Recoring under deck hardware. Plywood or Solid Laminate?

[Midday Gun]

Pretty simple question as per the title, if only the answer was the same!

So I realise this has been done to death and right now my eye are going square from reading all the websites / threads / other information on the internet on this subject, and the answer seems different everywhere I look.

I've attached a picture of the deck, in the highlighted areas the balsa core has been crushed by a combination of insufficient backing plates and overtightening the fasteners leaving the deck noticably deformed.
Fortunately the core elsewhere is still in good shape.

I want to replace the core in the damaged areas, and a couple of other spots where I will be putting deck hardware.
The easiest option is plywood which is pretty resistant to crushing, the other option is what builders do from the factory, solid laminate. However I've not seen anyone online doing anything other than putting new core, I've never seen anyone use solid glass in localised areas for deckhardware.

I plan to go in from the top and then redo the non skid afterwards.

So whats the current consensus on this? (If there is one)
Thanks!

 

[Blueknarr]

There are many ways to skin cats and solidify cores. Plywood is cheap, easy and relitively reliable. Therefore it is probably the most common. Personally, I have made core incerts of : plywood, timber,G-10, delron, thickened resin, and aluminum plate. Ideally, the anticipated load would dictate the core incert material.

 

[Midday Gun]

So the loads will be a winch on either side under the aft most circle.
These take halyard loads, reefing lines, and spinnaker.

They've been fine for thirty years in that they're still attached, however that area is where the deck crushing is most noticeable. They had no backing pads at all, just penny washers!

The second area forward of the winches has a bank of clutches and also a couple of clam cleats for pole down & kicker.

The area by the mast is for the organisers sending the lines from aft into the mast.

G10 I'm using for backing pads, its incredibly expensive here and hard to find in any meaningful thickness. Also hard to find in any size bigger than A4.

 

[missinginaction]

Once you get in there and excavate you'll see how much of the core needs replacing. Plywood and liberal use of epoxy should do the trick. The bigger issue is backing plates. That's why he core failed in the first place. I'm a power boater, rebuilt a small cruiser some years ago. Once the boat itself is repaired, consider substantial hardwood backing plates to spread out the load. I've used mahogany, white oak and even maple (which is not rot resistant but worked well because I sealed it perfectly). 5/4 stock and a planer helps with setting this up. I've used a technique taught to me on this site called making an epoxy grommet. Let's say you're using 1/4-20 machine bolts. First locate where you want the holes to be. Then overdrill the locations. In this case we'll drill holes 5/8" in diameter. Tape the bottoms of the holes with packing tape, just good old 3m stuff. Place a small amount of slightly thickened epoxy into each hole and let it kick off to seal the bottoms. Then fill each hole with neat (unthickened)resin and allow to cure. Once the resin is set up, drill your 17/64" holes for the machine bolts through the center of the epoxy. When you're done you have a bolt passing through the epoxy, completely surrounded by it. Absolutely no exposed wood and your core is sealed. I use nylocks and what are called "structural washers or extra thick fender washers" to finish the job. Structural washers look like fender washers, or penny washers but are 1/8" thick. As the name implies they are designed to carry a load. Made these cleat/backing plates up for my boat about 5 years ago, haven't had a bit of trouble with them.

18-8 Stainless Steel Extra Thick Fender Washers https://www.albanycountyfasteners.com/18-8-stainless-steel-extra-thick-fender-washers.html

Good Luck,

MIA

 

[Midday Gun]

Thanks for the advice.

I've actually already done a similar process to what you suggest on the rest of the deck bits on the boat, although I actually over drilled from the bottom and left the top skin intact before filling from the top.
Unfortunately whoever fitted this stuff in the past didn't think to do this as well.

The core I drilled out wasn't especially damp or rotten, just crushed with a noticeable indentation in the deck, so I'm hoping it will be possible to just replace it in the crushed areas.

Backing plates I already have G10 for, hardwood seems quite an expensive way to for something less strong & stiff, I can also bond in the backing plates with thickened epoxy which should increase the strength.

Really the question is just about the core, plywood is cheap & easy & should be stronger than the balsa, however I just want to make sure there aren't better options while I have everything apart.

 

[Zippydoodah]

The thing about using plywood IS that it's cheap. I would personally use marine plywood. Its treated so mo rot. It's also stronger and will last. Spmetimes going cheap means your going to have to redo it eventually.

 

[Midday Gun]

What about solid laminate?

I know some builders specify solid laminate in way of deck fittings, so if I have the deck up anyway then it might make sense to go that route? Just want to do it right and do it once.

