Engine strapdown

[zoidberg]

Thumbing through my copy of 'Heavy Weather Sailing' by the fireside, I noted the recommendation to ensure that heavy items be well secured from moving from their stowages, in event of a knockdown. Given that a 10'-11' breaking sea is almost guaranteed to roll my boat if caught beam-on, that had me thinking.....

....what about the engine?

I've seen breaking seas that size aplenty, around our tidal shores. One doesn't need a hurricane, or the Southern Ocean, to encounter a few of those around our headlands.

The 4 flexible mountings are secured, as I imagine most others are, by 8 lag bolts into wooden bearers. These are 40 years old and honeycombed with previous screwholes, although most have now been filled. Would those lagbolts - or the wood the threaded parts are screwed into - prove plenty strong enough, should the boat suffer a really violent knockdown? I have my doubts.

So, following the cue in 'HWS', I'm wondering about providing straps to ensure that a knockdown doesn't shift my engine.

What do others do....?

 

[LittleSister]

But what are you proposing to fix the straps to that is more solid than the engine bearers?

 

[TQA]

A couple of webbing ratchet straps should do the trick.

 

[sarabande]

I wonder about the effect of locking the engine solidly to the boat. Increased reverberation, issues with the propshaft, perhaps.

Could you use a wire cable to the engine lifting bolts, and run the cable not under tension down to a new strong point, rather like a car safety belt ? I assume you are considering how to prevent the engine becoming loose when inverted or nearly so.

 

[Robert Wilson]

And here's me worrying about the kettle flying loose

Look what you've done to my nerves now.

I suppose I should thank you for alerting me to another possible catastrophic incident. Just something else to add to lengthening list of "jobs must do".

 

[prv]

I wonder about the effect of locking the engine solidly to the boat.

I don’t think he was proposing to do so. Advice to run straps or cables around rubber mounts that might not be strong enough in tension (either as-designed, or due to deterioration of the rubber) has been around a while. The intention is to prevent the engine shifting far enough in a knockdown to cause leaks, damage, or even injury - not to lock it rigidly in place during operation.

Pete

 

[NormanS]

I worry about the sky's ability to support the weight of the full moon.

 

[zoidberg]

'prv' has grasped the idea.

A marine engineer, a 'Jester' and well-experienced in s/handed offshor'ing in small boats, advises that, should the engine shift on its mounts by as little as half an inch, it will probably be unusable due to violent vibration. There's no way to determine the strength of my 40-year old engine bearers and their capacity to resist being split open by 'sideways' shocks, and tables found via Google Images suggest the annular/ring thickness of wood holding my 3/8" lag bolts in place is as little as 3/32". That's not a lot.

So, while I'm working in there, I propose to add a little more 'belt and braces ( but wondered what others do ).

I could drill larger-diameter holes and fit the lagbolts into a 'pot' of thickened epoxy, as recommended by Gougeon/West Epoxy. Removing/adjusting them tighter could prove challenging.

I could design/fabricate some s/s straps to fasten over the protruding vertical heads of the engine mounts, and screw those to the sides of the wooden engine bearers. Those might well interfere with the proper function of the flexible mounts in absorbing normal vibration and lead to early failure.

I could fix small 4 ringbolts into the outer sides of the wooden bearers, then fit 4 small 'softee shackles' in 4mm Dyneema or PBO looped over the projecting heads of the flexible engine mounts. These tightened using 'spanish windlass' twists should ensure the engine stays in place if the bearers splinter a bit, but be flexible enough to permit the mounts to function as intended.

 

[pvb]

I think you perhaps don't have important things to worry about.

 

[dk]

In my 50 years of sailing and 25 years of reading about everyone's boat problems I've yet to hear of one lose its engine! If you're that worried remove it, replace the bearers, and reinstall using bigger fixings.
For sailboats having a proper means with which to cut away your rigging should your mast come down in the 360 is more important as that is a much more common occurrence.

 

[Deleted member 36384]

I ripped an engine out of a Sima 33, at very high RPMs. I was turning the boat on her keel when a line went round the prop and the engine stopped immediately. Investigation revealed that it had twisted in the cradle which itself was mounted to the the engine mounts, one mount sheared.

