Fibreglassing ply - advice please

Gordonmc

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There was a oops moment when my boat was being lifted last year.
The aft ship-lift strop caught the rudder which couldn't take the 15 tons-odd and broke in two places.
I have rebuilt the rudder, laminating 12mm marine ply and am now ready to encapsulate in glass.
-1411453017_20190303_115139_4008031_resized.jpg
I would prefer to use epoxy, so my question is about the best glass to use.
The intention was to use woven mat, 200gm or thereabouts but I gather woven will not bond with the ply very well and should only by used in a CSM sandwich.
CSM-only will need finishing either with tissue or flow-coat. Presumably I will need to use powdered CSM as styrene won't work with epoxy.
Anyone out there with a plan?
 

lw395

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Woven glass cloth is commonly used to sheath wooden dinghy centreboards and rudders, using epoxy.
That is what I'd use.
If you want best strength, then some unidirectional glass with all the fibres vertical might be good.
You could use more exotic fibres, but I doubt it's cost effective and you won't be looking to save weight?
I don't think inter-layer bonding is really an issue, as the epoxy should have good peel strength and none of the loads are trying to peel the layers apart.
 

VicS

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There was a oops moment when my boat was being lifted last year.
The aft ship-lift strop caught the rudder which couldn't take the 15 tons-odd and broke in two places.
I have rebuilt the rudder, laminating 12mm marine ply and am now ready to encapsulate in glass.
View attachment 76491
I would prefer to use epoxy, so my question is about the best glass to use.
The intention was to use woven mat, 200gm or thereabouts but I gather woven will not bond with the ply very well and should only by used in a CSM sandwich.
CSM-only will need finishing either with tissue or flow-coat. Presumably I will need to use powdered CSM as styrene won't work with epoxy.
Anyone out there with a plan?

The West System guide to sheathing might be worth a look

https://www.ecfibreglasssupplies.co.uk/images/pdfs/WSAPP02-A_GUIDE_TO_SHEATHING.pdf

I would use a plain weave fabric

My understanding is that the CSM For epoxy resin is difficult to use but I have not tried it
 

Iain C

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A few pointers from my experiences
-Make sure you've factored in the rudder will be bigger after sheathing.
-You can use a few layers, but you don't need to go full size on each layer. You can have smaller pieces over the stressed areas and less at the edges.
-Plain weave holds it's shape very well but does not like corners. Twill weave is great around corners.
-Consider machining out any areas likely to be bashed, and using thickened epoxy (with a high colloidal silica content) as a "sacrificial" piece. So if you take the ground and you lose a few layers of cloth, you're only exposing epoxy not wood.
-If you can get access to a vac pump, vac bag it to avoid voids and bumps.
-Cover the whole rudder with epoxy first, and add subsequent layers when the epoxy is in the "green" state (slightly tacky, will cut with a stanley blade) to ensure a chemical bond between layers
-If you have any bolt holes for pintles etc, drill out well oversize first, fill with thickened epoxy, and then drill when the rudder is finished

Good luck!
 

lw395

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According to West System, using polyester gelcoat or flowcoat over epoxy will work fine. Providing that the epoxy is fully cured, and the surface is then properly prepared then it should be as good or better than over polyester.

Applying Polyester Gelcoat over Epoxy

Depends what you want from it.
I've repaired gelcoat over epoxy without problems, equally osmosis bubbles are not exactly rare in dry-sailed dinghies which have polyester gelcoat and epoxy structure.
 

Bodach na mara

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The intention was to use woven mat, 200gm or thereabouts but I gather woven will not bond with the ply very well and should only by used in a CSM sandwich.
CSM-only will need finishing either with tissue or flow-coat. Presumably I will need to use powdered CSM as styrene won't work with epoxy.
Anyone out there with a plan?

I once read that CSM should NOT be used with epoxy as it doesn't agree with the powder.

I have sheathed a few small things with epoxy and glass cloth and have even used glass tissue in the final layer. The only reason I can think of for coating the article with polyester resin as a final coat is because epoxy is affected by UV, not a problem with a rudder. Actually, I have just done a repair on my own rudder (which is GRP) using polyester to mould on layers of glass tape and have finished off with two coats of VC tar epoxy followed by antifouling primer and antifouling.

Good luck with the weather Gordon I got lucky with a few warm days in February.
 

Lon nan Gruagach

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A few pointers from my experiences
-Make sure you've factored in the rudder will be bigger after sheathing.
-You can use a few layers, but you don't need to go full size on each layer. You can have smaller pieces over the stressed areas and less at the edges.
-Plain weave holds it's shape very well but does not like corners. Twill weave is great around corners.
-Consider machining out any areas likely to be bashed, and using thickened epoxy (with a high colloidal silica content) as a "sacrificial" piece. So if you take the ground and you lose a few layers of cloth, you're only exposing epoxy not wood.
-If you can get access to a vac pump, vac bag it to avoid voids and bumps.
-Cover the whole rudder with epoxy first, and add subsequent layers when the epoxy is in the "green" state (slightly tacky, will cut with a stanley blade) to ensure a chemical bond between layers
-If you have any bolt holes for pintles etc, drill out well oversize first, fill with thickened epoxy, and then drill when the rudder is finished

Good luck!

