Cored deck repair, epoxy or polyester resin?

[geem]

I fell out with the surveyor. Useless individual. I got a grand out of him as conpensation. He just said, at the time, that he had tapped stringers and could tell foam had seperated from glass, therefore needed replacing. Trouble is i am left with this on the survey report.

What's the boat? Somebody knowledge with the boat may know more about the stringer design.
I had similar experience with the surveyor when I bought my current boat. He refunded the whole survey cost. A useless individual

 

[Ceirwan]

Is this really true? Surely, both in the case of a deck and a stringer, both sides are in compression. It's not a beam with a load on one end.

Yes its really true.
With respect, I'm not going to go over basic engineering principles and try and prove it to you, that's how it works. There are lots of online articles, that go over it way better than I can.


In the case of a typical top hat style stiffener, then its a much smaller unsupported span than say the deck, which means that , the core is doing less work.
However lets say the core is made of oak or some other hard wood, or a very dense foam, (some are dense enough that you can even use them under a mast support) then that can add quite a bit of strength in its own right. If its a low density expanding foam, it won't do much.

If you have an older 60-70s era boat, then they probably overbuilt it anyway, but of course we can only help so much over the internet. you need a surveyor who knows his stuff.

 

[thinwater]

Without pictures ... yes and no.

If the stringer is narrow enough to be well supported by the sides, and the sides rise at a steep angle, the core does nothing. My F-24 has one very large, hollow beam that supports the mast.

On the other hand, if the stringer or beam is wide or if the sides form a shallow angle, the core may be structural. The outrigger beams (akas) on the trimaran are of a different shape and the foam is structural. Instead of vertical sides, they are at a lower angle to cut through waves. Exaggerated to its logical conclusion is a cored deck, where the core maintains the space between the skins and carries a the entire shear load.

[Look at the above drawing. Notice that in the lower drawing the lower skin is longer than the upper skin. The core or sides (web) have to be strong enough to maintain the spacing and prevent the two skins from sliding relative to each other. A common I-beam failure and is web sheer. It is most common failure with cored decks.]

This is not a yes or no question.

 

[IanCC]

Yes its really true.
With respect, I'm not going to go over basic engineering principles and try and prove it to you, that's how it works. There are lots of online articles, that go over it way better than I can.
View attachment 168146

In the case of a typical top hat style stiffener, then its a much smaller unsupported span than say the deck, which means that , the core is doing less work.
However lets say the core is made of oak or some other hard wood, or a very dense foam, (some are dense enough that you can even use them under a mast support) then that can add quite a bit of strength in its own right. If its a low density expanding foam, it won't do much.

If you have an older 60-70s era boat, then they probably overbuilt it anyway, but of course we can only help so much over the internet. you need a surveyor who knows his stuff.

Thanks for your patience. But am still not convinced. A deck or a stringer are not flat beams they are arches or parabolas or somesuch. I look at classic pack horse bridges and other arches made of individual stones or bricks. The undersides of which are not under tension otherwise they would drop to bits, they are under compression.

 

[IanCC]

What's the boat? Somebody knowledge with the boat may know more about the stringer design.
I had similar experience with the surveyor when I bought my current boat. He refunded the whole survey cost. A useless individual

Nic 31

Click image to see.

 

[IanCC]

Without pictures ... yes and no.

If the stringer is narrow enough to be well supported by the sides, and the sides rise at a steep angle, the core does nothing. My F-24 has one very large, hollow beam that supports the mast.

On the other hand, if the stringer or beam is wide or if the sides form a shallow angle, the core may be structural. The outrigger beams (akas) on the trimaran are of a different shape and the foam is structural. Instead of vertical sides, they are at a lower angle to cut through waves. Exaggerated to its logical conclusion is a cored deck, where the core maintains the space between the skins and carries a the entire shear load.

[Look at the above drawing. Notice that in the lower drawing the lower skin is longer than the upper skin. The core or sides (web) have to be strong enough to maintain the spacing and prevent the two skins from sliding relative to each other. A common I-beam failure and is web sheer. It is most common failure with cored decks.]

This is not a yes or no question.

Thanks for reply, really informative and really interesting. If you have a couple of minutes have a look at my last two responses to other contributions.
Regards.

 

[thinwater]

Thanks for your patience. But am still not convinced. A deck or a stringer are not flat beams they are arches or parabolas or somesuch. I look at classic pack horse bridges and other arches made of individual stones or bricks. The undersides of which are not under tension otherwise they would drop to bits, they are under compression.

A stone bridge is arched in the reverse direction of the post 62 illustration. Yes, they are are under compression, pushing against the banks. Different maths. Built of stone, yes, the bridge in post 62 is impossible. Yes, some stringers and arches have both skins under compression. The illustration 62 is a very common case and is the common worse case scenario.

You can and should always have a specialist confirm any thing serious a surveyor rejects. He has to be a generalist, looking at the whole boat, and cannot be fully knowledgeable in all things. He is not a composites guy, a marine electrician, a diesel mechanic, and a rigger.

 

[IanCC]

A stone bridge is arched in the reverse direction of the post 62 illustration. Yes, they are are under compression, pushing against the banks. Different maths. Built of stone, yes, the bridge in post 62 is impossible. Yes, some stringers and arches have both skins under compression. The illustration 62 is a very common case and is the common worse case scenario.

You can and should always have a specialist confirm any thing serious a surveyor rejects. He has to be a generalist, looking at the whole boat, and cannot be fully knowledgeable in all things. He is not a composites guy, a marine electrician, a diesel mechanic, and a rigger.

Thanks for reply. When i had the survey done i asked surveyor for name of a composites engineer and tried myself, but no luck. I am satisfied a curved deck is under compression when loaded from above and the two sides of the boat are held together by multiple bulkheads. The lower skin has to shorten to descend and is therefore compressed. Likewise my stringers.