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Itsabimmerthing

Keel joint hairline cracks

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Hi,

I have recently faired bottom, keel hull joint has fair amount of fairing in it.Bottom is painted with Light primer and then Durepox and final coat of durepox clear.

Last time I was cleaning the bottom I noticed hairline cracks on the portside fwd and more further back. They are clearly visible but can hardly be felt by hand. Keel was off last winter, structure is GRP hull with internal galvanized frame.

We had some races with pretty rough seas and few major broaches that could have caused these? My question is: Is this normal or should I do something about them other than fair them and touch up with durepox?

Thanks for the replies in advance!

 

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Unless it's weeping water into the bilge I'd check the torque on the bolts, fair it in and keep an eye on it when next hauled.

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I'm not an engineer but I suspect it would probably be best done on land with the keel blocked to take tension off the bolts.

You can find torque tables on the Interweb that will tell you the proper torque for your bolts - it is determined by the bolt diameter, thread count and material of the bolts.

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My understanding was/is that keel bolts do not need "re-tensioning" all bolts were properly torqued and bolts secured by double nuts.

Anyways, I understand small cracks are quite common? I do not like to see them though.

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Put it back in the water. Then you can't see them. Problem solved. 

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8 hours ago, Itsabimmerthing said:

My understanding was/is that keel bolts do not need "re-tensioning" all bolts were properly torqued and bolts secured by double nuts.

Anyways, I understand small cracks are quite common? I do not like to see them though.

Normally no but grounding can affect their tension.

It's just a check.

Hairline cracks are not uncommon, especially if the fairing was done with a hard filler.

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1 hour ago, SloopJonB said:

Normally no but grounding can affect their tension.

It's just a check.

Hairline cracks are not uncommon, especially if the fairing was done with a hard filler.

As SloopJonB said, the bolts can "stretch" (plastic deformation) during a grounding.

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No grounding. Was cleaning the bottom today and took a look, again. It is very very thin hairline crack, can not feel it by hand. Hopefully on the surface coating only and easy touch up fix. Boat will be lifted for winter soon anyway.

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Be careful with bolt torque guides as they are predominantly designed for bolting steel structures not comparatively soft boats together. 

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On 9/2/2018 at 12:21 PM, Alcatraz5768 said:

Be careful with bolt torque guides as they are predominantly designed for bolting steel structures not comparatively soft boats together. 

This is wrong. Bolt torque guides don't take into consideration the material being clamped, only the fastener's proof strength.

Of course if you're torquing a 1" fastener compressing a 1/8 sheet of glass, it will likely crack. The compression strength of the material being clamped needs to be greater than the fastener's tensile strength to be able to torque it to spec.

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10 hours ago, chinzw said:

This is wrong. Bolt torque guides don't take into consideration the material being clamped, only the fastener's proof strength.

Of course if you're torquing a 1" fastener compressing a 1/8 sheet of glass, it will likely crack. The compression strength of the material being clamped needs to be greater than the fastener's tensile strength to be able to torque it to spec.

That's a much better way of saying what I was trying to say. 

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13 hours ago, chinzw said:

This is wrong. Bolt torque guides don't take into consideration the material being clamped, only the fastener's proof strength.

Of course if you're torquing a 1" fastener compressing a 1/8 sheet of glass, it will likely crack. The compression strength of the material being clamped needs to be greater than the fastener's tensile strength to be able to torque it to spec.

The beauty of technical language! 

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18 hours ago, chinzw said:

This is wrong. Bolt torque guides don't take into consideration the material being clamped, only the fastener's proof strength.

Of course if you're torquing a 1" fastener compressing a 1/8 sheet of glass, it will likely crack. The compression strength of the material being clamped needs to be greater than the fastener's tensile strength to be able to torque it to spec.

Hairline cracks at the lead/GRP junction are almost unavoidable, eventually. Just from the differential thermal expansion if nothing else. Also GRP absorbs some water and expands a bit. 

As on the other thread, torque values for keel bolts is not a simple subject. It isn't just the compression strength of the material being clamped, it is the stiffness as well. And the fastener need not be torqued to any particular percentage of its proof strength, only some amount greater than the maximum load it will see. It's end fixing condition also matters: there needs to be backing sufficient in strength and stiffness under the nut, and an understanding that the other end is likely just a J bend cast in lead, the lead being far softer than the steel. So the fact that bolt torque guides do not take into consideration the material being clamped is in fact a problem. Those guides tend to assume you are clamping an infinitely strong, infinitely stiff material. 

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