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Old 18-05-2023, 23:43   #1
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Fibreglass sheathing required for large schooner in Lesvos, Greece

We are refitting a 90ft schooner in Lesvos for a Christian charity.
She had some quite large areas of rot in the wooden planks, which have now all been replaced.
She is sheathed with polyester resin and chopped strand mat.
We want to replace this with epoxy with biaxial cloth, or similar, preferably using a vacuum to install.
There are approximately 20sq meters to be done to a thickness of about 10mm.
Do you know someone in Greece who might be able to help us?
Please let me know!
Thanks
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Old 20-05-2023, 18:53   #2
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Re: Fibreglass sheathing required for large schooner in Lesvos, Greece

What is wrong with the existing fiberglass sheathing such that it needs replacing? It sounds like the new sheathing could add considerable weight to the hull. Depending upon where it was, it might not add much to the structural integrity of the boat, despite being a large expense. Whoever is advising you to proceed with this operation will likely have some suggestions -- perhaps a cousin of theirs? Where is the boat now? Moving a structurally questionable boat (it needs 20m2 of new sheathing!) more than a few miles on a calm day is probably not a good idea. Skeptic is a word with Greek roots. Be skeptical and careful.
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Old 20-05-2023, 21:18   #3
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Re: Fibreglass sheathing required for large schooner in Lesvos, Greece

Iím interested in the haulout method, is that a type of sled that slides over a series of logs? What is there to stop her from falling over in the cradle.
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Old 21-05-2023, 00:25   #4
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Re: Fibreglass sheathing required for large schooner in Lesvos, Greece

The sheathing was separated from the hull in approximately 8 square meters.
The other areas had to be removed for the rotten planking to be replaced.
It is an integral part of the strength of the hull, hence the thickness, it is not only a sheathing like cascover for example. She is more of a composite construction than traditional.
We will not be moving her until the works are completed.
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Old 21-05-2023, 00:26   #5
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Re: Fibreglass sheathing required for large schooner in Lesvos, Greece

Quote:
Originally Posted by skipperpete View Post
Iím interested in the haulout method, is that a type of sled that slides over a series of logs? What is there to stop her from falling over in the cradle.
Yes that's correct, just gravity!
When in place she is chocked up further.
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Old 21-05-2023, 00:55   #6
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Re: Fibreglass sheathing required for large schooner in Lesvos, Greece

If you are using epoxy and stitched fabrics you certainly won't need 10 mm of laminate. I would think that would be around 10 -12 layers of 600gm biaxial. I also would not want you to use biaxial on a planked boat. The usual method is to use double bias (which has the fibres running at + and - 45 degrees to the roll. Biaxial will have half of the fibres going the length of the planks, which is redundant structurally.

You can work out the needed laminate by working out the thickness of the polyester choppy laminate and getting some advice on an equivalent laminate in stitched fabrics. My guess is that it would be about half or 1/3 the thickness required.

I would not use vacuum on a large 20m^2 job. It depends where it is but I find vacuum bagging to be awfully wasteful and prone to rushing. (I built a 38ft carbon foam tri with vacuum) It does make great laminates but with your boat you can just add an extra layer of glass. If you tape the laminate up top then it should stick to a nice pre-prepared hull skin (roll on epoxy to the skin first). Then push the fabric down and roll on epoxy. You will also need to taper the current laminate with a soft pad or grinder so that the new laminate can flow load through the border.

I don't think the job would be that hard. If you take lots of pics, we could help you from afar.

cheers

Phil
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Old 21-05-2023, 02:05   #7
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Re: Fibreglass sheathing required for large schooner in Lesvos, Greece

Hi Phil,
This is very helpful, thank you, I will explain a bit further.
The existing polyester chop varies in thickness between 10mm - 19mm, it is thickest near the bottom of the keel and thinner approaching the waterline.
That's interesting about double bias v biaxial and makes a lot of sense.
The thinking behind vacuuming was to make sure the epoxy is as well bonded as possible to the wood, especially the areas on starboard forward, where the planks had diesel in them initially... they have been sanded, degreased and drying for a while now, we tried a small patch of epoxy as a test and it has stuck well.
If not using a vacuum we were planning to wet out the mat on a table first.
A lot of the area is concave so it would be hard to force the epoxy up and through the fabric, at least that was our thinking.
We were planning to precoat with epoxy as you suggest.
We have already tapered the existing laminate edges to approximately 1:12 ratio.
Thanks for the offer of advice, always helpful
Do you know how we can calculate the amount of epoxy + double bias based on the polyester / chop that was there? Is there a formula, or someone you would recommend asking?
Many thanks
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Old 21-05-2023, 02:14   #8
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Re: Fibreglass sheathing required for large schooner in Lesvos, Greece

