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Old 16-11-2015, 07:15   #1
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Repairing planked core

Hello helpful folks,

I'm knee deep in a deck repair. I noticed water migration indicating wet core, likely getting in from all the fasteners in the teak deck.

Carefully removed all the planks (im boring out the fastener holes and replugging, recutting the rabbets, and will ultimately reinstall with Teak decking system's purpose-made epoxy, no fasteners)

Next I cut out the top layer (5/16") of fiberglass where it was particularly soft. It revealed a core which is made up of 3" wide, 1/2" thick planks running longitudinally. The wood looks good but its clear water has been running between the planks. They are no longer adhered to each other or the deck fiberglass.

My plan is to let it dry out over the winter, then get a low viscosity (but not thinned like CPES) epoxy resin and just go to town filling between the channels in the core planks, etc, and then rebond the top layer of glass, attach the borders with generous bevel, then put a final layer of cloth over everything to make sure i've undone the leak potential of 1000 little deck screws.

A deck construction should be rigid and so while i'm tempted to consider something like g-flex (flexible epoxy) between these core planks, since a little flexibilty is often warranted with long lengths of wood running side by side, I am still leaning toward just a good strong resin, perhaps with low viscosity to seep well. Any thoughts?
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Old 16-11-2015, 07:56   #2
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Re: Repairing planked core

Hello;

You mentioned "core", so assumption is there's an inner ply of FRP? Penetrating epoy would soak the planks, as they should have been ; but will make a mess inside/between the lams. Ya need a way tosqueeze the inner lam tight to the core,as well as the outer skin..
Perhaps some strategically bored holes thru the planking at center points (most likely to NOT get any resin from between joints) to inject resin; then, thru-bolts to draw lams together? Once cured, simply fill the holes with thickened resin
The holes would allow inspection to determine just how far the inner has separated from the core.This makes gauging the amount of resin easier. Clamp/thru-bolt (or screw?) with boards wrapped in poly film and work from low to high point, using net set of holes to monitor resin squeeze-out (mindfulnof aforementioned voids)..

Outer lam the same time. Recommend slightly thickened resin to fill usual variations in surface. A few etra holes for the resin to push out where too much applied Easy enuff to trim once all done.

Plan, plan and plan some more. run thru the process mentally several times and have all mat'l. to hand. Then dry run again!'
Nothing like having to fetch something off the boat when ya have a hot batch of resin cooking in the lams

HTH,
Paul
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Old 16-11-2015, 08:13   #3
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Re: Repairing planked core

Cored panels stiffness often lost once the skip cut open. Hence the better method is to repair from below where the cut/seam can be laminated over afterwards.

You will want max adherence between the core and each skin. And then you want the skins to be continuous. It is in the compression vs. tension (and then shear in the core) but if you use wood core, shear will be sorted out plain vanilla. Once the skin is cut, it has zero resistance to tension/compression and the stiffness is gone.

It is not the core material stiffness nor the glue method that gives stiffness to a cored deck, it is the mechanical properties of the cored/sandwich technology.

I have seen cored panels fixed with epoxy injected then vacuum applied - this eliminates cutting the skin.

If the cut is on the top and not to close to the decks edge, the whole area can be sanded down to glass then roughened and glassed over with expoxy/glass 'overdeck', Then you can either paint it over or lay up non-skid or wood.

b.
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Old 16-11-2015, 08:29   #4
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Re: Repairing planked core

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Originally Posted by barnakiel View Post
... You will want max adherence between the core and each skin ...b.
INDEED!
In a cored sandwich panel, the (ususally light weight) core functions like the connecting web of an I-beam, separating the face skins at a constant distance, while the skins themselves function as the I-beam flanges.

Because the shear stresses in the composite material changes rapidly between the core and the skin, the adhesive layer also sees some degree of shear force. If the adhesive bond between the two layers is too weak, the most probable result will be delamination.
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Old 16-11-2015, 09:42   #5
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Re: Repairing planked core

Yep - all good points about what makes a sandwich deck so stiff.

