What Torque to Tension Chainplate Bolts?

[DuncanC]

This is a spin off of another thread I started a few days ago.

Short version:
My question is... What torque do you tension your chainplate bolts to???

Long version:
But... I have seen the answer, "to whatever the torque rating of the bolts is" and this isn't satisfactory, because (as I understand it) the friction between the chainplate and the hull is what takes the load from the shroud. So the limit of friction between the plate and the hull needs to exceed whatever the max possible load of the shroud is.

So you need enough bolts, torqued high enough, to produce a compressive force that creates a limit of friction (between chainplate and hull) that is higher than, say, the breaking point of the shroud cable or the breaking point of the chainplate eye or the shackle or whatever.

That means knowing what compression force each bolt will aplly when torqued to a given value, how bolts you have and the coef of friction between the boat and plate.

The hard part in that is the compression force of the bolts at their torque rating - the rest is easy to find.

So how hard do you have to tighten your chainplate bolts?... Or if it is too difficult to predict, then where is the table of magic answers that people use?

A boat designer must work this out at some point (I hope!).

Well done for reading the long version.

Thanks

Duncan

[a64pilot]

I think you'll find that the shear load on the bolts and the hull far exceeds the tension limit of the chainplate, and the tensile strength of the chainplate far exceeds the load the rigging can place upon it before the rigging fails, so as long as the bolts are simply tight your OK.
Until corrosion sets in and weakens the chainplates themselves.
But I'll watch this thread as it is something I have wondered about myself.

We are talking about cruising boats, not racing?

[model 10]

I find it hard to believe that it is the friction of the chainplate to the hull that keeps the rig up. I think it is the shear loads on the bolts and hull that you need to worry about. Because of that, bolt torque is probably not that important.

[Mirar]

You are probably sweating the small details too much. When you look at the shearing strength of the fasteners, compared with the shroud loads you will probably see that you have a lot of leeway. I remember when I built my 48ft Herrshoff ketch , hull material of soft red cedar, that putting anything near the correct torque on the 1/2 in. SS fasteners for the chainplates would simply crush the wood and squeeze out any bedding material used. I think shear strength is the important buzzword for you to look at, not torque values. Just my thoughts!
Greg

[hellosailor]

Duncan, I think you've got the engineering wrong.

You don't torque bolts to create a set amount of compression (what you call friction) you torque them according to the bolt size and material, in order to create loads IN THE NUT AND BOLT which will prevent the nut or bolt from loosening and coming apart from each other. (Nut and bolt or bolt and thread, doesn't matter.)

So while you might be right about creating friction, that concern would be addressed by using sufficient and properly sized bolts in the first place, and then simply tightening them down to the correct torque for those bolts.

Your approach would be a kludge after the fact. If you don't think your existing chainplates have been properly engineered, you've got more than torque to worry about. Measure the bolts, follow the torque ratings.

Or, redesign the chainplates entirely.

[lifeofreilly57]

Torquing the bolts on most chainplates to the bolts specified rating will usually crush whatever your fastening them to, be it a wood or fiberglass base, unless you have a steel hull and are fastening them to a steel hull section or bulkhead.
When torquing a bolt what your really doing is putting a pre-measured amount of pressure on the bolt (stretch) and the piece it's bolted to, in most cases the chainplates have multiple bolts so there is much more clamping force than needed, most times the number of bolts used are more for spreading the load over a wider area.
If your fastening on a fiberglass hull your most likely tightening it to just below the yield strength of the hull laminate.
How big are the chainplates? How many and what size bolts?
Commmon 303 or 304 stainless bolts around 3/8 have about 115,000lb shear strength, which could easily crush your laminate if torqued to the max.
Even in industrial settings where the bolts are much larger it's rare to see bolts used to their torque rating, most times doing so would deform the piece their clamping together, even when clamping 4" thick steel flanges together.
The closest I've been able to get someone to give me as a guide was to "tighten them by feel", don't crush the bulkhead.
Good luck, maybe you could contact the builder if they're still in business, if not you might be able to get some guidance from one of the builders still in business. I've been surprised by some of the advice offered by service reps who've offered information even though it wasn't their make.

[S/V Alchemy]

All the above answers are correct. The only after-market fix you might wish to consider (aside from pulling the chainplates and making sure the core is sealed from water ingress...and then bedding everything really well to keep it that way), is to consider putting a bushing or sealant in the holes through the knees themselves (assuming they are knees and not tie-rods or something). If you have room, a fender washer (about the size of a toonie) isn't a bad idea between bolthead and chainplate and nut.

I torque them once a year and sometimes remove them to look for signs of passivation or strain. I torque mine to about 35 ft/lbs. or the same as an Atomic 4 head bolt (hey, it's easy to remember and not so tight I can't get them off with hand tools! Hope this helps. Crushing the FG is bad.

[hellosailor]

A bushing, a fender washer...
Would it be unrealistic to think the builder might have originally included all the bits and pieces that were necessary to avoid crushing the frp when "proper" torque was applied?

[lifeofreilly57]

Originally Posted by hellosailor

A bushing, a fender washer...
Would it be unrealistic to think the builder might have originally included all the bits and pieces that were necessary to avoid crushing the frp when "proper" torque was applied?

