Cracks in Hull - Storm Damage

[nickc]

I believe you are supposed to put the fiberglass patches in smallest to largest, opposite of the diagram you drew. I’m not an expert but i’ve been doing quite a bit of research because I am getting ready to begin major repairs of my boat as well. I found a guy online who makes videos about how to do boat repairs, he is great and I have seen him recommended numerous places online. I will attach the link for his video on fiberglass repairs below.

https://www.boatworkstoday.com/videos/s ... ir-part-1/

[bilbo]

Nickc, that's what I thought too. Andy has a video for "one sided repairs" and he lays up his plies as you said. I've got some more grinding and prep to do, so I've got some more time for research before I get to that point, but the more I think about it if the large patch goes in first it might bunch up, no? Especially where the layup is thicker. Maybe not, I don't really know. This will be my first go at layup since I was in school 12 years ago, but that was a lab so very different.

[GreenLake]

I'm basing my recommendation on materials published by two different vendors of epoxy resins. While I think they are reputable sources for this information, I couldn't help wondering about a possible reason for this counter-intuitive approach.

Best I can figure out is that you avoid "stair steps" inside the repair if you go largest first. These little steps create places where there's a bit of unreinforced resin, which weakens the laminate. On the other hand, going largest first, all the "steps" are at the surface where you'll sand them smooth, and as long as the repair is shallow enough to allow the laminate to conform, there's no space that isn't filled completely with glass.

Also, if you go smallest first, you have to be perfect in sizing your patches, while in the largest first method, any excess is sanded off at the end, so you only need to make sure you do have a bit of excess, but don't care whether it's exact.

Different ways of stacking laminate to fill a shallow repair

Laminating-A-Patch.gif (24.21 KiB) Viewed 3477 times

What I can't answer is whether the difference reflects a level of theoretical perfection that isn't noticeable in practice, or whether it matters. I suspect the answer to that will depend on whether the laminate being repaired is overdimensioned for the loads it experiences. The closer it is to minimal required strength, the less infidelity in repair you would be able to tolerate. Some part of the DS hull are truly overbuilt, but the flat sections of the rear hull may experience forces that are closer to the design limit.

I have nothing riding on how either of you does this, but I felt like sharing my personal level of understanding on this issue. So, now you have all the info to make your own decision.

[bilbo]

Still undecided on the order of my layup. I've been doing research and there are compelling arguments both ways. I may end up flipping a coin. I bought the wrong fabric so I'm a bit delayed. I have 6oz straight weave. It will work fine for the repair near the transom where the hull is thinner, but will need a ton of layers for the repair near the keel as it's quite thick there. I'm going to get some biaxial glass that's a good bit bulkier and supposedly stronger.

[GreenLake]

For the bulkier part of the layup, you might also add a layer of mat in between the other layers. With mat, you do need to pay attention to whether it was manufactured to be used with epoxy. You will find that information on the package. Mat will conform better to small gaps, so it's useful in cases where you don't want such gaps to be pure resin. But it's good for bulk.

6oz cloth is very thin, I tend to use it as a final layer, so the pattern from the weave isn't so thick. You might add some heavier cloth to your arsenal, even for the thinner sections. Laminate with the biaxial stuff should be stronger than equivalent thickness of laminate with the cloth, but as the name says, it has a preferred direction.

My own hull repair used cloth and mat, but it may not be comparable as I used a flat sheet of laminate that I laid up on the bench as a backing plate. I have linked that thread above and looked back at what I wrote: the backing sheet was 1/16" from 3 layers of cloth/mat/cloth. I don't write what kind of cloth, but I typically stock both 6oz and something a bit heavier.

I glued the backing plate in place first, then did the repair using only mat, because I was no longer worried about strength and it became a matter of just filling the void.

I was forced to use an additional patch on the backside because in my case the two edges weren't in the same plane any longer (you might want to check yours, just in case). If you can tolerate a visible backing plate, I would suggest you consider one, if only to prevent your repair from sagging in the middle of the gap. Of course, you could use a stiff bit of plastic or something covered in peel ply as a removable backing plate. In any case, a permanent one should dispel all worries about insufficient strength. And unless you need one stiff enough to force the hull into local alignment, you might get away with a thinner one, perhaps down to 1/32". Just something to consider. (At that point, finally, the order of layup in the repair itself would become almost academic).

[GreenLake]

Still undecided on the order of my layup. I've been doing research and there are compelling arguments both ways. I may end up flipping a coin.

