Rotten wood inbetween grp near mast

[Rhylsailer99]

I have cables going through the boat to connect mast lights , they have been poorly fitted and over the years leaked water. Inbetween the 2 layers the wood is like a black slop that ive scraped back to where its almost dry Is it OK to push epoxy fiberglass paste inside the void to reinforce it thanks

 

[Rhylsailer99]

The boat is on dry now a d whilst working on the boat I have aimed a diesel heater blowing around the affected area.

 

[[178529]]

no. There are some really useful videos on how to fix this. I'll dig out one of the best from youtube Sail Life. Basically you need to re-introduce the sandwich strength back into the area of the deck.

As I understand it the most common method is to:

1) Identify (through sound, feel and/or drilling ) the extent of the soft waterlogged patches.
2) Cut the top layer of grp using a grinder or cutter that only goes through the outer layer (from the outside)
3) chisel away the balsa or plywood or whatever rotted stuff is in the sandwich.
4) Choose your replacement reinforcement. There are hi tech composite things, end grain balsa or plywood.
5) cut it to size and epoxy it down with resin and then fill with epoxy/silicoid
5) I have seen the next bit done two ways.
5.1) put back the old grp on epoxy filler over the new sandwich layer with loads of epoxy and epoxy filler, then fair and paint the edges
or
5.2) put new epoxy grp biaxial straight onto the new sandwich. fair and repaint the whole deck.

 

[john_morris_uk]

no. There are some really useful videos on how to fix this. I'll dig out one of the best from youtube Sail Life. Basically you need to re-introduce the sandwich strength back into the area of the deck.

As I understand it the most common method is to:

1) Identify (through sound, feel and/or drilling ) the extent of the soft waterlogged patches.
2) Cut the top layer of grp using a grinder or cutter that only goes through the outer layer (from the outside)
3) chisel away the balsa or plywood or whatever rotted stuff is in the sandwich.
4) Choose your replacement reinforcement. There are hi tech composite things, end grain balsa or plywood.
5) cut it to size and epoxy it down with resin and then fill with epoxy/silicoid
5) I have seen the next bit done two ways.
5.1) put back the old grp on epoxy filler over the new sandwich layer with loads of epoxy and epoxy filler, then fair and paint the edges
or
5.2) put new epoxy grp biaxial straight onto the new sandwich. fair and repaint the whole deck.

I would alter that to,

"if possible (and the inner underneath surface is covered by interior trim that is easy to remove and replace and it's all accessible), cut away the UNDER side layer of the laminate. If it's a large area that's got waterlogged and rotted, then ask again. If it's only the first 50 cm or so, cut it all away and rebuild the sandwich with BS1088 plywood bedded (upwards!) onto the underwear of the deck. Fill all voids with thickened epoxy and then laminate over the top with layers of matting and resin before putting the trim back."

From the top, it will look perfect and from the bottom, no-one will ever know unless they take the headlining trim off and it'll be stronger than before. You might have to put some saw cuts through the plywood to help it bend round any curve, or use several thin bits of ply and laminate...

 

[[178529]]

This is the start of his deck core replacement:

 

[[178529]]

I would alter that to,

"if possible (and the inner underneath surface is covered by interior trim that is easy to remove and replace and it's all accessible, cut away the INNER side of the laminate. If it's a large area that's got waterlogged and rotted, then ask again. If it's only the first 50 cm or so, cut it all away and rebuild the sandwich with BS1088 plywood bedded (upwards!) onto the underwear of the deck. Fill all voids with thickened epoxy and then laminate over the top with layers of matting and resin before putting the trim back."

From the top, it will look perfect and from the bottom, no-one will ever know and it'll be stronger than before. You might have to put some saw cuts through the plywood to help it bend round any curve, or use several thin bits of ply and laminate...

That's true. The only downside is that, for me, i've always found it hard to fibreglass from below and you have gravity working against you a bit. But absolutely, if you can do it from below then it will look untouched.

