Cored deck repair, epoxy or polyester resin?

[B27]

Why would sheer be even considered ??

The reason for core is to avoid the weight penalty of solid GRP ... to use two thin layers of GRP held apart by a core of light material. It is the same principle as the metal I beam ..... two bearers separated by vertical

It's a pretty basic point in structures.
If you take a thin bit of wood you can bend it
IF you take two thin bits of wood and put them together, it's twice as stiff if the two layers can slide over each other.
Bond them together with good shear strength, it's 8 times as stiff as the single bit of wood.
Foam cores will usually fail where they are bonded to the FRP, because that's where the stress is highest in the foam.
If the web of your I beam had no shear strength, the beam would reduce to two thin plates which would bend under their own weight.

 

[B27]

FWIW, some highly stressed dinghy rudder blades and centreboards are made by moulding the two skins then injecting the foam core.

 

[Refueler]

It's a pretty basic point in structures.
If you take a thin bit of wood you can bend it
IF you take two thin bits of wood and put them together, it's twice as stiff if the two layers can slide over each other.
Bond them together with good shear strength, it's 8 times as stiff as the single bit of wood.
Foam cores will usually fail where they are bonded to the FRP, because that's where the stress is highest in the foam.
If the web of your I beam had no shear strength, the beam would reduce to two thin plates which would bend under their own weight.

I will stick with the 4 years of GSK I was lectured on by experts and obtained one of the highest exam results of the year. GSK subject includes all aspects of Ships Construction covering sheering forces, bending moments, beam theory ... just to quote a few of the many in the subject.

 

[Ceirwan]

I will stick with the 4 years of GSK I was lectured on by experts and obtained one of the highest exam results of the year. GSK subject includes all aspects of Ships Construction covering sheering forces, bending moments, beam theory ... just to quote a few of the many in the subject.

I actually agree with you that the foam (expanding) can be used for smaller localised repairs.
But you are incorrect to say that shear strength is unimportant.
Yes the two cores need to be held apart so need compressive strength, but a structure like a deck when having a force applied (like someone walking), will have one skin in tension & the other in compression.

The core has to resist the shear loads of the two.



That's composites 101. If your course on ship construction covered cored composites, then they will havev taught this for sure.

 

[thinwater]

I have tested many skin/composite structures in the lab. The most common failure, by far, is sheer failure of the core. This is why balsa is so good structurally. Higher sheer strength than other cores. Builders foam would dramatically reduce the strength of the "beam." That said, most decks are considerably over built, so I'm not entering the debate. Only race boats, engineered close to the breaking point, are at serious risk in most areas.

The real problem, or risk, of just injecting something, is that the rotten balsa is probably compost, getting it dry is unlikely, and you aren't really going to get any real bond. Certainly, you don't know. The deck will certainly feel harder, but it may not have gained much strength through the repair. Would you smear wet soil on a surface before applying epoxy? Of course not.

It's not that hard to cut, clean it out, and do it right.

 

[CanePazzo]

A very interesting thread/argument because I am completely refitting my 1981 Oyster 46 and have had various ‘showers’ when removing all the deck fittings. I am very aware that I have a lot of deck core replacement to be done. Watching various YouTube channels, it seems that most people restoring boats of this age have pretty much had to replace the core on most of their decks. I will be using ready thickened West System epoxy for bedding down the replacement core and a thickened epoxy for reinstalling the cut out top skin.
Questions:
1: What is recommended as a replacement core material? It was balsa but once it gets wet, the wet travels. I was thinking some closed cell foam thing?
2: What on earth am I doing this for?

 

[IanCC]

I actually agree with you that the foam (expanding) can be used for smaller localised repairs.
But you are incorrect to say that shear strength is unimportant.
Yes the two cores need to be held apart so need compressive strength, but a structure like a deck when having a force applied (like someone walking), will have one skin in tension & the other in compression.

The core has to resist the shear loads of the two.

View attachment 166565

That's composites 101. If your course on ship construction covered cored composites, then they will havev taught this for sure.

I appreciate this is a resurrected thread from a month a go.

If you don't mind me asking. Why does the West System book say that the foam in cored stringers is of no structural value, or words to that affect? I can't imagine foam having much strength of any sort.

 

[bergie]

1: What is recommended as a replacement core material? It was balsa but once it gets wet, the wet travels. I was thinking some closed cell foam thing?

We used 3D Core:
3D|CORE™ SHEETS
Pretty easy to shape to fit our deck, and the honeycomb when filled with epoxy should keep water from traveling even if some deck fitting would leak.
The only annoyance was that the little hexagons break off easily, so you need proper support when cutting it.

2: What on earth am I doing this for?

Our boat used to make creaking noises when sailing in big waves. No such noises after the deck rebuild. So we got a much better stiffer deck as result.

 

[Boathook]

I appreciate this is a resurrected thread from a month a go.

If you don't mind me asking. Why does the West System book say that the foam in cored stringers is of no structural value, or words to that affect? I can't imagine foam having much strength of any sort.

