Rotten wood inbetween grp near mast

[boatmike]

Thanks. My query was largely the result of having recently removed a number of original skin fittings from a 1999 Moody. Two of the encapsulated plywood cores were rotten and needed replacing.

I am afraid some production boat manufacturers were as guilty as amateur owners for not carrying out good practice when fitting cleats etc. However well the designer specifies what the yard should do its down to the care taken by the yard in quality control. Sadly I have repaired a number of Moodys and Westerlies for exactly this failure. Really sad as the boats themselves are built well over spec and should last many years. Never seen a Swedish boat and very few French and German boats with such problems but then I have been retired for a long time now and I may be out of date. Having said that though the majority of problems I have seen have been amateur additions rather than original yard built problems.

 

[Ammonite]

That's fair enough but I still dont see why you cant use GRP instead of plywood and remove any chance of rot. If 12mm is too stiff, use 10mm. Why is plywood a better option?

 

[boatmike]

I give up. try casting a 10mm thick slab of solid GRP and try bending it. You may as well use a plate of stainless steel!

 

[john_morris_uk]

I give up. try casting a 10mm thick slab of solid GRP and try bending it. You may as well use a plate of stainless steel!

You can lead a horse to water but a pencil must be lead.

 

[Ammonite]

You can lead a horse to water but a pencil must be lead.

I give up. try casting a 10mm thick slab of solid GRP and try bending it. You may as well use a plate of stainless steel!

I fail to see how a 1/2" piece of decent marine ply epoxied to 1/2" of solid laminate and then epoxied over making it about 1-1/4" think is going to offer any flexibility over 12" but regardless my query was about backing plates in general not specifically the OPs situation and I apologise for the thread drift. I was merely trying to understand why plywood is so commonly used when it has clear drawbacks as well as certain advantages. The following article presents some interesting pros and cons about a number of different backing materials.
How big does a backing plate need to be? - Practical Sailor

 

[Ammonite]

Here's another one
The Critical World Of Backing Plates

 

[john_morris_uk]

We seem to have switched from discussing the best way to replace a rotted core to the best material for backing plates. Two separate issues.

 

[Ammonite]

We seem to have switched from discussing the best way to replace a rotted core to the best material for backing plates. Two separate issues.

Indeed. From about post #12

 

[Rhylsailer99]

If you have already decided what you are going to do there isn't much point in posting further but what you describe is a bodge, You might feel you have solved the problem but the integrity of the mast mounting plate will not be restored. The right solution is to remove the inner skin and cut out ALL of the existing ply reinforcement and replace as I previously suggested drilling new holes for the mast block. The purpose of this plywood block is to not only enable the fittings to be tightened without compression but to spread the load evenly over the area. the reason ply is used in favour of a more solid material is to maintain some degree of flexibility in the transition between the ply and balsa or foam core around it. otherwise any slight deflection would result in stress cracking where the solid material meets the balsa.

Cheers

I fail to see how a 1/2" piece of decent marine ply epoxied to 1/2" of solid laminate and then epoxied over making it about 1-1/4" think is going to offer any flexibility over 12" but regardless my query was about backing plates in general not specifically the OPs situation and I apologise for the thread drift. I was merely trying to understand why plywood is so commonly used when it has clear drawbacks as well as certain advantages. The following article presents some interesting pros and cons about a number of different backing materials.
How big does a backing plate need to be? - Practical Sailor

I would say some degree of flex is needed to stop cracks hence the use of ply.instead of solid grp.

 

[Rhylsailer99]

Not entirely clear where the plywood is but its normal to use either balsa or foam sheet in non structural areas. Ply is only normally used locally where the structure demands it like through deck chainplates, cleats or I suspect from your post as in this case mast mounting blocks. If this is the case cutting away the inner skin below the affected area and removing all the soggy ply is the best plan. stick a new reinforcing block up to the deckhead with crestomer paste or thickened polyester resin to ensure there are no voids. When cured layup over giving a good margin of overlap with the existing inner skin having ground it back to clean first, you can then drill through the mast fixings to refit the mast heel block on a sound base structure. All dependant on where the problem actually is of course but fundamentally wherever it is I agree with others. remove inner skin, remove all rotten core and rebuild from below. If its not a mast support block you can use foam core or block balsa in place of ply. Don't mess about replacing balsa or foam with ply though. always replace like with like. It was built that way for a reason!

 

[Rhylsailer99]

The circled area was the main / worst affected area between the grp

 

[Ammonite]

Are we talking about a rotted plywood core here or a localised plywood mast support as suggested by boatmike post #8?

If it is a core used throughout the coachroof then ignore my comments regarding grp. Replace like with like.

I had assumed we were talking about a localised / encapsulated backing plate.

If this is the case but the coachroof is curved rather than flat then I understand the need for flexibility and would use a correspondingly thinner piece (or pieces sandwiched together) of G10 (epoxy) or GP03 (polyester) board to give the same strength / flexibility as plywood and to bolt the mast plate on "wet" so the backing plate takes up the contour of the roof.

Plywood would of course do the same thing but you are going to have to epoxy fill and redrill something like 8+ holes for the mast plate and wiring and I put money on the first contractor fitting an additional cable gland in future not doing the same.

When referring to a regular flat backing plate used for cleats etc then to my mind something like a 6mm G10 board as suggested in the article I attached is a much better solution than 12mm ply.