What weight Chopped Strand mat

[Tmacmi]

The hatch for our anchor closet is constructed with fiberglass backed with plywood and then chopped strand mat.

The plywood has rotted and I would like to replace it. I'll cut out the chopped strand from underneath. Replace the plywood and then replace the chopped strand.

What weight mat would you recommend?

[coopec43]

Originally Posted by Tmacmi

The hatch for our anchor closet is constructed with fiberglass backed with plywood and then chopped strand mat.

The plywood has rotted and I would like to replace it. I'll cut out the chopped strand from underneath. Replace the plywood and then replace the chopped strand.

What weight mat would you recommend?

I presume you mean a hatch lid - not a locker?

If I was repairing it I would use 3 OZ CSM (Chopped Strand Matt) but I wouldn't replace the plyboard.

But in any case I'd start again, discard the rotted hatch and build a new one. (probably using a layer of WR (woven roving) as well.

Here is a site that might interest you.

Fiberglass Mat

[Spot]

Fresh core and a new backside composite sounds pretty good to me, especially if one can remove the hatch/cover and work on it 'sunny side down' and not have to deal with molds, gelcoat, or painting.

Has the reason for the plywood rotting been determined and remedied?

Next question is why limit the choice to plywood and chopped strand mat? There are non-rotting core materials and other cloths besides CSM.

A picture would help the forum see size and scope of the project.

All the best, I have some rotten core to deal with myself...

[tkeithlu]

Adding to the direction of the posts above, fiberglass cloth is a stronger choice than mat - the parallel comparison in wood is the difference between plywood and OSB panels. If you've got the opportunity to rework it, consider making your boat better than the original in the process.

The particular weight of cloth or mat may not be as important as how much resin is squeezed out from between the layers as you build it up. Think more fiber per cubic inch means more strength.

[hlev00]

Sorry, another non-answer to your actual question, but a suggestion.


One option worth considering is to use epoxy as it can be a bit more forgiving in its working time. On the other hand, epoxy doesn't work well with standard CSM because of the binder threads that are made to dissolve in polyester or vinylester, making CSM and epoxy too stiff to work around complex shapes which seem likely in a hatch cover. With epoxy or polyester, cloth should be fine for this application.




Harry

[guyrj33]

it's likely the top of the lid is cracked and allowed water to leak into the plywood.
I suggest replacing the core with divinycell core material and using fiberglass cloth for the back side.
Rebuid by first spreading a thickened layer layer of epoxy on the inside of the top layer, this it to seal the cracks in the top. When the first layer of epoxy has begun to kick glue in the core material and apply the back layers.
Then slide everything into your vacuum bag, pump it down and wait for the epoxy to cure.

[Tmacmi]

I guess I really should have titled this "How do I fix this hatch cover in easy steps and recommended ingredients and with no specialized equipment".

When I say easy steps and ingredients I mean things like

Epoxy or Polyester Resin
Type of fiberglass fabric
What type of replacement core.

Very simple stuff, this is a new world for me.

Here are photos of the cover. I have it off the boat and easy access to the underneath of it. You can see a hole in the fiberglass about halfway along the short side of the triangle on the underneath face. There is a small star shaped crack on the top side in roughly the same place.

The core is rotted.

Thank you!

[rslifkin]

I'd cut the lower panel out with an angle grinder, peel it off. Rip out all of the core. Re-core with foam. Bevel the edges of the removed piece and the shell, put the piece back in, fiberglass it back together. If you don't care about the seams being pretty, it's easy enough to do and it'll be plenty strong. Getting a strong and also pretty result is a bit harder, but still do-able.

[coopec43]

I assume there is a photo of the topside and another photo of the bottom side?

I'd go along with what rslifkin says but more specifically I'd us Divynicell (as has been suggested in another post above)

I'd use two layers of 3 oz CSM with polyester resin.

This video clip will explain how to go about your repair. (There are many more clips on YouTube)

https://www.youtube.com/watch?v=obWbqPXqggE

[Tmacmi]

Originally Posted by coopec43

I assume there is a photo of the topside and another photo of the bottom side?

I'd go along with what rslifkin says but more specifically I'd us Divynicell (as has been suggested in another post above)

I'd use two layers of 3 oz CSM with polyester resin.

This video clip will explain how to go about your repair. (There are many more clips on YouTube)

Thanks for the response! I'll look at the clip and search a couple more. The photos I posted are of the top and the bottom.

[Spot]

Tmacmi, just spitballing here, I was expecting more of an upside-down cake pan shape...let us know if I have top and bottom labelled correctly.

I would try to do the project using the hatch as its own container/mold- top side down, cut off back side, remove all core, reinforce hardware areas to be durable and 100% waterproof, replace rest of core, patch back with old CSM and woven tape or a layer or two of plain weave not to exceed the natural edges of the hatch. This is also assuming any other cracks in top or sides would be patched as well.

[Tmacmi]

Originally Posted by Spot

Tmacmi, just spitballing here, I was expecting more of an upside-down cake pan shape...let us know if I have top and bottom labelled correctly.

I would try to do the project using the hatch as its own container/mold- top side down, cut off back side, remove all core, reinforce hardware areas to be durable and 100% waterproof, replace rest of core, patch back with old CSM and woven tape or a layer or two of plain weave not to exceed the natural edges of the hatch. This is also assuming any other cracks in top or sides would be patched as well.

You have the diagram just right, the only clarification is that the hardware is already set in full fiberglass reinforced places.

Here is my thinking.

• Cut out 100% of current core
• Epoxy new plywood core in against top of hatch. (I know the protest about plywood. I just can't justify dropping $90 for Divynicell. I'll use outdoor grade plywood and carefully get epoxy in the edges.
• I'll use a couple layers of CSM with polyester resin

That make sense?

[rslifkin]

If you want to stick with epoxy for all of it, you can get CSM that works with epoxy. You may have to look a little to find it, but there's definitely CSM out there with epoxy compatible binders.

[Spot]

My experience is mostly in lighter weight composites so I lean towards epoxy and plain woven cloth 4, 6, and 7.5oz per sq yard. Two layers of any of those would be enough, maybe even more than enough for a small piece like this. There are some tricks that can be done like taping off the extents of the overlap and trimming the cloth when the resin is at its b-stage (soft set, flexible but not sticky). It makes a neat line that takes a minimum of sanding to clean up. The technical term in some circles is 'zipper cut lap'. Picture below is from https://compositecorner.wordpress.co...ow-to-cut-lap/

[coopec43]

I stick to polyester because it is about a ⅓ the price of epoxy but not quite as strong as epoxy. (but very strong anyway). My hatches are all CSM because CSM bends around sharp corners very easily. Where I have very high loads eg winches I use heavy WR/CSM lay-up. (Don't use polyester over epoxy)