To re-adhere everything, the "clamping" force will be my ~500 pounds of 10 pound lead bricks that are currently serving as trim ballast. There is temporary framing under the deck which this weight will push against to sandwich the whole works together, one section at a time. Although the through-bolt clamping trick is a great one, and I may use that in some areas.

My exact question is what sort of material would folks use to fill in the gaps between the planks of this planked core? On the one hand, assuming all laminates get properly re-adhered to the core, the thing is going to be super stiff again and shouldn't have any motion. On the other hand, it just feels "right" to use sometime with a little flexibility in between the planks.
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Old 16-11-2015, 10:06   #6
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Re: Repairing planked core

My Reliant has a fore 'n aft laid, planked core of 1/2" x 3 mahogany. When completely gutting and rebuilding Blue, '94 - '98, I removed the teak planking, set the saw at 1/2" and cut the upper glass skin away 3" in from the gunwale and cabin. Reliants are built with the upper and lower glass skins filled solid with a microballoon mixture for about 4" at the sheer and cabin side. The core had no damage whatsoever except 1 plank width at the stbd deck drain. The core had come loose from both skins in many places. Iscrubbed the core with acetone, and laid back in west system with battens and weights. In order the stiffen the springy decks for which Cheoy Lees are known, I then fitted and bonded, with epoxy and screws, 3/8 marine fir ply. Then put back top skin of 4 chop, 3 woven into tapered, 3" perimeter flanges. Painted decks. In 20 years, no problems and many compliments on the solidity of the assembly. The portside amidships hull/deck joint showed no impairment, even though hurricane Gonzalo put her up against a concrete seawall and mushroomed 3/4" bulkheads for 1 1/2" inward. They are tough boats, in spite of what is often said about Cheoy 'leakies'.
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Old 16-11-2015, 10:49   #7
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Re: Repairing planked core

Quote:
Originally Posted by chris95040 View Post
Yep - all good points about what makes a sandwich deck so stiff.

To re-adhere everything, the "clamping" force will be my ~500 pounds of 10 pound lead bricks that are currently serving as trim ballast. There is temporary framing under the deck which this weight will push against to sandwich the whole works together, one section at a time. Although the through-bolt clamping trick is a great one, and I may use that in some areas.

My exact question is what sort of material would folks use to fill in the gaps between the planks of this planked core? On the one hand, assuming all laminates get properly re-adhered to the core, the thing is going to be super stiff again and shouldn't have any motion. On the other hand, it just feels "right" to use sometime with a little flexibility in between the planks.
Few holes on the planking, carefully you don't want to puncture the lower skin, so that you can press the skin to see if it's unbonded. If so more holes and screw bolts into the holes (like 8mm) carefully so they press unbonded skin bit apart from the planking. Set a vacuum pump to suck the water vapour away throw hoses attached to additional holes, maybe 1 or 2 per sqm. After drying it out pour in slow setting low viscosity epoxy, about 1kg per sqm. Wrap the deck in a vacuum bag and set the vacuum pump on. Remember bag sealing, peel ply, some channels and a resin trap. When it's done laminate the topside.
That is if you wan't to do properly.

BR Teddy
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Old 16-11-2015, 16:15   #8
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Re: Repairing planked core

I think something along the lines of Blue Stocking's fix is your best bet.

The description of why a cored deck is strong is extremely relevant in this case because, in my experience, there is no way to get the lower skin completely rebonded to the core, and if you don't there will be weak areas in the deck, which can lead to weird stresses and possibly buckling and who knows what else.

Even if you were to use vacuum bagging, that is no guaranty that the resin is distributed to all surfaces between the inner skin and the inner surface of the core.

Not to mention the difficulty of getting the wood truly dry.

My approach would be, after drying, to flood the existing planks with resin, follow that with filling and fairing with resin and glass bubbles, then a layer of mat, then a layer of 1208 mat side up, then whatever coring you want to use, then whatever layup schedule you choose for the outer skin of the new core.

Essentially you'd just be making a new cored deck.

Comparatively, initially more expensive than you what you plan, but bulletproof. Much depends on your intended use of the boat (and of course the depth of your pockets, though a lot can be done with creative buying)...

Regarding your question about movement between the planks, I don't think this is a place you want that.
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