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Old 23-01-2017, 21:00   #1
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Help me understand how chainplate loads work?

So I've read Brion Toss's Rigger's Apprentice. Great book and it helped me understand how loads transfer through the mast and stays, but it doesn't really cover chainplates - how the loads transfer to the hull!

I have two designs which I'm trying to think through:
- Traditional SS plate thrubolted to a bulkhead or other vertical structure.
- Navtec U-Bolt tie rod, connected to a fully internal chainplate. Rigrite link with pictures: U-Bolt Type Chainplates

The main thing I don't understand is why do typical SS chainplates have 5-9 thrubolts? When a stay has many single points of failure above that (mast tang, the rod or wire itself, toggle, etc), why does it need redundancy in the 9th degree for the chainplate attachment?

Is the load evenly divided between those thrubolts (ex, 10,000 lb backstay on 10 thrubolts = 1k each)? Presumably this would only work if the holes drilled in the bulkhead were very tight - but even then it seems like 1 or 2 top thrubolts would take most of the load. So are the extra thrubolts just for redundancy - in case half the bolts rust out?

In Internet images of broken chainplates, it seems like the most common failure is SS corrosion in the chainplate itself, with it breaking clean through *above* the topmost thrubolt. So all those 9 thrubolts did was keep the bottom half of the broken chainplate attached to the boat. I haven't been able to find any pictures of a rig failure where the chainplate stayed intact *and* the bulkhead stayed intack but the thrubolts all broke.


Then when I look at the Navtec U-Bolt chainplates, it gets a bit more advanced. It's a beefy U-bolt through the deck to an aluminum tie rod block, which holds a headed tie rod captive in a socket, and then that tie rod is attached to the hull with a traditional thru-bolted chainplate. Since these chainplates are now fully internal, they're in one of the driest areas of the boat, with no oxygen starvation, so it seems like this design makes them a much less likely failure point.

So where is the weakest link now? The U-bolt legs passing through the deck. Or the SS tie rod head in the aluminum backing block. But the tie rod head can never have significant standing water or oxygen starvation, due to the design, so it seems that the tie rod head is actually not a big concern. The U-Bolts are though, at least for inspection, if I can ever get them out.
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Old 23-01-2017, 22:42   #2
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Re: Help me understand how chainplate loads work?

Rigging fatigues overtime. Standing rigging gets replaced on a more frequent schedule than chain plates, so you want chain plates to be designed to take higher loads and more fatigue cycles.

Also more bolts helps transfers load to the hull and helps reduce stress concentration.
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Old 23-01-2017, 23:14   #3
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Re: Help me understand how chainplate loads work?

The bolts aren't actually holding the load in shear, they are pressing the chainplates against the hull, and friction between the chainplates and the hull is taking most of the load. So while yes you would like the bolts to be strong enough to carry the load, they really don't have to be.

The chainplates however you want multiples of the strength of the wire. Both for corrosion allowance and because if something breaks you really want it to be above deck, if no no other reason than as a good place to attach a makeshift get home rig.
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Old 24-01-2017, 07:56   #4
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Re: Help me understand how chainplate loads work?

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The bolts aren't actually holding the load in shear, they are pressing the chainplates against the hull, and friction between the chainplates and the hull is taking most of the load.
Friction? Interesting. I wouldn't have guessed that. But I guess that makes sense - the coefficient of static friction is quite powerful. If this is true, then it's really important to tighten the thrubolts adequately.

I still don't understand why do some chainplates, that are intended to handle equivalent loads, have 7 thrubolts and some have 5? My backstay rated for 10,000 lbs has 7 thrubolts while the cap shrouds rated also for 10,000 lbs have 5 thrubolts. Maybe this is just that they want a greater margin of safety on the backstay since a failure there would be more disastrous?
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Old 24-01-2017, 08:09   #5
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Re: Help me understand how chainplate loads work?

Quote:
Originally Posted by Stumble View Post
The bolts aren't actually holding the load in shear, they are pressing the chainplates against the hull, and friction between the chainplates and the hull is taking most of the load. So while yes you would like the bolts to be strong enough to carry the load, they really don't have to be.

.




I'm no rocket scientist, but you may want to google your response's more carefully.

Shear loads are carried by the bolt shank..

Again free advice on the internet.....you get what you pay for.


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Old 24-01-2017, 09:12   #6
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Re: Help me understand how chainplate loads work?

Bolts are in shear and do carry the majority of the load, bolts and of course the glass they are thru bolted through.
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Old 24-01-2017, 09:37   #7
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Re: Help me understand how chainplate loads work?

The clamping force of the plate to the bulkhead is minimal compared to the forces in shear at the bolts.

The reason for many bolts is to increase the area of contact for the shearing forces. For example, one bolt would rip through the plywood easier than several sharing the load.
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Old 24-01-2017, 09:40   #8
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Re: Help me understand how chainplate loads work?

