction

And that, in the end, is why I‘ve gone on for 2,400 words about engine mounts. To celebrate.

First, to celebrate the vindication of our choice to spend the money on them.

But more importantly, to celebrate all the grave consequences we believe we’ve sidestepped by our choice to spend the time on them.

And the broken mount once again helps put our extended stay in Deltaville into a light we can live with. Because we certainly took a bullet three weeks ago when our latest troubles with Meander forced us to pull her out. But as we consider how our time here has revealed so much of what she was hiding, we are all the more grateful that so many other bullets have whizzed harmlessly by.

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