Mahogany ply repair.

[Saltram31]

I have had to cut out a section of about 8" x 5" deck, below the primary winch due to rot. The deck is 20mm mahogany ply and I'm building it back up with epoxy and glass mat. How many layers of mat would I need, to achieve the equivalent strength?
TIA.

 

[johnlilley]

If I understand your question you are considering replacing an 8" x 5" section of plywood on the deck with a solid laminate of fibreglass in its place.
To be honest I would not consider that as an acceptable structural repair for the deck particularly under a stress loaded section of deck. My reasoning is because you need to have a significant area of supporting structure beneath the winch and not just a square patch as, the connection of the patch to the remainder of the deck sounds very questionable. You need to ensure the patch cannot possibly lift out and the stresses must be further supported by the decks fore and aft of the area of damage . Obviously the precise method depends upon the location of the area of rot in that is it in the side deck or is it on the foredeck.
I would consider extending the area of replacement using a similar thickness plywood using a wide step joint around the perimeter of the patch to match a similar step joint in the original plywood and ensure the connection is made preferably over a deck beam if there is one in the vicinity forward and aft of the area of deterioration.
If there is a significant deck curve you will have to lay the section in two thinner panels glued together. Glue and screw the plywood down on to the beams and, if possible install a seam batten beneath the fore & aft connections between the beams.
To waterproof the area just sheath with a thin glass cloth and epoxy resin.
However, if I've misunderstood your question I apologise.

 

[Saltram31]

Hi John,I appreciate the detailed reply.
The area I'm repairing is not load bearing as there is a chock below it which runs fore and aft to the laminated deck beams. The winch is bolted through the chock with a large bronze bolt at the center and 4 threaded rods on the perimeter.
The rot occurred from years of rain water entering a screw hole below the coaming.
The area of the gap is below the 12mm deck, to which the 20mm ply is screwed and epoxied.(at build). Hope it make sense.I'll try and get pics tomorrow.
My idea for going the epoxy route is to prevent a further potential leak in that area. Please feel free to correct me.

 

[johnlilley]

A picture is worth a thousand words.....John

 

[PhillM]

Hi John,I appreciate the detailed reply.
The area I'm repairing is not load bearing as there is a chock below it which runs fore and aft to the laminated deck beams. The winch is bolted through the chock with a large bronze bolt at the center and 4 threaded rods on the perimeter.
The rot occurred from years of rain water entering a screw hole below the coaming.
The area of the gap is below the 12mm deck, to which the 20mm ply is screwed and epoxied.(at build). Hope it make sense.I'll try and get pics tomorrow.
My idea for going the epoxy route is to prevent a further potential leak in that area. Please feel free to correct me.

Epoxy is fine, but more as a glue than as layup to replace rotten wood. Personally, I would be looking at scarfing in new wood. By all means, use epoxy to glue in place, but use a decent (20 year) ply to replace the rotten area.