It does seem like the number of chainplate bolts on external plates is a bit redundant, (two would likely be enough theoretically) but the bolts are in shear and although they wont likely break until corroded, slippage could elongate the holes in the glass and/or bend the bolts. More bolts in close fitting holes= less likely to have slippage. Many boats are only 3/8-1/2" thick glass at the toerail/deck area.
Also, thru bolted external chainplates are a perfect situation for stainless corrosion if the sealant fails anywhere.
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Old 24-01-2017, 09:43   #9
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Re: Help me understand how chainplate loads work?

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It does seem like the number of chainplate bolts on external plates is a bit redundant, (two would likely be enough theoretically) but the bolts are in shear and although they wont likely break until corroded, slippage could elongate the holes in the glass and/or bend the bolts. More bolts in close fitting holes= less likely to have slippage. Many boats are only 3/8-1/2" thick glass at the toerail/deck area.
Minimize should be three on the same axis to prevent fatigue - two bolts and no adhesive bond between hull and plates = load can cycle between the two bolts and could reduce the lifespan of the chain plate by 70%!
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Old 24-01-2017, 09:43   #10
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Re: Help me understand how chainplate loads work?

Sometimes the additional fasteners can be simply nothing more than there was room for more. In aircraft anyway if you can show you far exceed min strength requirements you can get by with just analysis and no actual testing etc. I assume the same for boats. If I were the designer and was not real positive of the exact loads, but knew that seven bolts was way safe and there was room for them, I'd spec seven bolts and not spend a lot of time in analysis.
Now that sounds like BS answer, but sometimes when we start looking for highly technical reasons we are disappointed when we find out that it was nothing more than a highly experience Engineer exercised his professional judgement as said that's good enough
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Old 24-01-2017, 09:53   #11
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Re: Help me understand how chainplate loads work?

Quote:
Originally Posted by ur2slo View Post




I'm no rocket scientist, but you may want to google your response's more carefully.

Shear loads are carried by the bolt shank..

Again free advice on the internet.....you get what you pay for.


Question Stupidity
a. Like the old saw about free legal advise...

b. I wonder why it took me calculus and physics in high school, 4 years in college, and 30 years on the job to learn engineering, when I could have
just read one book and asked on the web?

so....

1. I hope it is obvious to all the the mechanisms are are complex.

2. Chain plates and bolts will not fail without corrosion if properly engineered, so why would we expect to find failures? When we find a failure, why are we surprised that it is at the point of maximum stress?
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Old 24-01-2017, 10:00   #12
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Re: Help me understand how chainplate loads work?

A few points I would consider;
As said above chain plates are massively overbuilt on cruisers and rightly so but do still get corrosion failure so I would be careful not to mix metals, all cast bronze is still the best (0 corrosion) but if SS keep to the same grade for bolts and plates. Mixing S/S and all'y, even below deck would concern me.
The number of bolts, is I think, more to do with the hull construction than the chainplates. On planked boats it was 1 bolt per plank. On solid construction it all depends how the hull is built, a solid core will need less bolts than a basa core.
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Old 24-01-2017, 10:11   #13
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Re: Help me understand how chainplate loads work?

Ok, so to sum up: the chainplate bolts take the load, as shear forces (perpendicular / 90 degrees). And the number of bolts in a chainplate are due to the medium they're in (wood core / FRP skin) rather than the strength of the bolts under shear forces.

And some excess redundancy is built in due to higher probability of the bolts rusting and difficulty of inspection (complete lack of inspection on most boats probably). Also thrubolts are relatively cheap, so why not throw in a few extra.

It does not seem to be the case that there are more bolts due to shearing forces being higher than inline forces - if that were true, then the shear forces on a clevis pin in the toggles above deck would have comparable force and be comparable to a single thrubolt.

I wish there were a rigging book that explained these forces (which I suspect are not much beyond high school physics, but it's been a long time since I took that). I suspect the only option would be to go to mechanical engineering textbooks.
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Old 24-01-2017, 10:11   #14
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Re: Help me understand how chainplate loads work?

Quote:
Originally Posted by ur2slo View Post




I'm no rocket scientist, but you may want to google your response's more carefully.

Shear loads are carried by the bolt shank..

Again free advice on the internet.....you get what you pay for.


Question Stupidity
Meh, my engineering degree is non-existent so you get what you pay for.

But as it was explained to me by a number of N Architects, the load is in shear, but the friction between the hull and the chainplates is carrying most of that load not the bolts themselves. The bolts are acting like a big clamp, which is why tightening to the proper torque is so important for them. Most of the load on the bolts is the tension pulling the plates into place, not the rigging loads directly.
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Old 24-01-2017, 10:46   #15
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Re: Help me understand how chainplate loads work?

Tesselate,

In one paragraph you mention the number of bolts in relation to the medium they are bolted. You got that part.

But to the toggle bolt, it would see the same magnitude of forces as the plate bolts. It's not like a single chainplate bolt, though. Because the fixture it bears upon isn't plywood. The area for shear force contact is smaller but it's resistance to shear is the same. Otherwise it would fail.
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