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Where is evidence of structural failure in the photo of the underside? Yes SS is expensive but would be a great backing plate. We don't know how thick the lay up is in this area either.
On my boat I believe is quite thick...
 

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I agree that the cracks are due to the underlying fiberglass flexing, but disagree that it represents failure of the laminate, or even failing of the laminate. Gelcoat is so ridiculously easy to crack, that there probably isn't a boat here that doesn't have a gelcoat crack here and there. So the laminate is flexing, but not beyond, or close, to the margins it is capable of repeatedly cycling. There is no indication from below that the glass itself has any issues.

But we agree that this flexing should be addressed because, well, just because. It's the right thing to do. There is only 2-3" between that winch pad and the locker edge, so 4-5 layers of additional glass under the pad isn't going to do anything, or spread much load in that direction. It would take 8 layers or so of 1708 to make 1/4", and more if using cloth.

What is required here is a thicker, stiffer laminate extending to the edge, and this is easily accomplished by by cutting a piece of 1/4" fiberglass, let's use G10, so that it fits the circular winch pad area and extends to the locker edge and fore and aft a decent distance. Now let's use thickened epoxy, say a tube of Six-10, and stick that piece to the underside. No need for sewing needles, etc - just use the existing bolts to hold up until cured. Of course, prep the surface for adhesion, but even adhesion isn't completely necessary here. There really won't be enough shear forces between the layers to matter, but adhesion is good regardless.

For this method, the only real skills and supplies needed are a small piece of 1/4" glass sheet (Mcmaster-Carr), a jigsaw with a few blades (the fiberglass sheet is going to dull them every foot of cut or so), and a tube of Six-10 in a caulk gun. Everyone can run a jigsaw and a caulk gun. No learning for the first time to use and mix epoxy, properly wet out stitched biax, and undergoing the frustrating learning process of glassing curved areas upside down (with epoxy - this is much easier with polyester). Also, the health issues with epoxy, particularly for a new user who hasn't experienced all the ways it gets on skin and through protective gear working upside down, goes almost completely away.

I do a lot of work in both polyester and epoxy, and carry both on board. Several versions of each, actually. They all have their places and uses. I'm not for or against either, and learned to glass with epoxy. I've probably done more epoxy work than polyester. People who have primarily used epoxy and are experienced with it often forget just how demanding it can be. There is no ability to control wetout or cure time, it wets out much slower than poly, and no ability to control gel time to better work overhead. When epoxy wetouts begin sagging and falling, there is little to be done other than start over again with fewer layers, or perhaps hang around for hours and hope to catch the very short gel time where they might be restuck and hold, but probably not. If needing multiple laminates, and not using peel ply, then unlike polyester the laminates need to be prepped between each layer. Get that wrong, and you have problems.

Mark
 

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Mark,

I think that we see the problem similarly, but each of us are suggesting our preferred method to remedy the problem.

In my case, I am trying to build a tapered structural platform with the G10 acting as a part of the structure. In your case, the G10 or other fiberglass board is acting as the primary structure with the Six-10 thickened epoxy acting as the web of the structure. If the OP used Six-10, then you are correct that the original bolts if well waxed or greased could be used to clamp the structure together until cured.

I do want to comment on why there is no visible signs of cracking on the underside of the deck. That side of the deck is heavily reinforced with woven roving that is visible on the surface. Fatigue and ultimate failure happens in the matt layers between the roving and in the veil coat and gelcoat layers. While gelcoat is brittle and will show hairline cracks pretty quickly, the pattern of those cracks follow what would be the expected path of flexure. The cracks stop at what would be the expected edges of that flexure. Once the matt breaks down, the laminate becomes increasingly prone to shear failure both from horizontal shear and from shear perpendicular to the reinforcing. Minimally that combination results in a delaminated area that will spread over time, potentially ultimately to the edge of the sail locker hatch opening.

My point being (and I think that this is consistent with your view) if the OP is going to bother to strengthen this area, there is no point in not trying to achieve anything less than a structural solution that whether it is through building up layers of glass and resin, or through using a reinforced epoxy like Six-10 both of which with a structural outer ply of a material like G10.

Jeff. .
 

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Where is evidence of structural failure in the photo of the underside? Yes SS is expensive but would be a great backing plate. We don't know how thick the lay up is in this area either.
On my boat I believe is quite thick...
See post #23 above for explanation of why you don't see evidence of cracking in the underside.,

Regarding your other point about the backing plate, done either the way that I am suggesting or that Mark is suggesting, a bearing plate won't be necessary or even useful. The G10 or fiberglass board that Mark or I are suggesting will adequately deal with shear distribution and will provide adequate bearing capacity for the washers on the individual bolts.

Jeff
 

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I don’t know how much bigger the footprint of the winches that the OP wants to install are, but if the bolt circle diameter is bigger that would also provide a bigger lever when the winch gets loaded/cranked and probably flex the gelcoat even more, so it would be even more important to stiffen that area.
 

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What does alternating directions accomplish? 1708 is 45* biax, so an alternating direction would give the same fiber orientation.

In this case, I do think a backing plate/structural thickening will be all that is needed. The cracks are in the thin inboard end that looks relatively flat all the way to the edge. The cracks go right to the end of the layup, so there isn't a way to extend glass past them. The other side of the winch is cored and doesn't seem to be hinging, so a 1/4" fiberglass plate cut to shape so it fills that entire winch bed area, and bedded in thickened epoxy will work fine.

Mark
Think about what you said. Biax is one direction, and then 45 from that. But not 90 and not 135. Biax is not radially symmetrical like plain woven fabric. If this is not done, it will be weaker in one direction than the other. This 45 angle makes it easier to lay on curved surfaces, and the fibers lay flat, without crimp like roving, but you can't lay it all one way and get full strength in all directions. I made that mistake... once.
 

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Think about what you said. Biax is one direction, and then 45 from that. But not 90 and not 135. Biax is not radially symmetrical like plain woven fabric. If this is not done, it will be weaker in one direction than the other. This 45 angle makes it easier to lay on curved surfaces, and the fibers lay flat, without crimp like roving, but you can't lay it all one way and get full strength in all directions. I made that mistake... once.
Still not understanding, but willing to. Attached is a picture of biax like 1708. The stitched strands run at 45* to each other. So if you lay two pieces over each other in alternating directions, the strands line up. They are indistinguishable, other than which side of the cloth the strands lie. The cloth is symmetrical in 90* turn multiples (alternating directions is 180* which is completely indistinguishable from original)

One could, I guess use multiples of 45* placement to get fibers in even more directions, but If one needs this for strength along biases, then biaxial cloth was a poor choice, and uni cloth should be placed along those load paths. Using biax at all is a waste there. Biax, whether 90 or 45 orientation provides strength in all directions for the purposes it is used. There is no need to choose placement, other than to conform to surfaces best.

Regardless, the loads encountered in the OP's issue of reinforcing a mostly flat surface will not be ones where biax fiber direction has any bearing.

Mark

 

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For those advocating additional fiberglass layup to the OP, none have fully suggested to him yet that the surface needs to be ground back to all glass with no flowcoat, as well as ground completely flat to accept cloth binding without gaps. This will be a very difficult job to perform given where the work needs to be done, and the shape of the area. It will require die grinders and a lot of tedious itchy upside down work. This isn't just a rough up with sandpaper by hand thing.

The alternative method is a rough up with sandpaper by hand, a gap-filling thickened structural epoxy, and a premade sheet of fiberglass.

Believe me, if I thought all of that prep and glass work and angst about fiber paths were necessary, I'd be pushing him hard that direction.

Mark
 
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