New inner skin over old roving before new core?

The core in my cabin top is wet. I cut a section of top skin away. The core is a patch work of plywood squares about 6" to a side. Under the plywood is spotty CSM (gun applied?) that is dry in spots. The lowest layer down is woven roving. There are areas where the CSM was thin to non-existent, or was dry and came up with the plywood. The result is I have an inner skin with areas of roving only that is open between the weave. So plywood was probably exposed to moist cabin air.

I want to create a new inner skin, before putting down new plywood, by laying up glass over the old roving. The patchwork nature of the core has creating a very uneven surface, i.e 6" square sections of solid CSM and roving, next to a 6" square of nothing but open weave roving. Suggestions on how to approach this? What glass? Filler first?

Because I will have to basically redo the entire deck, I'm partial to working with poly. Epoxy is still on the table for ideas though.

 

Welcome to the forum

Pictures, pictures, pictures.
They always help.

Sounds like you will need to smooth out the base. Very coarse grinding to remove most of the CSM. Fairing compound to fill in the valleys.

Are you going to rebuild as originally done with ply tiles?

 

I do not plan on replicating the tiled construction method unless there was a good reason to. Plan to use as large a piece of marine ply that I can get to fit the camber of the cabin top. Original ply looks to be 3/4". I found this link with details of the construction. It doesn't say what was put down on the interior though.

http://www.aquariuspilotcutters.com/apc-original-documentation/apc-specs-construction/

 

Just went out to take a pic but phone was dead. Will try to add one. I guess my immediate question boils down to, should I glass first with something like CSM that will be easy to conform to the old surface (some of which is pretty thin and won't take any grinding) or is fairing first definitely required.

 

How much chamber is there?
Will the 3/4 easily flex to it?
Your cabin top is very weak in its current state. It could deform rather than the plywood conforming to it.

Cutting the plywood reduces the amount of pressure required to forse it into a curve. It also reduces the amount of stiffness it contributes to the panel's (roof's) stiffness. The resin dams between the tiles should have prevented rot from spreading.
If 3/4 plywood won't easily confirm to camber, then you might consider foam core.

 

What Blueknarr said. The tiles of plywood contribute pretty much zero structural strength. You might as well use foam for ease of fabrication and to reduce a bit of CoG weight.

If the interior face of the deck (ceiling?) is in good shape, you might be able to get away with filling all the voids and dry weave and CSM with resin, laying the foam sheet down on it and then putting a sheet of like 1/8th ply on that and then clamping the edges and loading it with a few bricks or the like to apply pressure but not caving the roof. If the voids are really bad (like you'd need gallons of resin) maybe an expanding foaming adnesive like "Gorrilla Glue". Maybe? Hard to tell what will work best sight unseen.

 

That is a concern. One of the reasons I want to put down glass first before stressing it.

That was the theory. However, in my uninformed opinion, the inside skin was not up to the job of keeping warm, moist, cabin air from wetting the plywood over the past 4o years.

 

It would take many layers of glass to effectively strengthen exposed inner skin. The best treatment for inner skin will depend on type of recoring being done. Hence the lack of specifics on my part.

I read the link to original build. There were a few less than ideal techniques.

-Polyester was used instead of epoxy

• Poly doesn't bond to wood as well as epoxy
• Poly doesn't resist osmotic intrusion as well as epoxy

Tiles were not pre-sealedTiles sucked resin out of the "putty" holding them calcium carbonate (water soluble) was used as a thickening agent.

• Allowed water to migrate around plywood

 

Pictures uploaded to gallery but awaiting moderator before I can post them.

I did consider foam. It would be a matter of cost and if the strength would match the plywood. But then again, with the way the deck was made with squares, the original was probably not as strong as implied by the plywood material.

 

Live now - Pilot Cutter repair | Boat Design Net https://www.boatdesign.net/gallery/albums/pilot-cutter-repair.4412/

(P.S. welcome to the forum! When photos are for a forum thread such as this, rather than a stand alone project to display in the gallery, you could alternately use the "Upload a File" button below the reply area to attach photos to your reply directly, but an album to document the whole project is great too.)

 

It was not. the core provides no strength, foam or solid steel. It is just a web, like an I-beam, between the two layers of composite that provides the strength. Plywood is used because it is cheap (today) and all they had and didn't really understand how composites work (back then).

 

Doh! I did look for an option to upload photo, but only looked through the buttons shown at the top of the editor. Completely missed the "Upload a File" button down below. Thanks for the lesson!

 

The plywood squares are common, and yes it’s fairy strong if done correctly.

The problem is, as in most things in older production boat manufacturing, the skill level of the workforce wasn’t up to building the boat as designed. Nor did the care to.

You could rebuild it in the same fashion as the original design and it would hold up fine for decades to come.

Your attention to detail will most likely be a great deal better than person that did the work in the plant

 

After some thinking, I think the answer is in front of me, I just don't want to look at it. I've only removed a 3 x 3 ft piece of deck skin on the port side of the companionway hatch. Considering that the thin skin issue is on the inside, that means I should probably change my approach and plan on removing the inside skin and putting new core up from the inside for the rest of the cabin top (sigh!).

I'd be interested in hearing about what foam I should look into if I decide to go that route.

I'm thinking that if I stick with marine ply, instead of 6" squares, I'd go with 6" "planks" that run fore/aft.

 

Working overhead is a LOT harder...