Stringers AARGGHHH!

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OneWayTraffic
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Stringers AARGGHHH!

Post by OneWayTraffic » Fri Jan 24, 2020 4:54 am

I just needed to get that out. This is fustrating.

Boat is a C17 and I have a build thread C17 in New Zealand.

Photos will follow when I edit this post on my iPhone.

I spend 4 hours last night laminating two layers of my transom (12mm x2) and stringers. This is my second attempt at stringers.

First I thought I would show my method in case there is something I am missing.

I dispense epoxy into a cup (WEST with slow hardener, using pumps this time, and I checked the first few batches on a digital scale to confirm the pumps are on ratio.)
Then I mix for about 1 minute, maybe 2, scraping sides and stick etc.

The cup is paper lined with a 'bio plastic' called Ingeo, it is not waxed.

Then pour into a bigger plastic mixing container.

I then brush this on with a chip brush wetting the entire surface.
C9DCDEC7-1E4F-4014-9FC6-F755D894CB61.jpeg
I then get some more epoxy thickened with Cabosil (Wood flour is not available commericially in NZ.)

Brush that on one surface until it looks like this.
AE901D4D-D348-48F3-8A5F-3A523775FF3D.jpeg
Photo clamping board with glue.

Then put one over the other and weigh down.
4BD99848-333D-4137-9333-21C08620D0C6.jpeg
I followed this exact same procedure for my stringers, and one of them (call this bad stringer) has a good half metre near the front where there is daylight between the layers! I drilled small holes visible here and in the end I could get a saw blade through! I then got my holesaw and took some plugs out of both stringers.

The plugs on here from my 'bad stringer' show delamination between the layers of plywood. This is the same stringer with the daylight near the front. I could pull these apart by hand.The epoxy on the left plug shows no sign of being in contact, it was smooth.
2F765C77-934B-488F-9F85-3670B19D32C0.jpeg
The plugs on the right from my 'good stringer' show a nice solid bond, but who is to say that there isn't voids in that stringer somewhere?
3E58DFB1-F7E4-4133-8B61-4B56F2342B48.jpeg
My options at this point are

A) to attempt a repair, cutting out the whole front of the bad stringer and regluing, taking several plugs out of both stringers checking and epoxying back etc, then when I finally put this in the boat wrapping the whole stringer with so much biaxial that it won't matter what it is made of.

B) Buy some more plywood and try again, but if I do this I really need to rethink my lamination. How does one ensure that the stringers are bonded everywhere? And if not, how close to everywhere do I need to be? Obviously rocks every half metre or so isn't cutting it.

If it makes a difference, I have far more glass than I need (it was economic to buy a 100m roll of double bias) but am curently out of plywood, except scraps.



OneWayTraffic
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Re: Stringers AARGGHHH!

Post by OneWayTraffic » Fri Jan 24, 2020 5:03 am

If I take the repair and install approach, how much glass do I use in case of unseen voids?

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Re: Stringers AARGGHHH!

Post by OneWayTraffic » Fri Jan 24, 2020 5:07 am

What my stringers look like.
Attachments
3D5C7139-8BC2-43B6-A113-5E671780593E.jpeg

Rmarsh
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Re: Stringers AARGGHHH!

Post by Rmarsh » Fri Jan 24, 2020 5:37 am

shopping.png
shopping.png (3.18 KiB) Viewed 319 times
When laminating plywood pieces like the stringers and transom with thickened epoxy, try using a notched spreader.
Bob

OneWayTraffic
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Re: Stringers AARGGHHH!

Post by OneWayTraffic » Fri Jan 24, 2020 5:44 am

Actually I did that right after the second photo.

I think it is the way I weighted the stringers. Rocks every half metre doesn't cut it. If I rebuild from scratch (I am still waiting for advice here) I will need to do something different there. Buy a 2 by 4 and screw the stringer to it? Seems a bit of a waste but I have already mucked up two lots of stringers. :(

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Re: Stringers AARGGHHH!