What he ^ said.
I sheathed a 14" plywood dinghy with Gurit SP106 and plain woven cloth. if I had to take it back to bare ply I would start again or cry.
I didnt have any major problems overlapping and wrapping round the bow, so dont be too worried about it not liking corners..
 

Quandary

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I once read that CSM should NOT be used with epoxy as it doesn't agree with the powder.

I think you misunderstand the problem slightly, the mat used with polyester has a resin binding to hold it together until it is wetted up, (otherwise it would bunch up rather than layer) which is compatible with that type of resin but not with epoxy resins, for them you use a powder bound mat, both types are stocked by most grp suppliers but where only one is available it is usually the one made for polyester. The powder bound matt seems to be a little more difficult to wet, it does not hang together quite as well as the older type.
 

Tammany

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Woven cloth is ok. Make sure you coat the bare wood with the resin before laying on the cloth however. This method & cloth is also used in the model aircraft industry too.
 

William_H

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Polyester resin could be OK for this provided that the layers of GRP are so thick that it becomes a GRP rudder laid on a wooden male mold ie at least 6mm thick. That way the GRP can hold its own in strength regardless of adhesion to the wood. (which will be poor). If however you see the rudder as being merely a wooden rudder sheathed in glass then yes epoxy is the best. (essential) The problems in both are in getting the glass to conform to the curves especially the leading edge and bottom. The trailing edge can be layers of glass clamped together to make a thin edge.
As said twill weave glass or use ordinary cloth cut on the bias so any bend is diagonal to the weave.
If as it might seem you are going for a thick GRP then woven rovings is the name for really thick woven cloth. This should be oK with epoxy or polyester as there is no binding agent. ol'will
 

Boo2

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According to West System, using polyester gelcoat or flowcoat over epoxy will work fine. Providing that the epoxy is fully cured, and the surface is then properly prepared then it should be as good or better than over polyester.
Why would one prefer to use gel or flow coat over epoxy rather than more epoxy, may I ask ?

Boo2
 

lw395

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Why would one prefer to use gel or flow coat over epoxy rather than more epoxy, may I ask ?

Boo2

It's cheap, easy to repair and readily available in lots of colours including matching your boat.
It's UV resistant and tougher than paint in many ways.
It's easy to slosh on in a thick layer. Much less work than filling, fairing, epoxy coating and painting with something UV resistant. Depending on what the project is.

For a yacht rudder I'd make it in epoxy, then coat it in epoxy (possibly gelshield type). Below water would be antifouled, above water painted.
 

William_H

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Why would one prefer to use gel or flow coat over epoxy rather than more epoxy, may I ask ?

Boo2

Now I may be wrong but I think what you call epoxy paint is actually polyurethane paint (2 pack) True epoxy as in resin will deteriorate quickly in UV so although it can be pigmented it is not a good paint. Polyurethane is great hard paint in many colours but not necessary if rudder is to be anti foul panted. ol'will
 

TQA

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Err there should not be UV problem The OP is working on a keel.

I have vacuum bagged stuff in the past and even if I had access to the kit I would not bother.

I find small rollers work better than the other methods as you don't pull the cloth about.

TOP TIP Check to see your foam roller is suitable for epoxy
 

lw395

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Vacuum bagging is mostly about using the minimum amount of resin, for minimum weight etc.
But it can have other advantages, like you could do both sides of the rudder in one hit without fear of all the resin sagging.
I don't think it's needed here.
 

Quandary

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When we bought our Finngulf 33 part of the deal was that for a couple of years we would offer demo. sails to enquirers from Scotland. The first to take advantage was a keen racer and he turned up with part of his crew and with his sailmaker, Chris Owen. The were out on quite a boisterous day doing more than just having a wee sail. Chris made various recommendations regarding sail mods and choices but also recommended that the rudder power could be improved. The boat had a very deep Jefa grp rudder to go with a sophisticated lead keel.
Chris advised that we would get more power from the rudder if the aerofoil was made fatter. His customer went on to buy a Finngulf and raced it for a few years, instead of fattening the rudder profile they applied a flanged fin around it close to the top like you see on aircraft wings, this was easier to do and to undo if it was not successful. I copied this on our boat and rated it a success.
What I am getting at with this tale is that fat rudders can be faster than skinny ones. If you look at modern aircraft the aerofoil shapes are not that different from the profiles that high performance rudder makers like Jefa select though the speed is 100 time greater, of course the medium they operate in is a lot heavier. So I would not worry if a rudder shape changed as long as a proper aerofoil profile is maintained.
 
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