 

[Steve Clark]

End grain balsa has a higher compression strength than cross grain plywood. So you will be going sideways.
If the Balsa hasn’t seen a ton of water and rotted, you might consider this: add laminate to the top of the house to build up the depressed area and spread the load around. Possibly add laminate to a larger area than the base of the hardware to further distribute the load. 25-50 % isn’t inappropriate for highly loaded equipment. Drill the bolt holes for thick walled G10 spacer tubes, bond them in place with epoxy.
Glass over the top of the tubes ( a step I have learned the hard way. The bond between the skin and spacer always cracks and lets water in) cut an 1/8” aluminum or G10 backing plate 1/2” bigger than the winch base for the back side. Assemble with polysulfide bedding compound of your choice. Avoid 5200 or Silkkaflex, known to us as “**** the next guy.”
If the core has rotted, the best replacement os Coosa foam or Penske board, which are the same thing. No core replacement really handles the crushing load of nuts and bolts, so spacers are the only way to go
Solid glass also works, but you still need to have an oversized backing plate and laminate the solid plug to the outside skin with oversized patches.
SHC

 

[fallguy]

I can tell you what I am doing in a new build for cleats.

Hull is 12mm corecell. Deck shoe is Gurit pvc and corecell M. Using some offcuts as the hull proper is M.

Decored an area a bit larger than the 8" cleat base. Plan to use a solid piece of stainless for backer on the inside. Those pieces are designed to decrease psi loading to what I think is reasonable. So for my 8" cleat, I am using 3" wide by 12-16" plates. A 3" by 12" plate is 36 square inches. So a 5000# pound pull is reduced to 140 psi. My goal was to make it so a single cleat would hold the boat, but I think that might be a little difficult to achieve.

Here we have laminated the inside and decored for the cleat base and will glass over. I am a bit nervous the cleat will delam the glass still. The only way to prevent this will be metal sleeve inserts. I won't do that right off. I will wait and see.

On the aft portion of the boat, it plywood where the cleat will go. I plan to just use the plywood there and made no special arrangements except the plywood is 1/2" to match the hull and so I backed the cleat area with another piece of 1/2" ply. This was done with ply on purpose for possibly adding a rod holder and you can see the fuel line hole on top. Those were already overbored and filled with thixo and rebored. The cleat goes under the area of the forward bucket. I expect the cleat may hog in a bit overtime. I would then fix with a thixo mix in a repaint session.

 

[Blueknarr]

The way you are thinking of simply filling the void with many layers of resin and glass will provide a solid base; if heavy and expensive. If you go this route, and have already purchased your planned G-10, drop it in the bottom of the well and then add glass over top to match thickness.

The usual way to spot reduce a cored hull to solid is to work from the inside. Remove inner skin and core. Bevel the core to ease inner skin transition. Re-skin with a thicker lamination schedule. This leaves a recess on the inside where the panel isn't as thick. As a side benifit, the mounting bolts and nuts may not project past the interior skin's original plane.

 

[Midday Gun]

I'm no fan of working upside down with any kind of laminating, plus the top skin has deformed, maybe I would spring back with the core removed, I'm not sure.

It seems like marine ply is going to be the best option then, its only localised areas that need replacing under the hardware.
So cut top skin off, remove core, taper edges of top skin, new core with lots of thickened epoxy, re laminate top skin with biaxial, new non skid, done?

 

[DCockey]

I would personally use marine plywood. Its treated so mo rot.

"Marine" plywood is usually made with rot resistant hardwood veneers and is not treated with chemicals. Some boats use chemically treated Douglas fir or pine plywood.

 

[fallguy]

I'm no fan of working upside down with any kind of laminating, plus the top skin has deformed, maybe I would spring back with the core removed, I'm not sure.

It seems like marine ply is going to be the best option then, its only localised areas that need replacing under the hardware.
So cut top skin off, remove core, taper edges of top skin, new core with lots of thickened epoxy, re laminate top skin with biaxial, new non skid, done?

Yup. But cabosil could be used instead of ply. Or you can just repair and sleeve or bushing the fastener and put the backing on reverse.

 

[Midday Gun]

Yup. But cabosil could be used instead of ply. Or you can just repair and sleeve or bushing the fastener and put the backing on reverse.

So the area under each piece of deck hardware filled with cabosil thickened resin, and then glassed over the top?

While it would certainly be strong enough in compression it sounds like quite an expensive job in terms of resin use. Presumably thickened polyester resin wouldn't be up to the job?

@DCockey, its more down to what is easily available. Marine ply should do the job for another 30 years.

 

[Blueknarr]

Thickened poly works quite well. Add some kitty hair (1/2 inch chopped fibers), and only lay in 1/4 inch at a time to avoid excessive heat and shrinkage.