Post by Rmarsh » Fri Jan 24, 2020 6:06 am

Looking at the picture of the transom lamination, and the amount of thickened epoxy looks a little skimpy. When spread with the notched trowel you should get approx 1/8" high raised lines of thickened epoxy over the whole area. When you apply the weight to hold it down, there should be a very small amount of "sqeezeout" at the edges.

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Re: Stringers AARGGHHH!

Post by Jaysen » Fri Jan 24, 2020 6:17 am

So I’ll bite...

Three things come to mind
1. Wood sucked epoxy out of joint
2. Not enough glue
3. Not enough pressure.

2 and 3 seem most likely based on what you’re saying.

2. You want glue to squeeze out before you apply much pressure. Epoxy is a “gap fill” glue and wants a bit of space. This means you need a thicker cover than if you were using LPU (if I recall you’re a woodworker type). This also means you don’t want too much pressure on the glue up.

3. Just use screws. Get some drywall screws, drill a pilot every 18”. One row in middle. One row 2” in from each edge between the middle row. (You need more rows for transom). Slap a healthy layer of glue, assemble you’re pieces, then HAND SCREW the center row in until the screw head just snugs up the joint. Once the center is done hit the edge rows. at 6hr take a razor blade and clean the squeeze out from the edges. Once the glue has about 18hr setup remove screws, drill out the screw hole and fill with thickened epoxy.

That’s just what I’d do. I can’t remember if it was Jacques, Cracker Larry or someone else that told me how to do it that way. I’m sure someone will point out a flaw though.
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Jaysen wrote:
Mon Apr 29, 2019 3:44 pm
I tried to say something but God thought I was wrong and filled my mouth with saltwater. I kept my pie hole shut after that.

OneWayTraffic
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Re: Stringers AARGGHHH!

Post by OneWayTraffic » Fri Jan 24, 2020 6:47 am

Rmarsh wrote:
Fri Jan 24, 2020 6:06 am
Looking at the picture of the transom lamination, and the amount of thickened epoxy looks a little skimpy. When spread with the notched trowel you should get approx 1/8" high raised lines of thickened epoxy over the whole area. When you apply the weight to hold it down, there should be a very small amount of "sqeezeout" at the edges.
I see what you mean looking at that photo. That was about half the glue I used for the transom clamping board. The other half I mixed up after the photo. It was hard to coordinate photos with clean gloves. Obviously there were places in the stringers needing more glue (or better use of the glue I had! :oops: )

I would say about 300ml (10 fluid ounces) epoxy for the glue for the transom clamping board plus fillers. About two lots of WEST five pumps each.) And another 200ml to wet out both sides.
The clamping board is roughly 1.8m by 0.3m (6' by 1') so that's ball park within gougen estimates of glue use. Of course they have better technique.

I will probably use more glue next time regardless.

OneWayTraffic
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Re: Stringers AARGGHHH!

Post by OneWayTraffic » Fri Jan 24, 2020 6:50 am

Jaysen wrote:
Fri Jan 24, 2020 6:17 am


3. Just use screws. Get some drywall screws, drill a pilot every 18”. One row in middle. One row 2” in from each edge between the middle row. (You need more rows for transom). Slap a healthy layer of glue, assemble you’re pieces, then HAND SCREW the center row in until the screw head just snugs up the joint. Once the center is done hit the edge rows. at 6hr take a razor blade and clean the squeeze out from the edges. Once the glue has about 18hr setup remove screws, drill out the screw hole and fill with thickened epoxy.

That’s just what I’d do. I can’t remember if it was Jacques, Cracker Larry or someone else that told me how to do it that way. I’m sure someone will point out a flaw though.
Thats fairly labour intensive, but not as much as cutting stringers three times. :lol:

I did actually consider screws, but I was worried about using too much pressure. Hand screw into a pilot hole would work.

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Re: Stringers AARGGHHH!

Post by OneWayTraffic » Fri Jan 24, 2020 6:54 am

I must admit that I really don't have the heart to cut new stringers right now, even if I had the wood to hand. What I really want to do is rescue them somehow.

I'm going to sleep on it. Tomorrow I will start working on the jig. If anyone else wants to chime in with advice I'm all ears. The stringers won't be going in the boat for a few months in any case.

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