As I wrote, I have noting riding on whichever decision you make. However, I always like to learn and find out whether a piece of information is just something that is endlessly repeated by one source cribbing from another or whether it represents a better understanding of an issue.

In this case, the more detailed and comprehensive written guides (not videos) that I've consulted have the large-patch-first method, while I found many of the more basic guides advocating the "obvious" small-patch-first solution. In some cases, I had other issues with the info presented by those basic guides, and they gave no reason for their approach.

When I wrote my earlier post I had forgotten the actual reasoning given for the large-patch-first method and came up with one of my own. Because your remark made me curious, I went back and looked at one of the sources and they suggest that sanding the patch may work differently. If you see my graphic above, you'll notice that sanding would mostly cut through the edges of each layer in the largest-patch-first method. However, in the smallest-patch-first you are effectively sanding only the top layer, and if that's not flat, you'd sand through that. Now you've reduced the strength of your largest patch. Anyway, something like that.

As much as I love to chase such things down, it may well be the case that once you've finished your repair, that particular choice isn't as much a contributor to success as many other things: incorrectly mixed epoxy, insufficient dewaxing of the repair site, puddling of liquid epoxy, or conversely starving the repair and not fully wetting out the laminate. So yeah. But I'm still curious about what other rationales are being claimed pro and con.

[bilbo]

So far I've not found any "official" publication other than one from West System that details the order of layup, and it says start with big patch. Lots of internet expert opinions though.

I found a local composites supplier where I can get resin, cloth, and just about anything else I could ever need. One of the benefits of living somewhat near the coast I guess. They are cutting some 17oz. biaxial cloth for me and I'll be picking it up later this week. In the meantime, I made my backing plates out of the 6oz. cloth I had and used some thickened epoxy to put them in place.



Also, does anybody recognize this sticker? It was on the boat when I bought it and I've no idea what the story is behind it. The boat also has a DS Association sticker from 2014 so I figure maybe someone here knows what it is.

[GreenLake]

So far I've not found any "official" publication other than one from West System that details the order of layup, and it says start with big patch. Lots of internet expert opinions though.

I was under the impression I had found that in other places when I first researched it many years ago. But I can now also only find it at West. However, it's hard to think of a more reputable source. If I had the proper tools for destructive testing, I would for sure want to try for a head-to-head test.

BTW, I thought of another reason why biggest first can be useful: it would be best at spanning gaps in any situation where you don't have a backing plate. But by the time you ave a gap big enough for that to matter, you might be using a baking plate anyway...

[GreenLake]

I found a local composites supplier where I can get resin, cloth, and just about anything else I could ever need. One of the benefits of living somewhat near the coast I guess. They are cutting some 17oz. biaxial cloth for me and I'll be picking it up later this week. In the meantime, I made my backing plates out of the 6oz. cloth I had and used some thickened epoxy to put them in place.

I like buying from local suppliers over chains and online. Good luck for you to have one nearby.

Your backing patch setup looks very professional!

[bilbo]

I found TotalBoat also uses the big patch first. I didn't find it in a document anywhere, but they did it that way in the how-to video I watched. When I was in engineering school, I got to play with composites quite a bit for different projects. I wish I were into boats back then, I could have done the testing. They had a few different testing stations for tensile, bending, fatigue, shock, etc. They had a vacuum cannon that was fun to play with. Basically, you pull a vacuum on a box with a long tube attached. The composite goes in the box and a cartridge goes in the opposite end of the tube. The cartridge is released and flies down the tube, then impacts the composite being tested. We used to smash all sorts of things with it.

The place I ordered from is called US Composites in West Palm Beach, FL. Their website and ordering process is a bit dated, but it was nice being able to talk to a real human being when I called them. They can ship their product, but I'm not sure how reasonable it is.

Last week I had company over so not much time to work on the boat. I did use some thickened epoxy to make the repair areas smooth. Yesterday my plan was to sand and prep the repair areas after gluing the backing plates on and do the patch today, but I was ahead of schedule so I decided to go ahead with it. I got ahead of myself, and after mixing the epoxy I realized I hadn't wiped the area down with solvent. So I lost a minute or so of my pot life there. I also neglected to put plastic around/below the repair area to keep excess resin from running down the boat hull. Another few minutes of working time lost cutting and taping some plastic. I really should have just waited and I wouldn't have been in a self-imposed rush. Now I've got quite a bit more sanding and cleaning to do on the hull. Lesson learned.

