What Torque to Tension Chainplate Bolts?

[model 10]

Originally Posted by 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.

There is something to be said for the proper sized holes and a dab of sealant.

[S/V Alchemy]

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?

Not in my experience. Old boats were built to old, inadequate codes. When I bought my 1973 sloop, it had brass gate valve seacocks barely better than the hardware store, untinned wiring everywhere and household grade Romex on the AC side.

Did I mention boltholes straight through the core material with no epoxy to keep the wet out? No backing plates? Glass fuses?

About the only '70s state-of-the-art material that's still working is the butyl bedding tape. That stuff is unkillable and works fine after 40 years.

[billr]

One thing to keep in mind when discussing torque on a bolt is the materials involved and the ranges of environmental conditions the system has to operate in. Changes in temperature, and humidity (for some materials) will change the dimensions of the bolt and the material you are bolting through. If you are using longer bolts, this can become a real engineering problem, particularly if the system is used in an application subject to vibration.
Billr

[DuncanC]

Thanks everyone for your posts!

For some clarification - My chain plates are exterior and bolted through the hull. My sailing type is cruising.

My project is not actually on my chain plates, although that is something that I will have to go over soon. I am planning to get a series drogue and need proper fixing points for it. The recommended way is to put 2 chain plates at the back of the boat (running horizontally instead of vertically).

What I am thinking about is sizing my new chainplates so that I know they can take the load.

I'm not going to be torquing to the bolt torque !! Rather i want to know how to estimate compressive force for a given torque. That can help with over torquing as well as est. the friction.

In any case I looked up the physical properties of West System epoxy (which I usually use) and did some had waving to estimate the shear str of my existing chain plates if they were epoxy glued to the hull - its a LOT.

I'm still no totally sure how the strength of a chainplate situation is understood from the hull perspective....

Lets assume (for a min) that the friction idea is fantasy. So the shear of the bolts is the load bearer. That would mean that the hull is suffering compression from each bolt. So from the hull perspective I would look at the compression str of my hull material and spec the number and size of bolts accordingly, with some safety margin.
Does this sound right?

There is also the spacing to think of. I could look at my existing chain plates for a guide there.

[a64pilot]

In aircraft maintenance there are two torque charts, one for tension and another for shear. Often "thin" nuts are used on a bolt in shear load as well.
I looked for such a shear torque chart for SS bolts used in non aviation applications and couldn't find one, only torque charts I could find did not specify shear or tension load, but as the numbers were high, I suspect tension.

[a64pilot]

The friction idea is not fantasy, it does exist. Just in my opinion it's not the majority of the strength, the majority of the strength comes from the shearing of the bolts in the FRP hull and the FRP hull itself in shear.
Another opinion of mine is the hull strength and shear load strength of the plates far exceeds the strength of the chainplate itself, and the chainplate strength far exceeds the wire that attaches to it. Other wise we would occasionally see whole sections of hull torn out in an extreme overload case, and I assume that has to be very rare?
On edit, I also assume that when chainplates do fail it's always due to it having been weakened most likely due to some form of corrosion, and not good metal that fails?

[DuncanC]

thanks a64pilot. I agree with what you say.

I am confident now to go ahead. I am intending to use an adhesive bedding compound (i.e. epoxy) between the plate and the hull (steel to concrete) to improve the 'friction' part.

I do have another niggle though. the bolts go through a hold in the hull. If that were a 'perfect' fit for the bolt then the area in contact would be higher for each bolt and the load on the hull better distributed. But I can't make 'perfect' holes!
I could use something like an epoxy filler when I insert each bolt to fill gaps and get a good matching of the bolt surface to hull. Has anyone tried this or knows a better solution?

FYI my hull is ferro cement, if that is relevant.

Someone told me that when put under load the bolts will crush/deform the hull very slightly till they have a nice fit...? Sounds dodgy to me!

[Cheechako]

Hmmm.... ferro. The load on the hull would be shear I imagine...? (is that possible?)the bolts are trying to pull thru the hull in the aft direction. I guess you could call that compression, the bolt is trying to compress the cement. I think friction is a consideration to some extent, and could definitely work for you on cement. If I was doin that on cement I would want a long chainplate with lots of bolts. OF course the trouble with that theory is the more holes the more stress breakage possible. Maybe long chainplates with wide hole spacing?

[hellosailor]

Duncan, it sounds like what you want to install are really spreader plates, not chainplates at all. Chainplates transfer a load perpendicular to the hull sides. Spreader plates will prevent your drogue from tearing out mounting points in the transom.

Or are you planning to install horizontal chainplates along the side of the hull, extending out at each side of the transom? I haven't heard of doing it that way but there's always something new.

