Discussion in 'Class Societies' started by DUCRUY Jacques, Dec 1, 2009.

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### DUCRUY JacquesJunior Member

Hello,

I am new in this forum : please, excuse my bad english (I am French).

I had a question about the fastening of a chainplate on a plywood bulkhead : how to calculate the required nomber of bolts ?

Is the following formula correct :

n = L / (d t Sb) where

n is the nomber of bolts
L is the breaking load of the shroud multiply by 4/3 (in N)
d is the diameter of bolt (in mm)
t the thickness of the bukhead (in mm)
Sb is the bearing stress of plywood (in N/mm²)

If this formula is good, It seem to me that the more "mysterious" is the choice of the bearing stress : I had read that the bearing stress is the product of the compressive stress by a factor in relation with the ratio t/d
but in ISO 12215-9, the calculation of bearing stress is independant of the ratio t/d.

What is the good method ?

Jacques

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### MikeJohnsSenior Member

Hello Jacques and welcome to the forum

Yes your formula looks correct, and bearing stress is the local ultimate comprssive stress for your material. It's not a rato of dimensions.

I think you would be better working to a minimum of 1.6 times the braeaking load of the attached stay preferably 2.
When the chainplate is bolted onto a bulkhead you use a backing plate of the same area. The friction from these plates when bolted tightly will distribute nearly all the load anyway.

I hope this is of some help
cheers

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Also don't forget to make sure the spacing of the bolts are not too close together nor too close to the edge of the plate.

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### DUCRUY JacquesJunior Member

But, for this calculation, what is the safe bearing stress for marine plywood : I had read values between 20 and 5 N/mm² ?

Best Regards

Jacques

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### fastwaveSenior Member

t/d and bearing stress

I am not a specialist structural engineer but here is what I know.

The t/d is important when it comes to bearing stress and bolts. The maximum achivable depends on t/d. Usually you should not go below t/d of 1 if you want to get near the max. If you clamp the bulhead between 2 plates then this changes and you can go thinner.

Be carefull about relying on the clamped plate to distribute the load. Although it is perfectly valid people tend not to inspect and maintain this and if the bolts are slack then it is a huge reduction in safety factor if it is relied on.

Using a safety factor of 2 over the breaking strength of the rigging is excessive. The rigging has a SF already. What your should aim is that the rigging breaks before the bulkhead, then you do not get a big hole in the boat.

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### warddSenior Member

another point

make the holes at least a slight press fit if possible

this will eliminate the possibility of one bolt bearing the full load then failing and causing a cascade of bolt failures

and look into class 5 or 8 bolts

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### Tom.151Senior Member

And yet another...

Consider creating a compression tube of extended epoxy in the plywood so that there is no crushing of the wood when the bolts are tightened. Even with backing plates it's a good idea. Many if not all of the best wood/epoxy builders consider the sleeving of all boltholes to be mandatory.

I prefer a nice big fat compression tube to better spread the shear loads of the bolts into the ply. It simulates having a much larger diam bolt going through the plywood.

8. ### mark775Guest

"Using a safety factor of 2 over the breaking strength of the rigging is excessive." Then what? You did state that "What your should aim is that the rigging breaks before the bulkhead." !.6?

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