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I agree with PDQ and Overbored. Those are stress cracks that extend beyond the rim of the winch. It will need to be reinforced with fiberglass that extends beyond the recess for the winch. (I had to do a similar overhead repair for a babystay bearing plate. Its not all that bad if you prepare properly.

If I had to do this I would grind the area below the repair to get rid of all of the paint. I make up 4-5 circular layers of biaxial cloth each larger in diameter than the last, and with couple layers that are perhaps a inch or two in diameter larger the recess in the deck below the winch. Number them starting with the largest one first then smallest on up to the next to largest diameter. I would make adisc of G10 that is slightly larger in diameter than the bottom of the recess with a couple holes that align with the old bolt holes, This will be the last piece to go on and will clamp the other layers in place until they cure. It will also create flat surface for the washers.

To do the layup, I would wet out each of the discs of fiberglass leaving them flat on a piece of plastic. (slow cure epoxy hardener) As soon as they are all wet out. I would apply a coat of epoxy to the surface and then mix thickened epoxy and trowel it onto the wet epoxy to bridge any gaps., The thickened epoxy does not need to be perfect or very thick. Then I would stack each layer of the wetted out cloth on the G-10 disk one at a time in the order second biggest to smallest and end with the biggest.

I would use a couple pre- threaded sailmakers needle and some whipping cord through the holes in the G10 from the bottom, and pass the needles through the deck. Then push the whole assembly in place while someone on deck tied the whipping cords together, and uses a couple wedges or a Spanish windlass on the whipping chords to clamp the G10 hard up against the laminate layers. From below you can place some wax paper over G10 and squeegee the reinforcing beyond the G10 against the underside of the deck. Leave the wax paper in place so you get a smoother finish. Alternately use some peel ply and put the wax paper over the peel ply and trowel through both of them. Lastly, if the edges of the laminate do not stay in place against the deck, use tape some scrap corrugated cardboard over the wax paper so that it holds the edge of the laminate in place against the deck.

I suggest doing a couple dry runs to make sure that you have the tools and the materials all laid out so you can do this easily. The actual layup with be scarily quick, maybe 10 15 minutes.

While we don't actually show the lamination, around 17 minutes into this video
you can see how we used string and wedges to clamp the G-10 and laminations in place.

Jeff
 
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Farr 11.6 (Farr 38)
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Wow, that is a complicated procedure for a simple solution to this winch base issue. Not what I would do, but nothing technically wrong with it. This really is a one hour job with simple materials.

One thing I will note is that epoxy seems to gain mystical qualities here. There is nothing about this that requires any physical aspect of epoxy to accomplish, so it could be done less expensive, with more timing, temperature, and mixing error margins, and with much better health safety using polyester resin. Particularly for someone who hasn't worked with fiberglass or epoxy before. Also nothing mystical about G10, because any sheet fiberglass will work fine. Absolutely everything about this reinforcement is in compression, where the properties of epoxy and G10 sheet are meaningless.

Not to mention that there really is no structural issue in the existing setup. The only issue is a small amount of flexing under high loads over time has caused the brittle gelcoat to crack on the surface. Fixing this is fundamentally a good thing to do, but the winch is not going to pull out or break the boat if it isn't addressed.

And once it is addressed, the much, much more difficult work of fixing the esthetic-only gelcoat cracks still needs to be done. After all, that winch mount would last forever as is and never be questioned other than those surface finish cracks.

Mark
I agree with much of this, but not all of this. I do agree that this as essentially a one or two hour repair as well.

But I respectfully disagree that "there really is no structural issue in the existing setup" , To explain my opinion, when you look at the photo from above the winch you can see that the pattern of the crazing reasonably definitively shows that this is flexural (stress) cracking. Fiberglass is a fatigue prone material so that by the time that you see that much stress cracking, you can be reasonably assured that the laminate has been weakened to one degree or another. I would be less concerned if this was in the middle of the deck with out any adjacent openings, but the photos appear to show that this is within a few inches of the opening for the sail locker lids. As fatigue further weakens the laminate the damaged area will spread towards the opening and eventually the edge condition will fail.

To minimize flexure, an area larger than the winch needs to be strengthened in bending and stiffened to minimize deflection. Conceptually, I am recommending reinforcing an area that is several inches larger in diameter than the winch bases which appears to be easily accomplished in this case using the method that I described.

(I will note that my boat has Lewmar 42 ST's for primary winches. The bases for those are also on the coaming and while the top of the fiberglass is smooth and without stress cracks after 40 years, the fiberglass in this area is over 3/4" thick (no core) and that thickness extends roughly 6" fore and aft of the winch and several inches to either side before tapering down to the normal 3/8" thickness of the rest of the coaming.)

I agree that polyester probably could be used for this repair. I personally find epoxy easier to work with since there is a more reliable working time, and so you can prepreg and layup a lot of layers at once. I have not been able to reliably do that with polyester resin,

But the other reason that I like epoxy for a job like this, is that I want the whole assembly to work as a beam using the deck as one flange and the g10 (or other fiberglass) acting as the other flange. Polyester tends to form poor secondary bonds. While it may in fact develop a sufficient bond with the existing laminate to act as a beam, I prefer to count on the higher adhesion of the epoxy.

Polyester is definitely less expensive (roughly half the price) but this project takes so little resin that the difference in cost should be minimal. In the flip side, On this point I profess some bias since, I personally keep epoxy resin and the pumps around, but don't keep polyester.

I agree that G10 is not necessary if the layup is done well. Similarly with the G-10, I usually have scraps of G10 around big enough for this kind of project so its my go-to, I like the stuff. But any flat surface will work to clamp the laminate when it is curing. In a recent bulkhead repair, I used door skin plywood screwed down over wax paper to hold the laminate in place and create a flat smooth surface while the laminate cured. I have also made a backing plate by laminating three layers of cloth (mat-roving, mat) between a release waxed pieces of glass. That would also work instead of the G10 as a flat surface to clamp the laminate to the deck while it cured, and which would ultimately become a part of the final layup.

I am no expert on boat construction or GRP repairs. As I wrote I would simply add a thick stainless steel backing plate below and may a similar plate on top of the GRP.
I would agree with the first sentence. Regarding the second sentence, 'adding a thick stainless steel backing plate' Stainless steel is hard to work with and expensive and so would be about the last material I would use. But beyond that the stainless steel plates alone would not really solve the flexural problem since the existing fiberglass surfaces are uneven and the stainless steel plate would simply create a stress riser at the edges.

In order for the problem to be solved, the area needs to be strengthened against bending. For the stainless steel plate to contribute to that, there would need to be laminate below the plate that extended out onto the adjacent laminate on all sides and beyond the stress cracking. By the time you did that, the stainless steel plate becomes unnecessary. The plate above the deck also does nothing useful since the winch base itself provides the bearing surface and shear resistance that is needed above the deck.

Jeff
 

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Farr 11.6 (Farr 38)
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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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