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Old 12-11-2020, 13:51   #1
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Layers of cloth per inch question

I'm sorry if this is not the right form to post it, but I wasn't sure where else to put it. I have a repair question. I was repairing some through hull holes on my boat and I seem to have put the patch on a little thin, at least in the middle. I was trying to build up a section of 3/8 inch hull and I had somewhere between 17 and 20 layers of 10 oz woven cloth with epoxy resin and that doesn't seem to have built it up quite thick enough. Am I thinking about it wrong? Should I really have used something like 25 - 30 layers of 10 oz cloth just to get 3/8 of an inch?

in any case now I have to decide if I want to grind away the whole thing from the inside and out and redo it, or if I should just add another five or six layers with resin over the old patch.
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Old 12-11-2020, 15:08   #2
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Re: Layers of cloth per inch question

What are you using to get that many layers for a 3/8th inch thick hull ? I did a 1/2 inch thick 1.5 through hull hole with 4 layers of stitch matt, on the outside, then 4 layers on the inside ! When I ground it flat (to hull curve) I filled slight voids with thickened resin ! I am asuming you did a 12 to 1 taper on both sides, which should give you a taper on each side of about out about 4 inches ?
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Old 12-11-2020, 15:15   #3
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Re: Layers of cloth per inch question

10 oz woven. Nothing special.
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Old 12-11-2020, 15:16   #4
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Re: Layers of cloth per inch question

https://www.fibreglast.com/product/1...erglass_Fabric

Then chase down to the Documents tabs and Product Data Sheet:

10 oz/sq yd 38" and 50" Wide .014" Thick, 16 x 14 Plain Weave Frequently used in mold building, sandwich core panels, and high strengthlayups.
8 plies of 10 oz fabric will produce a strong 1/8" laminate weighing 1 lb per sq ft. This Style 7500 fabric meets AMS-C-9084 Type
XII-A (Class 1 & 2).

Crosscheck:

https://www.hexcel.com:82/calculator...alculator.aspx

Cured Ply Thickness Calculator

Reinforcement areal weight, grams per square meter
340 (10oz/yd *33 to convert to g/sm)
Resin content, percent weight / weight
50 (hand layup)
Resin density, grams per cubic centimeter
1.2 (guestimate, 1.1 to 1.5 range)
Fiber density, grams per cubic centimeter
2.54 (Googled)
Void volume fraction (void % divided by 100)
0.00 (I know you are that good!)

Cured Ply Thickness, mils
16.425
Cured Ply Thickness, millimeters
0.42

Fiber Volume, percent
32.09

.016" per layer * 24 layers = .384"
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Old 12-11-2020, 15:29   #5
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Re: Layers of cloth per inch question

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...8 plies of 10 oz fabric will produce a strong 1/8" laminate ...
That settles it then. I appreciate the info. Anyone want to try to talk me out of just adding more layers even though the original patch has already cured?
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Old 12-11-2020, 15:39   #6
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Re: Layers of cloth per inch question

People usually alternate mat and roving to build up thickness. If the layers you've already applied provide enough strength, there's no real need to add more. You could fill in the divot with some epoxygoop and be done with it. (Chop up the cloth you have into little pieces, add epoxy until it's like a crab cake and slap that in there.)
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Old 12-11-2020, 18:39   #7
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Re: Layers of cloth per inch question

I'm hoping that even at less thickness than the original layup (looks to have been alternating layers of roving and chopped strand mat with polyester resin), the fact that it's 10 oz mat and epoxy might make up for it. I'm still going to add the additional layers, but they'll only be attached by the mechanical bond, whereas of I went an idiot, the whole thing would be chemical (after the first layer obviously).

I just want to see if the response I get is more like, "It's probably fine," or, "For the love of god no don't do it the lazy way or you'll immediately sink on putting the boat in water."
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Old 12-11-2020, 18:56   #8
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Re: Layers of cloth per inch question

It's fine. You don't need to add strength, just keep the water out. The hole that was there didn't have any strength.
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Old 12-11-2020, 19:52   #9
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Re: Layers of cloth per inch question

Not saying you need to put as much strength in the hole as is surrounding it, but since you asked....

It takes a lot more fiberglass to build up thickness than you would think. The rate I read in another forum, and since (roughly) proved to myself, is .0018 inches thickness per oz. of fabric, in normal hand layup trying to get a good 'glass/resin ratio.

Never used woven roving, so maybe that soaks up more resin.
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Old 12-11-2020, 19:58   #10
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Re: Layers of cloth per inch question

I believe it now.

It is a tiny tiny bit low over the inner area of the bevel just around the hole as well (maybe 2 ml) so I'm going to add some layers, but I feel a lot better about it now.
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Old 12-11-2020, 20:11   #11
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Re: Layers of cloth per inch question

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Originally Posted by kmacdonald View Post
It's fine. You don't need to add strength, just keep the water out. The hole that was there didn't have any strength.
A very sound point! I agree wholeheartedly... fair it out if you wish for cosmetic reasons, but don't worry about strength. Spend the worry on meteor strikes and being trampled by unicorns in stampede.

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Old 12-11-2020, 20:33   #12
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Re: Layers of cloth per inch question

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A very sound point! I agree wholeheartedly... fair it out if you wish for cosmetic reasons, but don't worry about strength. Spend the worry on meteor strikes and being trampled by unicorns in stampede.

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I can't tell if this is sarcasm.
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Old 12-11-2020, 20:42   #13
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Re: Layers of cloth per inch question

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Originally Posted by cyrano138 View Post
I can't tell if this is sarcasm.
Nope, not sarcasm... kmac made a insightful comment with which I agreed.

The latter bit was meant to be amusing, relating to the degree of risk involved with your repair.

Sorry if I was confusing.

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Old 12-11-2020, 20:49   #14
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Re: Layers of cloth per inch question

I knew I shouldn't have spent money meteor-proofing the deck.
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Old 12-11-2020, 22:24   #15
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Re: Layers of cloth per inch question

The hull was made with alternating layers fiberglass cloth and mat. The mat builds thickness faster and therefore stiffness. The cloth is the strength part. With cloth only you don't need the same thickness as cloth/mat to achieve the same strength.
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