pictures showing wood repairs in progress, they are now completed.
Drawings show the approximate area where the sheathing has been removed and the double hatching shows where the timber was rotten.
(The rot came from fresh water leaks internally / on deck.)
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Old 21-05-2023, 02:32   #9
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Re: Fibreglass sheathing required for large schooner in Lesvos, Greece

Rough formula for hand lay up is 66% resin for 33% glass by weight.
This applies to chopped strand mat, which you should never use with epoxy.

On multiaxials you can, depend on skill level reach up to 50%:50% glass to weight ratios.

If you have large overhanging areas, I suggest more layers of a lighter biax instead of fewer of a heavy one or it will tend to fall down during application.
It might be easier to work with overlapping vertical strips of fabric instead of long horizontal runs. On the thickness you want to achieve it won't make a huge strength difference, as none of the fiber run longitudinally in the +-45 fabric.

Absolutely wet the wood out with catalysed epoxy resin before lamination, but don't let it cure before lamination. Just give it a little time to soak in and add another coat if it's sucked up to fast.
Than proceed with your prewetted biax.
Don't built up to much thickness in one go, or the exotherm reaction heat might "cook" your laminate.
Better add 2-3 layers and let it cure to a slightly tacky touchdry (most reaction heat is gone by then) and then proceed with the following layers.

If it's hot, use a slow hardener and/or cool down the unmixed resin and hardener in a fridge. This will give you a lot more working time.
We built a 6m structural beam this way in hot summer conditions.

Side note, Biaxial is a generic word for stitched two layered fabrics and it exists in a lot of different fiber orientations.
The +- specifies the actual fiber orientation. So, a 0,90 Biaxial is not what you want, +-45 is very much what you want.

Lots of success.

PS. Just saw the pictures, so good for you that you have vertical but not overhanging areas.
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Old 21-05-2023, 03:29   #10
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Re: Fibreglass sheathing required for large schooner in Lesvos, Greece

Not to drift the thread, but nobody calls the 0-90 cloth "biaxial"; it is plainly called "cloth" or "woven roving", because it has one axis, 0, with the warp travelling along the axis and the weft going at 90 degrees to that.
The "biaxial" is so called because the strands are going at plus or minus 45 degrees to the axis of the cloth, but meet each other at 90 degrees.
There are also twill weaves and harness weaves which are not biaxial, even though strands cross each other, but 'Biaxial' is a very specific thing.
I would use biaxial cloth for the sheathing, but first I would ask whether the juice is worth the squeeze: it's going to be a huge and expensive and difficult job, and pre-wetting panels big enough to be worthwhile with epoxy is usually done on an impregnator: doing it by hand outside will be tough, even with professional help.
Is the rest of the boat worth the work on the hull, and is the budget substantial enough to cover the inevitable cost overrides?
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Old 21-05-2023, 03:43   #11
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Re: Fibreglass sheathing required for large schooner in Lesvos, Greece

Just a comment. Having a nice smooth sanded surface to bond on to is not always the best. Timber has a longitudinal grain, horizontal in your case, that is good. Adding a series of cross cuts, across the grain vertically and at 45 degrees with a box cutter will assist in mechanical bonding especially in the areas that were previously soaked in diesel. When bonding to a substrate the rougher the better.
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Old 21-05-2023, 03:51   #12
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Re: Fibreglass sheathing required for large schooner in Lesvos, Greece

Bay of Fundy boatshops started polyester fiberglassing old wooden planked fishing boats about 1980 & still do. The method involved stripping to bare wood,repairing rot & sag,fairing ,etc. while the hull dried for a couple months. The mat & roving layers were SS stapled to the hull to hold it in place while wetting out with resin. Finished layups were 3/8"- 1/2" thick.
These were mostly workboats,but some were approaching "yacht " finish,depending on customer. The end result is a hand-laid fiberglass hull over a wooden "mold). There are hundreds that are still in use after 30+ yrs.Click image for larger version

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Old 21-05-2023, 03:58   #13
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Re: Fibreglass sheathing required for large schooner in Lesvos, Greece

I agree with a lot of what Franziska says but not all.