This is the only instance of loss of engine security that I am aware of. Having read a lot of voyaging books and various editions of Total Loss, the engine coming loose and breaking free is not something that I recall, so it’s probably not a likely risk.

The point of my own story is that in my experience where a massive load was instantly applied to the engine, it still remained fixed to the boat. I doubt that a rolling boat would produce the same forces.

 

[sarabande]

It's a good point, BOB.

Does anyone have any idea of the forces acting on a boat, either from

a roll

falling off a wave

being hit by a wave ?

Perhaps the worst case for an engine is the boat rolling 90 degrees, then falling sideways into the trough of the wave. A guess - 5G ?

 

[jamie N]

My engine weighs 83kgs. That's around about 20kgs/ mounting bolt, 10kgs. per screw into the engine bearers. Double the loads for a REALLY violent 2g 'event', and one's still only getting a strain of 20kgs. per screw.
The engine, which is 8hp, gives about 65kgs of thrust IIRC, so if it's able to run without breaking away from its mounts, it's not really going to be compromised I feel.
Blowing Old Boots's example shows much greater loads onto the bearers than the above scenario, which for for my engine is like having a 6Kw reverse shock, would be far more likely to damage the mounts.
I shared the fear of the kettle also, and made my own fiddles to overcome the 'tragedy' of a dented kettle, rather than paying Origo £33 EACH.

 

[sarabande]

I think you perhaps don't have important things to worry about.

With respect, pvb, when anyone who has survived umpteen years flying up to 6 miles above rather solid ground in thin aluminium tubes at rather fast speeds starts to think about engine bearers that look like Swiss cheese, then perhaps there might be some rather valid reasons for wondering if the boat engine might need a bit more than duct tape to hold it down in a rollover. .

 

[jamie N]

With respect, pvb, when anyone who has survived umpteen years flying up to 6 miles above rather solid ground in thin aluminium tubes at rather fast speeds starts to think about engine bearers that look like Swiss cheese, then perhaps there might be some rather valid reasons for wondering if the boat engine might need a bit more than duct tape to hold it down in a rollover. .

 

[sarabande]

My engine weighs 83kgs. That's around about 20kgs/ mounting bolt, 10kgs. per screw into the engine bearers. Double the loads for a REALLY violent 2g 'event', and one's still only getting a strain of 20kgs. per screw.
The engine, which is 8hp, gives about 65kgs of thrust IIRC, so if it's able to run without breaking away from its mounts, it's not really going to be compromised I feel.
Blowing Old Boots's example shows much greater loads onto the bearers than the above scenario, which for for my engine is like having a 6Kw reverse shock, would be far more likely to damage the mounts.

That's a start, Jamie, on some figures, but I know from some forensic work years ago that shock loadings for a very short time, perhaps milliseconds, can be much higher. Just trying to get some idea how an engine, perhaps a bigger one than yours, might behave under short sharp lateral forces.


e.g. My engine, Yanmar 3cyl, weighs in at 151 kg (with all the accessories), and has a height of 540mm, and width of 440mm.

Assumptions:

CofG is at half height
Force of 5G for 0.5s in horizontal direction athwartships. (I know that's crude figs, but a start)

Anyone care to do the rotational/leverage figs of the forces acting on the bearers?

 

[Aja]

In my 50 years of sailing and 25 years of reading about everyone's boat problems I've yet to hear of one lose its engine! If you're that worried remove it, replace the bearers, and reinstall using bigger fixings.
For sailboats having a proper means with which to cut away your rigging should your mast come down in the 360 is more important as that is a much more common occurrence.

+1
Donald

 

[TQA]

Go along to a rollover derby at your local stock car track. Assimilate the fact that engines seem to stay in place.

Now stop worrying.

 

[sarabande]

I suspect that car engines are fixed to the subframe ( or whatever it's called) in a very different way from boat engines to bearers.

Boat engines seem to sit ON bearers; car engines seem to sit IN the subframe.

 

[ffiill]

I think you perhaps don't have important things to worry about.

Then there are the water tanks,the twin fuel tanks,the gas bottles,the spare jerry cans of fuel,the spare gallon of oil,the batteries in my case x3 heavy duty.
I decided that given the sailing I do not worth bothering about.