Chain Plates

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raro3

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I have an old Roberts 34 with external chainplates bolted through the hull, each with 3 x 6mm s/s bolts. The standing rigging is 8mm and I'm concerned that the bolts may not be beefy enough. I don't race, so there's no huge load on the rig. It will be a real pain the change the bolts as the yacht was refitted (not by me) without access to them on the inside, but nevertheless I'd rather the mast remained in the vertical position. Anyone who can advise me on this? Cheers
 
Doesn't seem anywhere near enough to me. On our 30 footer they're secured with 6 x M8.
Are you sure that there isn't a web of metal extending from the plate into the boat which is then secured inside.
 
There's a steel backing plate inside for each of the 3 chainplates on each side of the boat, through which the 3 bolts are secured
 
I would suggest you find someone with the same boat (which is probably what the original post was about), better still contact the designer or builder.
 
It seems a little flimsy, but in theory it should be Ok: The three bolts should have a minimum shear capability of about 5 tons, whereas the wire should have a maximum tension of about 1 ton, even with some shock loads. The wire itself will have a tensile strength of around 5 tons. It looks to be alright, as long as the material through which the chain plates are bolted is of adequate strength.
 
I have much lower figures for SWL (shear) for M6 stainless of 2KN which is 200Kgf
3 x that is only 600Kg. UTS (breaking strain) is about 4 times that, about 2.5 Tonnes.

That all of course neglects any corrosion problems or damage to the fixings, or indeed any quality issues with the screws.

I reckon that if that's the only thing holding them you need to do something.
 
Rule of thumb-I think you should be able to 'carry' the weight of the entire boat on the combined lower shrouds on each side.

6mm bolts acting in sheer,with the fair chance of some water penetration/salt/crevice/anairobic decay around each through hole...-I would certainly want to create access to the backing area and knock out a couple for a good looksee at the very least.

Presumably there is more than enough 'meat' on the chainplates to drill out and countersink the holes to accept new 10mm bolts?
 
Blimy, it sounds much stronger than the set up on my ald Atlanta. Just 4 x 6mm bolts through the wooden deck with a thin steel backing plate and a plywood pad to spread the load. In my case I would have thought that the effort need to pull the whole lot through the deck was a ton at most??
I always thought that plates through the hull must be more reliable though take the point that bit is the diam of the bolt which is critical here.
Dare I put the sails up again??
 
Thanks for all the advice. As suggested I wrote to Bruce Roberts and, believe it or not, I already got a reply (that happened before when I had a question for them, pretty impressive eh?). Anyway, they said the 3 x 6mm bolts sounds OK, but suggests I add another one for good measure. If I'm doing that, since it means cutting an access panel, I may just as well upgrade them to 10mm. By the way, the chainplates and backing plates are very heavy duty, no problems there.
 
andamanman,

I take it your "elderly Roberts 34" is a steel one? Though it doesn't really affect things.

Your bolts are only acting in "shear" if they are slack. Which they should NOT be.

If they are tight - which they should be - they are acting in "tension"

They press the chain plates tightly against the hull causing a massive amount of friction between them. It is this friction which stops the plated moving.
High tensile bolts are capable of being done up much tighter than ordinary ones, thus holding them together even more.

Provided the bolts are in good nick ( and the area round them is too) then you would be amazed at the effort needed to move them.
There is a popular misconception about shear strength of bolts.
If you are talking about shear strength problems, then you are looking at pins - like a clevis pin - not bolts
BTW. rivets work in the same way as bolts.
 
I had one friend with a 21ft boat where the chasin plate bolts let go. I think there were 4x6mm. I don't really know why they let go but some sort of corrosion or stress failure I presume.
Anyway if you can either add more bolts or larger bolts it would certainly be a good thing. (and add to you peace of mind) These kind of bolts are often compromised in strength when the thread is cut and add to that the possibilities of different types of corrosion it would be good to beef it up. Racing or not your boat will often be subject to laying over in a gust which is where the stay needs to be strong. olewill
 
When I dropped the stick of my Holland 25, I replaced the chainplates (one had failed) and discovered that 4 of 6 1/2 inch bolts were cracked - they were all inside the boat, clamping the plates through a 3/4inch ply bulkhead into a 50x6mm backing plate. Both chainplates were cracked too.
 
My Roberts, unlike most of them, is GRP not steel. It was completely refitted here in the UK before I bought her late last year and all the bolts are new.
 
I would agree with your figures and conclusion apart from the fact that the situation is indeed complicated by the fact that friction between the plates and hull are also factors as someone else said. However, for this purpose this should be disregarded and the shear strength should be calculated to be sufficient with slack bolts, which, after time could, due to corrosion, elongation of the bolts, or other factors be the case. Its also true that there are big differences between set screws and bolts. If a bolt is used the plain diameter under the head should be sufficiently long to go through the plate, hull, and backing plate, but, have a packing washer under the head to ensure the nut does not run out of thread. Under these circumstances the set up is marginal but probably OK. A set screw with the thread right up to the head should not be used in shear however unless well oversize. If these are setscrews, not proper bolts I would do something about it immediately. /forums/images/graemlins/frown.gif
 
Sheer on a bolt is far higher than the wire can take ... the sheer figure on a bolt increases substantially when its tightened and fixed with plates either side.

You would be surprised what can hold together in sheer when properly tightened.

A Kenter shackle on a ships anchor has a spile pin that acts in sheer ... the pin is soft and surprisingly small in diamater. That shackle may be carrying a physical drop weight of 20T anchor or more and itself ..........

I would suggest making access hatches internally though ... doing it when you have all the time in the world - can be neat and cosmetically OK .... doing it in an emergency means damaged surroundings and unsightly mess.
 
If it helps, Avocet's (27' boat with a masthead rig) backstay is held on the same way but with 4 set screws - each 1/4" UNF and stainless. Although I accept that when properly clamped, there will be a great deal of friction, if it was me sizing the bolts, I wouldn't rely on it. You can't get that much clamping force with a 6mm "A2" stainless set screw. Even if you do, there's always the chance of relaxation in the joint. If it does loosen - for any reason, the situation could be worse than 3 bolts in shear because the holes might not line up preceisely so one bolt could end up taking a larger share of the load. I think I'd be inclined to size them on the basis of all of them being able to equal the ultimate tensile strength of the shroud but in shear. The only thing to be careful of in that situation is that you don't weaken the chain plate too much by opening out the hole to 8 or even 10mm.
 
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