The most important thing is to be careful when drilling into a cored deck for fixings. You should drill a bigger hole than needed, fill and seal with thickened epoxy and let cure. Then drill through the thickened epoxy for the fixing.

 

[Poignard]

https://www.westsystem.com/wp-content/uploads/Fiberglass-Manual-2015.pdf

Chapter 5 page 26: "Repairing Core Related Damage"

 

[boatmike]

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]

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!

Its very close to the mast which makes me nervous of removing grp . The rotten area seems to be around 6"x6" Square area as some holes ive drilled to look are good wood underneath.

 

[Ammonite]

Im happy to be corrected but wouldn't it be better to use GRP sheet than plywood? Ive never understood why most yards insist on using ply which is bound to rot at some point in a boats life if you're going to drill holes in it

 

[john_morris_uk]

Im happy to be corrected but wouldn't it be better to use GRP sheet than plywood? Ive never understood why most yards insist on using ply which is bound to rot at some point in a boats life if you're going to drill holes in it

Sandwich construction in GRP is all about stiffness and weight. Frequently the core material is balsa wood and so long as the GRP isn’t penetrated and remains impervious to water, it’s a very stiff and light construction. Where extra strength is needed or where fittings need to be bolted through its common to use plywood (sometimes some quite substantial pieces) instead of balsa. Ply is dimensionally stable and resists the crushing forces of the bolts used to fix things down as well as providing a localised ‘stronger area’ for the cleat or track or whatever is being fitted.

Laying up such an area in solid GRP would be possible, but it would be expensive in materials and to what benefit? It would almost certainly be heavier in an area you don’t want to add weight.

Most boat builders learned NOT to use sandwich construction below the waterline a long time ago. Sandwich construction is normally limited to topsides and deck.

For the OP, cutting away the underside of the sandwich and grafting in a new bit of ply and glassing over again with epoxy and woven matting will end up with a bit of deck stronger than before and the repair (so long as there is some sort of headlining covering his work below decks) will be invisible.

 

[Ammonite]

Sandwich construction in GRP is all about stiffness and weight. Frequently the core material is balsa wood and so long as the GRP isn’t penetrated and remains impervious to water, it’s a very stiff and light construction. Where extra strength is needed or where fittings need to be bolted through its common to use plywood (sometimes some quite substantial pieces) instead of balsa. Ply is dimensionally stable and resists the crushing forces of the bolts used to fix things down as well as providing a localised ‘stronger area’ for the cleat or track or whatever is being fitted.

Laying up such an area in solid GRP would be possible, but it would be expensive in materials and to what benefit? It would almost certainly be heavier in an area you don’t want to add weight.

Most boat builders learned NOT to use sandwich construction below the waterline a long time ago. Sandwich construction is normally limited to topsides and deck.

For the OP, cutting away the underside of the sandwich and grafting in a new bit of ply and glassing over again with epoxy and woven matting will end up with a bit of deck stronger than before and the repair (so long as there is some sort of headlining covering his work below decks) will be invisible.

Thanks for the detailed explanation but thats not what Im referring to. Apologies if I wasn't clear. I understand the arguments for Balsa / Foam cores etc but not plywood backing plates. Yes, its dimensionally stable but it also rots so why not use a piece of pre-made GRP sheet as a backing plate i.e cut it up and bond it in place as you would plywood?
Putting a plywood backing plate where your skin fittings are going to go, glassing it over and then drilling a large hole in it seems a daft idea, albeit cheap and effective in the short term. A bit like brass seacocks. Im happy to stand corrected and realise that plywood has been used in this way for years but it doesnt seem a great solution to me. Maybe its die hard wooden boat builders not wanting to let go ;-)

 

[john_morris_uk]

Thanks for the detailed explanation but thats not what Im referring to. Apologies if I wasn't clear. I understand the arguments for Balsa / Foam cores etc but not plywood backing plates. Yes, its dimensionally stable but it also rots so why not use a piece of pre-made GRP sheet as a backing plate i.e cut it up and bond it in place as you would plywood?
Putting a plywood backing plate where your skin fittings are going to go, glassing it over and then drilling a large hole in it seems a daft idea, albeit cheap and effective in the short term. A bit like brass seacocks. Im happy to stand corrected and realise that plywood has been used in this way for years but it doesnt seem a great solution to me.