The foam is non structural so is 'used' to form the grp into a shape that is structural. An example of an 'I' beam was mentioned in an earlier post.

 

[IanCC]

The foam is non structural so is 'used' to form the grp into a shape that is structural. An example of an 'I' beam was mentioned in an earlier post.

Thanks for response.
So was my surveyor wrong when he tapped the stringers forrard and said, the foam is detached from the fibreglass inside the stringers and therefore they need replacing ?

 

[Boathook]

Thanks for response.
So was my surveyor wrong when he tapped the stringers forrard and said, the foam is detached from the fibreglass inside the stringers and therefore they need replacing ?

A lot depends upon how the foam is used and the shape of the stringers. Some decks are 2 flat panels with the foam separating them and the foam can add some structural strength. Other decks have ribs where the foam is mainly a light way of making a shape. My boat is so old it has a mix of foam, balsa and even ply to make the ribs / stringers. The ply does seem to used mainly where items are bolted through.

 

[Ceirwan]

Thanks for response.
So was my surveyor wrong when he tapped the stringers forrard and said, the foam is detached from the fibreglass inside the stringers and therefore they need replacing ?

To answer this and your above question.

It depends, if you are just using the foam as a form, and all the strength is coming from the glass, then the bond between foam and glass is not important, its just a lightweight thing to get the shape you need.

However with denser foams and other core materils, they also add strength to the floor / stringer / etc.
In the case of these ones, then yes the bond between the core and the material is important.

If you watch one of Sail Lifes videos, they had rotten core in the floors and stringers. Rather than ripping it all out, someone was able to do an FEA analyis for him to determine how many layers of epoxy and glass to lay over the existing structure to replace the lost strength of the core.

 

[IanCC]

To answer this and your above question.

It depends, if you are just using the foam as a form, and all the strength is coming from the glass, then the bond between foam and glass is not important, its just a lightweight thing to get the shape you need.

However with denser foams and other core materils, they also add strength to the floor / stringer / etc.
In the case of these ones, then yes the bond between the core and the material is important.

If you watch one of Sail Lifes videos, they had rotten core in the floors and stringers. Rather than ripping it all out, someone was able to do an FEA analyis for him to determine how many layers of epoxy and glass to lay over the existing structure to replace the lost strength of the core.

Thank you for response. Really interesting and makes sense. I haven't watched video yet, but I wonder if there is an easy way to decide which way mine are inclined. 1979 boat, i know the lay up or at least what it is meant to be. Food for thought.

 

[IanCC]

However with denser foams.... then yes the bond between the core and the material is important.

I don't just see how this can be true. I am no engineer, but it would seem to me that foam of whatever density has a lot less shear strength than the glass. It would also seem to me that that stress is at a maximum at the join between the glass and the foam. I can't imagine any foam withstanding that.

I found the vid you recommended. Very interesting. His stringers, however, are ply and conform to the West model. However it would appear that 6mm skin thickness is satisfactory so will chaeck mine.

 

[geem]

I appreciate this is a resurrected thread from a month a go.

If you don't mind me asking. Why does the West System book say that the foam in cored stringers is of no structural value, or words to that affect? I can't imagine foam having much strength of any sort.

Thanks for response.
So was my surveyor wrong when he tapped the stringers forrard and said, the foam is detached from the fibreglass inside the stringers and therefore they need replacing ?

If the foam was simply used to create the stringer on an otherwise solid glass hull, then the foam has no structural role. It is simply used as a former for the grp layup that makes the stringer.
Foam core or balsa is used in decks because of its stiffness when sandwiched. You achieve incredible stiffness for very little weight. Two relatively thin grp sheets with foam between are way stiffer than just a simple solid grp lay-up

 

[IanCC]

If the foam was simply used to create the stringer on an otherwise solid glass hull, then the foam has no structural role. It is simply used as a former for the grp layup that makes the stringer.
Foam core or balsa is used in decks because of its stiffness when sandwiched. You achieve incredible stiffness for very little weight. Two relatively thin grp sheets with foam between are way stiffer than just a simple solid grp lay-up

Thanks for response.
Yes, i do get this, but am not sure if the foam in my stringers is more than a former and needs to be reinforced with extra glass or replaced.

 

[IanCC]

but a structure like a deck when having a force applied (like someone walking), will have one skin in tension & the other in compression.

The core has to resist the shear loads of the two.

View attachment 166565

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.

 

[geem]

Thanks for response.
Yes, i do get this, but am not sure if the foam in my stringers is more than a former and needs to be reinforced with extra glass or replaced.

Ask your surveyor why he thinks it needs replacing. I have seen stringers with rotted out balsa core and there was no problem with hull stiffness.

 

[IanCC]

Ask your surveyor why he thinks it needs replacing. I have seen stringers with rotted out balsa core and there was no problem with hull stiffness.

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.

 

[IanCC]

Deleted.