Help me understand how chainplate loads work?

[Steven UK]

Originally Posted by Stumble

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.

+1 I have designed and redesigned lots of castings with this thought process in draft machinery (tractors) where the bolted face was in shear. Also the engine to bell housing on the engine on your boat is this way (the dowels only position). Of course I don't know sh1t about sailboat design.

Regards...

[Stumble]

Originally Posted by Eigenvector

I totally agree. However due to cost, it's really hard to pop a laid up structure out of the mold with these types of discontinuities. It can be done. The general rule I always quote to clients is: "Fast, Cheap, or Good. Pick Two." And when it involves FRP or injection molding, pick One.

Eigen,

One design that I worked on was a titanium T-Rod inserted into the hull laminate stack. It could be built as part of the original hull layup, would last forever, and if you run the numbers for a G5 threaded rod it doesn't take a very thick rod to carry the loads so it didn't distort the layup very much.

[rhubstuff]

ur2slo: If the chainplate bolts take all of the load, as you say, would it be OK if you installed them only hand tight? After a few tacks, the bolt holes would elongate to the point of destruction. What prevents this is friction between chainplate and hull. Your theory would be correct if the chainplate were attached with some sort of clevis pin arrangement, not bolts. (This is actually possible.) Interestingly, the 12-meter I used to crew on has the main upper shrouds attached directly to the mast step, passing through the deck over a curved fairlead, with deck and hull structure acting as a sort of spreader. Turnbuckles are attached directly to ears on the mast step, so the hull takes no upper shroud tension at all.

[ur2slo]

Originally Posted by DumnMad

I'm afraid you are the one with the theory.
No theory to us - we know how friction grip bolting works. They transfer the load by shear friction. Used in all sorts of areas of the construction and manufacturing industries. Google "friction grip bolts."

Did.....Bellville washer came to mind...same principle?

Btw, I first encountered them working in the aerospace field.

Lockheed C-130 Herky Bird uses em in the wing attach BOLTS, along with lots more things..........

I been bolting & field engineering stuff longer than most engineer's been alive.

But I'm still saying the bolt shank is taking the shear load and transferring it to tension load via the chain plate.


I'm a gonna leave it at that cause I firmly believe that if you put 4 engineers in a room with a problem, all you will get is 5 engineers........and more problems.

I appreciate the banter and am going to talk to some old dudes I kno to quell MY curiosity

As always Question Stupidity

[valhalla360]

Originally Posted by Rapanui

You are transferring the load from a high modulus and strength material (shroud or stay) to a low modulus and lower strength hull material (grp etc). To do this you have to spread the load over a larger area to keep the stress down to allowable levels. To this end, clamping of the chainplate to provide static friction is essential. The more bolts you use the more even and stronger the total clamping force. If you loosen all the bolts to the chainplate and take all the load on the bolts in shear, the bolts would take the load no problem but the bolts would eventually cut into the glass and elongate the holes.
The other factor is stress concentration, any discontinuity in the structure will increase the strain and hence stress markedly over theoretical stress Force/area. Spreading the load over a wider area reduces both the theoretical stress and the stress concentration factor. Ideally a chainplate would taper down in thickness towards its edge to reduce stress concentration.

Bingo!!!!

It's not about the steel. If the bulkhead was a thick piece of steel a single bolt of appropriate size would work just fine. Plywood & Fiberglass are drastically weaker than steel, so you need to spread the load out (whether you ascribe to bolt in sheer or clamping friction is a separate matter).

[JD-MDR]

I just got a Cape Dory 30K and the backing plates are so rusted I'm afraid to raise the sails even at the dock. I'm hauling out next week and plan to start the process. The iron plates have rebar welded to them which is fiberglassed to the hull. does anyone have any suggestions to improve this design, Aluminum? or stainless?. I don't want external chainplates. I would like to keep things the same. Sorry to piggyback off your post but its good info to share and I haven't figured out how to post yet. Thanks

[a64pilot]

So we highly polish the chainplates, then thru bolt them onto an uneven fiberglass surface hoping the fiction coefficient of polished SS is high enough to carry tons of load.
Really?