If you’ve got access to 12mm GRP sheet (or whatever thickness is required), I guess it’s an option you might consider. I try to ensure that things I bolt through the deck don’t leak?
Another option might be to do what you do when you put fittings in an area where there’s balsa? Chisel out the balsa or ply and cast a plug of epoxy and drill through that. The epoxy is impervious to any leaks or ‘sealant failure’.

 

[Ammonite]

GRP sheet is readily available for amateur repairs but even if it weren't i imagine boat builders wouldn't have a problem sourcing it! Removing some of the core material epoxing and drilling is a solution and great for repairs / additional fittings etc but from a production line perspective its a pita. Maybe Im missing something? I cant imagine cost is the issue given the area involved but maybe it is?

 

[Rhylsailer99]

GRP sheet is readily available for amateur repairs but even if it weren't i imagine boat builders wouldn't have a problem sourcing it! Removing some of the core material epoxing and drilling is a solution and great for repairs / additional fittings etc but from a production line perspective its a pita. Maybe Im missing something? I cant imagine cost is the issue given the area involved but maybe it is?

I've drilled 8mm.holes and found the rot is in an area about 6" Square so im going to stuff the void with fiberglass and resin mixed together and pack it down .

 

[Boathook]

West systems do a milled (I think that's what they call it) glass fibre additive for epoxy. That combined with some microfibre and a slow hardener will hopefully fill all the little voids.

 

[boatmike]

I've drilled 8mm.holes and found the rot is in an area about 6" Square so im going to stuff the void with fiberglass and resin mixed together and pack it down .

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.

 

[Ammonite]

I agree it needs to be repaired properly but isnt plywood actually stiffer than fibreglass, which is why epoxy / ply boats are so stiff and light. If that is the case whats wrong with a GRP backing plate? I suspect theres very few people on here with GRP boats that havent had a leak from a ply backed hull or deck fitting and it doesnt take that long for rot to set in.

Edit: just to be clear Im agreeing with boatmikes approach but just questionning whether it might be a better long term solution to use a GRP backing plate?

Alternatively, anyone have any experience with this?
EKOPLY - Market Leader in Recycled Plastic Sheets and Boards

Sadly in doesnt look like it can be glued which probably rules out epoxy and most sealants :-(
http://www.ekoplysheets.co.uk/EKOply Specification sheet.pdf

 

[boatmike]

Having spent a good deal of my professional life designing and building composite structures I can say with some degree of certainty that plywood is an ideal material for this purpose and no its definitely not more rigid than solid glass reinforced polyester resin. The average core on a small yachts deck is roughly 1/2 in thick. try bending a piece 1/2 in thick solid cast GRP 12 inches square (i am assuming his mast reinforcing plate would be approx this) and you won't get far I assure you. 1/2 marine ply is made up of several laminates and while still fairly rigid is more compliant than solid resin reinforced glass fibre. As for the number of rotten wood cores experienced, the only ones I have seen is where some numpty has drilled into it from outside and let the water in as appears to be the case here. When fitting anything through any core drill well oversize and apply a liberal amount of sealant around the bolt or screw to seal the wood down the hole, not just at the surface. Better still "paint" the inside of the hole with epoxy before you fit the bolt and let it cure first. You should never have a problem with properly fitted parts if the job is done properly. As far as squirting resin down the hole as the OP suggests it wont adhere properly to either top or bottom laminates or the wood around it. It will therefore not contribute to the integrity of the original mounting plate and form a solid plug of material that is not part of the structure.

 

[Ammonite]

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.