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Old 30-05-2013, 14:19   #1
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Glassing thru-hulls side by side

I need to glass over 2 thru-hulls that are side by side.

I plan on grinding a 12:1 bevel back from the holes, then layering circular 10oz cloth patches over the beveled area.

However, the 12:1 bevels will over lapp each other by a coupe inches from each hole, and I'm not 100% how to go about this.

Do i glass over 1 at a time? or try and do both together?
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Old 30-05-2013, 14:44   #2
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Re: Glassing thru-hulls side by side

This is not the space shuttle fuel tank, just do it.
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Old 30-05-2013, 15:30   #3
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Re: Glassing thru-hulls side by side

I would do them both at the same time and overlap where you have to. I would suggest you use a 1808 knit or alternating layers of mat and roving as this is what the boat is built with. Cloth is more of a finishing material but if that is all you can get you can use it I just find it harder to work with and get good results. You will also need less material and get a faster build up with mat/roving. I also would use polyester or vinyl ester resin and not epoxy as once again this is what your boat is built of and it is easier to work with and get a good job. Do half from the inside and half from the outside to get a good overlap on each side if possible. Good luck and let us see the finished project.
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Old 30-05-2013, 16:35   #4
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Re: Glassing thru-hulls side by side

If the holes are close enough together so you'll be overlapping a bunch of layers, grind out the hull in an oval to include both pukas. Cut the fiberglass material in an oval to cover both holes with one piece. Lay the biggest piece of fiberglass in first and then successively smaller pieces. Roving builds thickness faster but don't know of the stitched matt/fabric material is available for roving. If it isn't, makes it harder to layup. Working with the stitched material is way easier than separate matt and roving/cloth.
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Old 30-05-2013, 16:50   #5
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Re: Glassing thru-hulls side by side

FYI the knit materials are usually a non weaved roving with a mat layer attached to one side. So basically they are a mat/roving layer The 1808 I suggested is common and is a 18 oz roving with a 8 oz mat. The knit materials can have a bias such as +- 45 or 90. This is useful for aligning the glass with loads and stresses. In the case above for a small repair the bias does not matter much but the +- 90 would be best. The Op may ask around local yards or repair shop for some scraps that would work for a small repair like this. Generally speaking I like to lay in the small pieces first and overlay with the larger but for a small repair like this either way will work. I just find you get a cleaner smoother finish with the larger pieces last
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Old 30-05-2013, 17:17   #6
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Re: Glassing thru-hulls side by side

The problem with laying in the large pieces last is you almost certainly will grind off portions of them when you fair the area. That means you will no longer have continuous threads running the entire dimension of the area. That will reduce it's strength somewhat. Whether grinding off the continuous layer of cloth/roving is enough to have any significant effect is the issue. It's definitely harder to lay in the big piece first but Iit's worth the piece of mind to me to suffer through it.
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Old 31-05-2013, 06:10   #7
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Re: Glassing thru-hulls side by side

Thanks for the ideas, I think I'll grind it out in an oval and start with a the larger patch 1st.

Now I just have to build a support so i can lower the cradle pad for access.
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Old 31-05-2013, 20:29   #8
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Re: Glassing thru-hulls side by side

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Originally Posted by roverhi View Post
The problem with laying in the large pieces last is you almost certainly will grind off portions of them when you fair the area. That means you will no longer have continuous threads running the entire dimension of the area. That will reduce it's strength somewhat. Whether grinding off the continuous layer of cloth/roving is enough to have any significant effect is the issue. It's definitely harder to lay in the big piece first but Iit's worth the piece of mind to me to suffer through it.


