deck stepped mast delamination concave repair

[brian cc27mkiii]

looking for recommendations on repair material.
i have removed the tabernacle fiberglass skin and wet core from my c&c27mkiii and found what was left of a piece of marine ply that was sandwiched between the skins under the mast step.
do i use another piece of marine ply? stainless plate? or build up solid glass?
any recommendations would be appreciated thks.

 

[Tom J]

I think some marine plywood sealed with epoxy [several coats] would last the lifetime of the boat and conform to the original construction specs.

 

[DArcy]

I would strongly suggest going with FR4 (or build up your own solid glass laminate) under the mast step and replace the wet balsa core around it. My C&C27 had this job done before I bought the boat and they used solid glass laminate. It was rock solid when I sold the boat 21 years later.
Don't use wood anywhere there are deck penetrations or high compressive loads, go with solid laminate.

 

[jssailem]

There is also Coosa.

• high-density, closed-cell, polyurethane foam reinforced with woven roving continuous strand fiberglass

Lightweight Fiberglass Coosa Board | Coosa Composites

Coosa Composite structural panels are made of high-density, polyurethane foam reinforced with fiberglass. Our panels are the perfect plywood replacement!

coosacomposites.com

 

[DArcy]

I thought about Coosa board but it looks like it has a much lower compressive strength than solid glass laminate and would not recommend it under a mast step.

 

[brian cc27mkiii]

thank you all for your input my concern with solid glass is will it withstand the compression load over time? we know ply is very good with compression but water intrusion is inevitable at some point. many composites and honeycomb products are great torsionally but don’t have the compression strength.
much pondering to be done!!!

 

[jssailem]

water intrusion is inevitable

This is not accurate. Yes it can happen, but not an “absolute”. You need to design a solid base. Install all hardware with good caulking and proper installation. Monitor and maintain your boat and you can protect the boat for long term service.

 

[DArcy]

thank you all for your input my concern with solid glass is will it withstand the compression load over time? we know ply is very good with compression but water intrusion is inevitable at some point. many composites and honeycomb products are great torsionally but don’t have the compression strength.
much pondering to be done!!!

Well made solid glass (like FR4/G10) has much higher compression strength than plywood. Roughly 60,000 psi for FR4, vs. around 7,000 psi for plywood.

 

[jssailem]

While not the fiberglass strength the Coosa product has some good properties.

COOSA BLUEWATER 26 SERIES COMPOSITE BOARD – Bluewater 26 is Coosa’s strongest and stiffest composite panel with the highest strength-to-weight ratio. This board offers 26 lb. density per cubic foot and is 30% lighter, or more, than plywood. Bluewater 26 is used for structural components such as transoms, bulkheads, flooring, and wall units. Sold by the 4′ x 8′ board.​

 

[Don S/V ILLusion]

The title implies the mast step has a concave depression from rotten core material. This may be only a symptom rather than the cause.

You need to thoroughly inspect the bottom of the mast support particularly where it rests on the bilge keel to ensure the problem is limited only to the deck area. Every boat I’ve seen with such a concave mast base suffered from compression post rot and/or the effect of some idiot designing a wooden post support.

 

[DArcy]

You need to thoroughly inspect the bottom of the mast support

Very good point Don. My boat did not have this problem but another C&C27 at my club did and had to replace the compression post due to rot. The cable entry right behind the mast step can leak and lead to water in the compression post.

 

[brian cc27mkiii]

yes thank you Don the top of the compression post and bulkhead appear to be solid as does the rest of the stable post but the cabin sole at the base of the post is damp with some dry rot. i will use the camera to get a better look at the support under the post and cut back a bit of the sole to check the bottom of the post. still trying to figure my way around this interior liner thing.
wow thank you for the numbers on the FR4 D’Arcy iwill look into that when i head to toronto for supplies.