I chose to wet the pieces individually on a flat work surface before applying them to the hull. The boat is on its side, so the repair is not flat. Then I picked up the piece and laid it on the hull. It was difficult to keep the smaller patches from stretching and losing their shape. Once on the hull, it was hard to reshape them without introducing air into the laminate. One video I saw showed them laying up all the patches together on the flat board and then transferring all of them at once to the hull. I think I'll try that on my next patch as it will allow me to get rid of most of the excess resin before putting it on the hull. This one was rather resin-rich in my opinion. I have two to go, so hopefully by the last one I'll figure it out!

[GreenLake]

The longer I stare at this diagram, the more I'm convinced it's the difference between gluing/laminating flat surface on flat surface vs. "edge on".

Laminating-A-Patch.gif (24.21 KiB) Viewed 3290 times

The first joint, for the largest patch, would be a glue joint without chemical bonds, while the next layers are all part of a single laminate. If you go big first, the glue joint is nice and continuous throughout without any gaps and transitions. That should give you the best conditions for what is the weakest part of your repair.

Love that you actually have experience in composites.

Building a patch on the bench should make it easier to wet out. If you work on a transparent support medium (plastic sheet) you can draw concentric outlines on your bench to help with placement. As you wet out the laminate, it also becomes transparent and lets you view the marks.

There are all sorts of warnings about not applying gel coat on top of an epoxy repair. I've also read that the issue is reduced if you mix accurately and clean any blush. I've not observed issues myself. However, System Three sells SB-112 epoxy which is formulated to be gelcoat compatible and can be used as an intermediate layer. I found that not necessary, but I've been using other System Three resins for various repairs. On the contrary, when I tried to paint the hull it turned into a complete disaster.

[bilbo]

I did another patch today. I built the whole thing on the bench, and added a layer of 6oz. Straight woven cloth over the whole thing. That was much much easier, and the straight weave “cover” helped immensely to keep the patch layers in place while rolling it down. Without it, the smaller patches on top wanted to slide around and disintegrate. One more to go.

This “field work” is a whole new thing for me. All my previous experience was in a lab, and we used VARTM almost exclusively. I only hand-laid a couple of prototype parts. This is much more difficult.

I intend to gelcoat over the patches when I’m done. I’ve seen lots of anecdotal evidence that it will hold up. I guess worst case is the gelcoat fails and I will have to remove the gelcoat and build it out with epoxy, then paint the bottom. Water shouldn’t hurt the epoxy.

[GreenLake]

Good. It sounds like you are gaining useful experience.

On the theory question. I finally found a discussion where this very same question is addressed. The rationale, as I suspected, is that you want to get the discontinuities out of the repair. Which matters most if you have limited design reserve. Luckily for us, the DS was overbuilt and so is more forgiving.

Here's the link to the discussion.

One of the other nearby posts in that thread shows a diagram that illustrates the discontinuities with the smallest-first approach.

[bilbo]

As of yesterday, the repair is functionally complete. I ended up turning a lot of glass and epoxy into dust but everything's finally sanded down just below the gelcoat surface ready for finishing. It's really not that bad, the flap wheel makes quick work of it and the RO sander smooths everything out, but wearing all the PPE in Florida weather isn't much fun. I could sand for about 10 minutes before having to clean and dry my goggles. You can see on the repair to the right in the second photo where things got away from me and I have more sanding to do. I put a coat of epoxy over the repaired areas to protect them from moisture while I get ready to put the gelcoat on.




I have some gelcoat tint that should be arriving today and I can mix that up. I'm going to try to get close to the color of the existing gelcoat but since this will be under water I'm not super worried about an exact match. From what I've read it looks like the most straightforward way for me to do this is to brush it on and build over the thickness of the existing gelcoat, using wax in the final coat. Then sand it back. One thing I'm not sure on is the time between layers of gelcoat. I'm assuming I only need to wait for it to gel vs. fully cure before adding another coat but I still need to verify that.

[GreenLake]

Thanks for the update. On the recoat time, I found this, which agrees with your expectation, but doesn't give a time. Probably because cure times vary a bit based on conditions.

Gelcoat without wax can be recoated without sanding once it has cured to a firm, yet tacky state.

The amount of catalyst needed depends on temperature and to some extent on how thick your layers are. I definitely recommend you do a sample to prove to yourself that you've got the correct ratio for your conditions. It took me a few tries to get the process down.

If you have a stiff sheet of plastic that covers the repair, you may use that over the last layer to get a smooth surface and reduce sanding. I've done that on occasion for repairs. Other times I've just relied on formulations that came with the wax.