[SV DestinyAscen]

Originally Posted by a64pilot

So we highly polish the chainplates, then thru bolt them onto an uneven fiberglass surface hoping the fiction coefficient of polished SS is high enough to carry tons of load.
Really?

Reminds me of early commercial jet aviation when they were bonding metal with resin impregnated cloth and two rows of rivets and calling it a day.

Its a complex system. If friction was the primary force you would see more chain plates that are simple wedges. Why bother with multiple bolts distributing load when you can just have a simple geometric cutout, insert a wedge and call it a day.

[thinwater]

Originally Posted by Tessellate

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.

Yes, it is far beyond that. HS skips deflection, fatigue, and materials in general.

Yes, it is mechanical engineering. You'll need more than one book, since this involves materials, deformable bodies, statics, and dynamics. These are separate courses.

Or you can simply design by experience and rules of thumb, which is the more common case.

[Cheechako]

Let's face it, all those bolts in the chainplate in shear are part of a system that relies on one pin in the clevis in shear (probably about the same size as the bolts or one size larger) !
I would imagine the largest potential load to be seen by the upper shroud/cable system on the mast is during a knockdown in which the mainsail fills with water.... then comes back upright.

[DumnMad]

Originally Posted by a64pilot

So we highly polish the chainplates, then thru bolt them onto an uneven fiberglass surface hoping the fiction coefficient of polished SS is high enough to carry tons of load.
Really?

"tons of load" is not scary. I've designed a shear friction connection with 200 tonne WLL and 236t ULS.
Of course you don't polish the contact face, but even so, the coefficient of friction does not have to be that high if your clamping force is way higher than the rigging load. The load capacity is proportional to the clamping force times the coefficient of friction. Its why your have so many post-tensioned bolts.
The chainplates spread the clamping force to protect the fibreglass. Once you know the allowable load on the fibreglass before plastic yielding has its way its a simple ratio calculation.

[four winds]

This is an interesting discussion.

Even as an architect and teacher of statics at University in another life, it seems I may be on the wrong side of the reality on this issue. Frankly, it would take me a month of refreshing to be able to run the numbers through the formulas at this point. But in a general sense I thought I understood how this connection would be loaded.

Intuitively one can easily see the reason for multiple bolts is because of the considerable difference in tensile properties between steel and wood, or steel and fibreglass.

But just how the load transfers between the two is more difficult to grasp.

I would be curious to hear from those who have serviced or replaced chainplates if they noticed any evidence that a plate had shifted upward or that the holes showed any sign of deformation.

[a64pilot]

Originally Posted by thinwater

Or you can simply design by experience and rules of thumb, which is the more common case.

I think that is way more common than we realize, its professional judgement, and more often than not I am more comfortable with a known designer with decades of experience than I am with a wiz kid and a computer

[Cheechako]

Originally Posted by four winds

This is an interesting discussion.

Even as an architect and teacher of statics at University in another life, it seems I may be on the wrong side of the reality on this issue. Frankly, it would take me a month of refreshing to be able to run the numbers through the formulas at this point. But in a general sense I thought I understood how this connection would be loaded.

Intuitively one can easily see the reason for multiple bolts is because of the considerable difference in tensile properties between steel and wood, or steel and fibreglass.

But just how the load transfers between the two is more difficult to grasp.

I would be curious to hear from those who have serviced or replaced chainplates if they noticed any evidence that a plate had shifted upward or that the holes showed any sign of deformation.

There are plenty of pics of old chainplates here:
https://www.google.com/search?q=bad+...lfO63nuzs4M%3A
I don't see a preponderance of elongated holes, however, many of the broken ones are at hole locations, which would indicate to me that slipping likely occurred...? (how can the metal pull apart if the plate doesn't move?)

[a64pilot]

I think you do polish the whole thing don't you? Isn't the purpose of polishing to remove stress risers, not to make it pretty?

They break at the holes likely because that is where they are weakest, especially if the hole is not de-burred, back to stress risers