I haven't seen a bushing in any of the chainplates I've worked on, remember the bolts are used to produce clamping force to "clamp" the chainplates to the hull or interiror structure.The amount of clamping force produced by even a small amount of torque on the bolts is impressive, that's why it's good to be careful when tightening any bolts clamping a piece to any FRP structure.
Most chainplates I've worked on were either bolted to a bulkhead or similar interior structure or the hull of the boat, on newer boats and some models of older cruising boats I've seen dedicated structures molded into the grid to take the loads.
The advice to inspect them from time to time is good advice.
The boat my wife and I bought last year has interior cabinetry over the chainplate mounting point, my guess is that they've never been inspected since the boat was built, guess I'll be doing some woodwork soon.

[hellosailor]

reilly, I'm no engineer but I think the whole "clamping force" is a misconception. The chainplates don't have to be "clamped" to the hull, crazy glue could do that. They are typically being held in place against the shear on the bolts, which are acting pretty much the same as a shear pin in any other connection would, aren't they? Like a cotter pin in your rigging terminals, or the shear pin in an outboard prop shaft. If the bolts aren't tight, the chainplate still stays attached to the hull. If they shear--it comes loose.
The clamping? Just [sic] a convenient way to keep things from wiggling around, versus the constant motion of the rigging.

I'd be curious to hear from some riggers and engineers as to what the real issues are. Maybe this means we could all save some dead weight by going to pop rivets instead.(VBG)

[Scott Berg]

I replaced the Chain Plates on CHARDONNAY (outside, bolted to the hull directly) and spoke to the NA who designed her for advice. He said to bed the plates and bolts well, use the full size backing plates I had fabricated, and make them as tight I could easily do with a regular socket wrench and not to sweat it. We used hi grade SS bolts and washers/lock washers/hex nuts. We finished the inside where it shows with acorn nuts also (oh, and 5/8" SS Acorn Washers are a shade dear!). As all the loads are in shear, there was no advantage in over tightening them like you would if they were holding the load in tension instead. So just bed them well and tighten with a wrench.

[hellosailor]

"SS Acorn Washers "
Acorn washers? who knew?
Who's been hiding these things from me for all these years?!

[model 10]

Maybe this means we could all save some dead weight by going to pop rivets instead.(VBG)[/QUOTE]

Maybe not "pop rivets" but hollow bolts. Blind rivets (pop rivets) might come loose and you couldn't tighten them.

[lifeofreilly57]

Originally Posted by hellosailor

reilly, I'm no engineer but I think the whole "clamping force" is a misconception. The chainplates don't have to be "clamped" to the hull, crazy glue could do that. They are typically being held in place against the shear on the bolts, which are acting pretty much the same as a shear pin in any other connection would, aren't they? Like a cotter pin in your rigging terminals, or the shear pin in an outboard prop shaft. If the bolts aren't tight, the chainplate still stays attached to the hull. If they shear--it comes loose.
The clamping? Just [sic] a convenient way to keep things from wiggling around, versus the constant motion of the rigging.

I'd be curious to hear from some riggers and engineers as to what the real issues are. Maybe this means we could all save some dead weight by going to pop rivets instead.(VBG)

On the racer/cruisers I've had and maintained, without the chainplates "clamped" tight enough to prevent movement they would have continually worked on the holes the bolts were going through, eventually elongating them and compromising the bulkhead/mounting surface they were attached to. Most of the chainplates that are run through the deck to an interior mounting point are not usually mounted using bedding compound or 4200 like the externally hull mounted type on some cruising boats. In these applications the clamping force, although low was enough to prevent this type of movement. On a couple of boats I worked on to turn over ( to pay for my daughters tuition) just this type of situation arose. Upon disassembling them I found that the bolts were not properly tightened, the chainplates and bolts had worked the holes to the point that the mount had to be replaced, the chainplates were alright but the mounting surface had to be replaced. Not an easy job since on one boat it was part of the interior furniture that was tabbed into the hull.
They don't need to be torqued to death, like it was pointed out in a previous post a hand wrench is good enough, you'd be amazed how much force a bolt can produce when tightened with just a 3/8" socket wrench.
In this application the bolts are doing both shear and clamping jobs.
I work for an OEM engineering company that builds industrial machinery and have seen the results over over tightening bolts on mating surfaces, it's very rare that a bolt is used to it's full torque limit, most are sized to have a huge safety factor. Had to replace a 4" thick flange on a 1500hp extruder because a plant mechanic torqued the flange bolts to their torque rating and turned it into a potato chip, that wasn't cheap.
As for gluing? Not sure if I'd go that route, ever try to remove anything bonded with 5200? Usually it peels off whatever surface it is attached to with it. The new Corvettes have some of the body tubs glued, or as they say, bonded together instead of using bolts since they feel it gives a stronger more consistent joint and those tubs are structural members. Something to think about when cruising at 150mph, it's not your fathers superglue. Yikes. Fortunately we cruise at a much more leisurely pace.
Anyone out there work for an OEM boat builder? I'd like to hear what their take on it is.

[Cheechako]

My gut says torqueing chainplate bolts to bolt specified levels is exccessive on the material (hull) between the chainplate and the backing plate. The bolts are in shear and they hold the plate in place once the bolt seats on the upper part of the hole thru the hull. With enough bolts the firction may be enough to keep the plate in place without the bolt seating against the hole in the hull I Imagine. At any rate , I wouldnt tighten the bolts severely.