[transmitterdan]

Jordan series drogues can apply a load to the mounting point about 60% of the displacement of the boat. When considering attachments imagine the boat hanging in the air from your chosen attachments. If you are sure the boat will not fall then you should have no issues.

The drogue needs the equivalent of chain plates at the stern to transfer the load to the hull. Chain plates do not rely on the bolts in shear. Most hulls would be cut by the shear loads. Backing plates are needed to spread the load to prevent the bolts from going into shear.

Recommend discussing with a ferro expert before designing drogue attachments. Ferro does not respond well to tensile loads. You might rip the stern out if not designed correctly.

[DuncanC]

The idea is to run chain plates horizontally, at the transom, with the tang poking out aft.

The idea has already been done a few times

Custom stainless steel chainplates on the stern of aluminum sailboat Morgan’s Cloud to hold the bridles of a Jordan Series Drogue.

Working out the shear yield of the bolts is easy.

I don't have any figures or way to check the measured strength of my cement so I found this UN Fisheries and Agriculture Dept. report to use as guide.

I cross checked against this report for the Canadian Fisheries Dept. This report also recommends that the steel reinforcement adds so little to the compression strength of ferro that it is usual to ignore it and use the compression str of the cement mix.

I think the force of the bolts against the cement is compression because the transom won't give way (lets hope!) so the cement between the bolt and the transom is being compressed between the bolt and the transom.

As for the risk of losing the transom...
What I am planning will be at least equivalent to twice my largest rigging chain plates. Early on I had an idea to extend the interior/backing plate forward, to the first bulkhead. Then bend the end of the backing plate 90 deg to allow me to bolt through the bulkhead. This would also disperse the load onto the bulkhead.
I could run the whole setup as far back as 2 bulk heads and repeat!

But after i started calculating strengths I decided this was perhaps a bit overkill.

[hellosailor]

Oh, ferro. That could change lots of things.

Why not use a bridle instead? No holes in the boat, no spot loads. Just a clip over the bow and some strops to keep the bridle positioned below the rail on both sides of the hull. All force would then be in compression on the entire body of the hull, and no penetrations would be needed.

[DuncanC]

hellosailor - Thanks for the question. The series/jordan drogue is trailed from the stern. You use a bridle (well a split bridle) from as aft and as wide as possible. That is why the horizontal chainplates at the transom are handy as they are wide and aft.

A rope goes from the port chainplate to a free rotating shackle - and the same from the starboard.

The reason for the exterior plates is also to avoid chaffe. It is recommended that the chaffe is so great that the bridle will risk being lost during use.
So if you splice and eye (around a metal eyelet) at the end of the bride the chaffe wont occur.

One onther thing I have noticed. Last night I decided to go out and look at my transome (what a difference look at it makes). My hull comes up past the deck to form a small wall around the boat. This is about 30cm high and 7 or 8 cm thick. This is much thicker that the hull (3ish cm). A bolt through the 'wall' would disperse its load of much more cement.
Also consider the catastrophic failure mode - the chainplate is ripped out. If this occurred in the 'wall' then the damage to the deck-hull cocoon would be less (or nothing) compared to it ripping out of the hull proper.

However you suggestion did make me think of using a chain strop from ondeck, through a fair-lead and out to the rope bridle. I could then use my post cleats (they could just have extra backing plates and easily be over engineered for the job). Some quick calcs show that my panama fairlead has about 60% more contact area with concrete that 12 x 20mm bolts - so good for the concrete. They are also in the 'wall' so I have the benefit of the extreme failure mode being outside of the waterproof cocoon.

One other note on failure modes. I want the fixing to the boat and the passage of the load out of the boat to have a wide safety margin so that they can easily take any load I will throw at them. However I will design in a 'safe' failure mode by joining the strop to the boat via a shackle with a breaking strain much lower than the boat fixings (but still above the max load) so that in some kind of extreme or unpredictable failure the shackle will go before anything else.

[cburger]

As part of the refurbishing of Moon Dance I installed new externally mounted chain plates. When the old ones were removed several of the bolt holes were slightly elongated so on installation of the new plates the contact surfaces between the plates and the hull were bedded with 5200 prior to bolting to hopefully spread the loads and relieve point loading of the bolts against the glass. The next owner will have to see if this has improved the setup. As the fasteners are carriage bolts and there was no way to get a torque wrench on the nuts the assemblies were hand tightened.

[hellosailor]

"Also consider the catastrophic failure mode - the chainplate is ripped out. ... in some kind of extreme or unpredictable failure the shackle will go before anything else. "

Precisely. With a properly engineered system, something intentionally sacrificial should always fail first.

cb-
If you were using carriage bolts, why couldn't you use a torque wrench on the nuts then? Presumably you had some kind of wrench on them to tighten them. Just no room for access?