One only lays the large piece first in new construction, never in a ground back repair. This is a common newbie mistake. The reasons are fiber continuity and surface contact. If you place the big piece first in a repair, all of the fibers in the repair will end up bowl shaped, just as the repair was dished out. This means they will be at an angle to the original fiber, making any matching of the original lam for fiber continuity impossible. It also means the entire repair must rely on the adhesion of a single ply to the original surface. If this ply delams the whole repair will fail. If you start with the small pieces first, each ply makes contact with the original surface around its perimeter. The fibers are parallel to the surface and much easier to align properly for fiber continuity. Generally one glasses close to fair with structural laminate and then finish with a couple of matts, so you can grind fair without getting into the roving or DB. When building new the opposite is done for the same reason, big piece first when glassing in structural members. This is a much stronger method of repair and dramatically less likely to fail.
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Old 02-06-2013, 15:16   #9
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Re: Glassing thru-hulls side by side

Makes total sense, i just happened to check the thread again before i started the epoxy work; glad i did.

Thanks
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Old 02-06-2013, 21:25   #10
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Re: Glassing thru-hulls side by side

Those damn amateurs at West systems don't know what they are doing!!!

West says to lay in the largest piece first when repairing a boat. Not only do you not risk grinding away those continuous strands but you are getting the maximum bonding area to the original hull. The beveled area you are bonding to is six or more inches wide all the way around the patch. Definitely going to get the strongest


Quote:
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One only lays the large piece first in new construction, never in a ground back repair. This is a common newbie mistake. The reasons are fiber continuity and surface contact. If you place the big piece first in a repair, all of the fibers in the repair will end up bowl shaped, just as the repair was dished out. This means they will be at an angle to the original fiber, making any matching of the original lam for fiber continuity impossible. It also means the entire repair must rely on the adhesion of a single ply to the original surface. If this ply delams the whole repair will fail. If you start with the small pieces first, each ply makes contact with the original surface around its perimeter. The fibers are parallel to the surface and much easier to align properly for fiber continuity. Generally one glasses close to fair with structural laminate and then finish with a couple of matts, so you can grind fair without getting into the roving or DB. When building new the opposite is done for the same reason, big piece first when glassing in structural members. This is a much stronger method of repair and dramatically less likely to fail.
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Old 02-06-2013, 21:38   #11
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Re: Glassing thru-hulls side by side

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Those damn amateurs at West systems don't know what they are doing!!!

West says to lay in the largest piece first when repairing a boat. Not only do you not risk grinding away those continuous strands but you are getting the maximum bonding area to the original hull. The beveled area you are bonding to is six or more inches wide all the way around the patch. Definitely going to get the strongest


Agreed, they don't know what they're doing. First person to make me have the light bulb go off over my head with a drawing just like the one I posted was Carl Schumacher (RIP). We were having this same old argument. I was already on his side of it, but couldn't explain why. He shut up the other side right quick with that drawing. A real genius.
Later I discovered all sorts of chemical engineering literature that (of course) supports his stance. It's a no brainer, and really kinda laminating 101, in the higher echelons of real laminators in any case.
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Old 02-06-2013, 22:58   #12
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Re: Glassing thru-hulls side by side

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Those damn amateurs at West systems don't know what they are doing!!!
NEWS FLASH!!!
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Old 02-06-2013, 23:05   #13
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Re: Glassing thru-hulls side by side

They make a pretty good glue though, so not complete amateurs.
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Old 02-06-2013, 23:35   #14
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Re: Glassing thru-hulls side by side

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They make a pretty good glue though, so not complete amateurs.



Damn fine glue-makers, no doubt about it. When was their most recent literature on techniques put out, twenty years ago?
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Old 03-06-2013, 04:55   #15
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Re: Glassing thru-hulls side by side

Another reason not to use the largest piece first is as clearly seen in minaret's drawing. All the fiber ends are pointing out and exposed to the water allowing moisture to wick into the repair. With using the smaller pieces first and finishing off with a couple of mat layers (as the boat was built) when you smooth the repair down you will only be exposing some random fibers that down not go far into the laminate.

Most will agree WEST is a good resin but it was not developed for fiberglass repair. WEST stands for "Wood Epoxy Saturation Technique." The Gougeon Bros are great at marketing but are better known for building cold molded Wood boats.

All I can recommend is how I would do this on my own boat and I would start with the smaller pieces (as all pros I know do) and use polyester resin (just like the boat is built with). I would however finish off with a epoxy barrier coat as this will be under water. Guess the OP can review all this and make up their own mind as to how to proceed.
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