>> >Many years ago we built some 12-14' skiffs with extruded polystyrenecore.
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>> > They worked pretty well, but one of them had some major delamination which proved to be caused by paint solvents passing through pin holes and dissolving the core. They also suffered a lot in high impact areas.
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>> Infusion would not leave pinholes, right?
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>Yes. The laminates we were using were very light, and woven so pin holes were near inevitable.​

Sorry for my confusion. You vac bagged and infused these panels? Or wet layup? And vac bag?

Can one expect pinholes in every light skin weight infusion?

>> >The light skins would deflect the core, which would not spring back fast enough so the skin would debond. We also use polystyrene for the impact absorbing bows on most harrys. With a layer or 2 of glass over it, depending on the size and density of the foam), it works well.
>> >The problems are the low impact, shear and compression strength and modulus so it can't be used in high load areas (between the rudders on the lee hull or anywhere that is walked on or sees high local loads, like the hull bottoms), which does not leave much.
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>> Do you remember what type of polystyrene you used? There are many grades, but if it was construction insulation, it was probably only 15psi. Do you have any interest in seeing if higher grades are worth it? If you exceed the psi (kPa) it deforms.

>​15 psi is vacuum, so it was stronger than that. It was putruded blue Dow styrofoam, if that helps.​

Dow has lots of different variations of there products, so its hard to say. My understanding is Dow typical residential construction, 'utilitary' brand, Styrofoam sheet is 15psi compressive strength. (4x8 sheet in the big box stores). Afaik it is vac bagged and does not deform noticeably in the process. The 15psi is a minimum, so perhaps it is actually stronger. It may have been 25psi, as they have some variants that are 25, like the 'SM' subtype.

There are higher grades, also for wall insulation, and under concrete slabs.. They vary from 25 to 30 up to 100psi. Generally also following the 40, 60, 100 pattern.

I found this thread
https://www.boatdesign.net/threads/extruded-polystyrene-xps.24213/
In this post
https://www.boatdesign.net/threads/extruded-polystyrene-xps.24213/page-5#post-504270
Groper seems to suggest the 60psi (415kPa) HIGHLOAD 60 grade XPS can cut it with 676gsm cloth.

Seems like that's a lot of the boat?

>> You don't spec any lower grades than H80 either? Like H60 or H40? Is it not worth it? In the engineering software, can you just substitute a different material to see if it works? I think the engineering info is in the sheets I linked. Or is this a 'we know what works, so we do that' thing?
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>​Mostly it's cost vs weight. there are plenty of places you can use the lighter foam, but the weight savings are not huge, the cost difference (here) is.​

So the lower spec, lighter H40 and H60 are more expensive? Weird. I was thinking they would be cheaper

>> >The greasy surface is hard to bond to, but a hot wired surface is not much better as it leaves a smooth(ish) surface with none of the partially open bubbles that increase the bonding area.
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>> Good point. Although, one would be treating the surface for an infusion, so i imagine it might have enough extra grip? And one could sand the HotWired surface. At least the grease is gone.
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>​Sanding the Dow stuff was not much fun. The surface was hard, then it was gone, so a lot of divots and unevenness.​

Do you think adding infusion channels and holes should give enough gripping surface? It has to be more grippy than the factory flat surface?

>> >I agree that H80 is costly. Some savings can be made by using it in squares with say 50mm/2" spaces between them where the skins are joined.
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>> I do not understand this. Is this a hollow box section? Can i get more details?
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>​Take a sheet and cut 2" wide pieces out of it at 500mm centres in both directions. Angle the edges ​and glass it in. The skins meet where the cut outs are. The vac bag needs to be applied carefully or you will get big pubbles of resin in the spaces where it did not sit down tight.

Perhaps there is a disconnect. When you said costly, I thought you meant spendy. I now think you meant heavy. So I think you mean this is a weight savings technique. Which is very neat.

Or is this a money saving technique, and i missed something? Can you reuse the cut off parts?

>> I need to get an intelligent infusion kit from you and get started doing this. Do the foam holes follow the feed and vac lines over the part?
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>​I don't have a kit, try your local materials supplier.

The 10m camper mentioned a canoe tender. Is that and the intelligent infusion instructions available?

>> I appreciate your tolerating my questions. I hope they do not bother you too much.
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>​Not at all. I can, and frequently do, talk boats, building and rigs all day long. Keep it coming.​

How blessed are you then! Challenge accepted!