Bollard backing plate is plywood

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January 2, 2021 at 17:23 #26806
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Keymaster

Here’s a warning about a problem we discovered while rebedding deck fittings.

When we pulled the bollard from the front deck to rebed it, we discovered a somewhat soggy 3/4″ plywood backing plate below deck. In Rainshadow, hull #126, this plywood was partially glassed over below deck, and partially exposed. The exposed backing plate seemed solid from below, but once we had the bollard removed, we discovered the plywood was rotted in the core around the center bollard bolt holes due to the sealant at the bolts failing. Like all deck fittings we’ve removed, presumably for the first time since C&N installed them, the sealant was degraded to be just dust now and no longer doing it’s job.

Since the plywood plate was partially glassed over from below, we could not easily remove it. Our solution was to:

1) From the underside, we used a 2″ hole saw to cut away from under the deck the area of rotten plywood around the worst rotted bolt holes. We were careful to not drill through the deck FRP, just the plywood. For those bolt hole areas that were not rotted, we gained access from the deck side bolt clearance hole and we used an angled tool on a drill to dig out about 1/4″ of plywood surrounding the clearance hole. Doing this from the top was easier than using a hole saw from below because these were areas where the plywood backing plate had been glassed over and so was buried between fiberglass sections.

2) Fortunately there was still ample solid plywood left  so we could epoxy and screw on to the plywood a new 1/2″ fiberglass sheet that spans between the vertical plywood stringers. This added a lot of deck strength under the bollard, without adding new fastener holes through the deck.

3) We used filled epoxy to refill all bollard bolt holes (and cavities created by removing rotten wood) to better protect the plywood backing plate.

4) Drilled new deck clearance holes for the bollard bolts, this time through the solid epoxy cores in the plywood that won’t be destroyed if a future leak develops.

5) Reinstall the bollard using butyl as the bedding compound. This required longer bolts to reach the underside of the new fiberglass backing plate.

Fortunately we caught this failing plywood backing plate before it was a complete loss. I encourage others to check around the bollard for deck cracks, or softness. Better yet, just rebed the bollard if you have not done so recently.

Marilyn and Van

February 1, 2021 at 15:32 #26810

Rhapsode
Participant

Ours was an aluminium plate. It had started to corrode around the holes for the bolts. We’re replacing the foredeck teak and whilst doing it decided to replace the aluminium with a SS plate.

February 1, 2021 at 22:27 #26811
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Moderator
Keymaster

Oh yes, I forgot to mention that – we had an aluminum backing plate below deck that was below the plywood “core”. Ours was corroded too, so the new FRP plate is the replacement for the aluminum plate as well as the deck strengthener.

Thanks for clarifying the existence of the aluminum backing plate.

Marilyn, moderator

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