I don't pre-wet large sections of glass. I only pre-wet what I can easily carry the glass to the job and your job is way too large for that. As for the job being tricky - it won't be, it will just be laborious.

I have successfully laid up 600gm 0/90 biaxial overhead. I needed the help of my sons. We put on caps and gloves and paper suits and glassed the inside roof of our 38ft cat interior bridgedeck in less than 1 day.

I would not pre-wet but I would precut the glass. Then I would tape the glass at the top edge after the timber has been wet out with epoxy. It will drape down over the wet out timber. Then, gently pad down the glass onto the wet timber with gloves. Then when all the glass is tacked down use rollers, not squeegees, to roll epoxy into the glass.

You will need some helpers. A couple to get the glass all in place and one to mix resin. Then one or two to roll the resin into the glass. Like Franziska says, you want to get a couple of laminates down. But if you do too many then the laminate will fall off the hull as it becomes too heavy. I would try to get the whole side done in one go myself, but you may instead have to cut the job into sections.

As for a laminate guide - I would be thinking of somewhere about 1/2 to 1/3 of the thickness of the polyester glass. The polyester is way over engineered already. There is no way that the polyester resin is able to transmit the shear force from the timber to the thick polyester laminate - polyester and timber do not have great bond strength. What this means is that the interface between the polyester and timber will be much weaker than the polyester laminate. So the weak point is the bind between the poly and timber and you don't have to worry about replicating the thickness in epoxy.

Chopped strand mat is such awful stuff mechanically that you have have lots of it to do the same job as nice fibreglass. One of my books has the tensile strength of a polyester CSM laminate at 100 MPa whereas a good epoxy stitched fabric laminate can be over 500MPa. So you can use much less of the double bias. Probably 1/2 to be really safe - about 5 layers (5mm) and then fill the rest. You also get a much much better bond between the timber and glass with epoxy, so the sheating will be able to take the load without shearing the glue line. I doubt you will find an engineer who will authoritatively tell you what to do with laminates. But if you get to half the thickness with a double bias/epoxy laminate, you will be fine and stronger than what is there now.

I can't see in the photos, the 1:12 feather at the edge of the old laminate. It is essential that this be done well. When all is complete, you will need a 7 inch sander/polisher with 40 grit to feather the edges of the epoxy laminate.



Cheers

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Old 21-05-2023, 04:00   #14
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Re: Fibreglass sheathing required for large schooner in Lesvos, Greece

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Old 21-05-2023, 04:16   #15
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Re: Fibreglass sheathing required for large schooner in Lesvos, Greece

Quote:
Originally Posted by Benz View Post
Not to drift the thread, but nobody calls the 0-90 cloth "biaxial"; it is plainly called "cloth" or "woven roving", because it has one axis, 0, with the warp travelling along the axis and the weft going at 90 degrees to that.
The "biaxial" is so called because the strands are going at plus or minus 45 degrees to the axis of the cloth, but meet each other at 90 degrees.
There are also twill weaves and harness weaves which are not biaxial, even though strands cross each other, but 'Biaxial' is a very specific thing.
Down here in Australia, at least since I built boats, 0/90 is called biax. Woven roving is very different. Woven roving is not stitched together but has the glass rovings woven (hence the name) like normal cloth. Biaxial (0/90 to the roll), Double Bias (+-45 to the roll), Triaxial (+- 45 and uni) and Quadraxial (+-45 and Biaxial stitched together - all stuff we have used a lot to build cats and tris) are stitched and much higher spec than woven roving because the strands are straight and not woven. I only use woven cloth for low strength plywood sheathing. All higher spec fabric is stitched. It may be different in different countries but this page from a local retailer gives a picture.

https://trojanfibreglass.com.au/prod...gory/permetre/

It certainly looks like European retailers don't use biaxial to mean 0/90 so I may have to be careful with terminology. It is always better to use double bias (+-45) over wooden strips. I would also disagree with Tupaia, glass really doesn't like ridges under it. Ridges can leave a little void either side, so I would sand the timber with my 7 inch sander polisher with a 40grit disc on it. That gets it smooth enough but with enough grip.

cheers

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