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Single or double-sided taper when glassing over a hole in a thick hull?

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Hello,

I removed a very old depth sounder and am now ready to glass over the hole. Due to my hull thickness, it seems like I am going to have to remove quite a bit of material to get the desired 12-to-1 taper. I'm just looking for a quick sanity check to make sure my approach is sound. I have done a little bit of fiberglass work -- mostly on a stitch and glue dinghy. But I would consider this to be my biggest project yet. I've watched the videos and read the West Systems repair manual -- but grinding away 12" of my hull still makes me a bit nervous :)

Here are the specs:

 1. polyester and fiberglass boat built in 1965 (Tartan 27 Yawl)

 2. solid fiberglass bottom

 3. hull is 0.44" thick where the depth sounder was removed

 4. the hole is 3/4" in diameter

To create a 12-to-1 taper I would need to create a taper that is 5.28" around the outside of the hole. So that means the diameter of the beveled circle is going to be 11.31". I'll probably just make it an even 12". Does that sound unusually large?

Another option would be to taper from the the inside *and* the outside. On the inside I could maybe create a 2" wide bevel around the hole. So I think that means on the outside I would only need to taper 3.28" around the hole for a total diameter of 7.31".

Another option would be to just plug up the hole. Perhaps install a proper mushroom head thru hull with some good sealant and cap it on the inside. That is probably less work, but more likely to leak. I do not think I am going to install anything else there soon and it is well below the waterline.

So my inclination is to fiberglass over the hole with fiberglass and epoxy. I am also inclined to only bevel from the outside of the hull because it has the following advantages:

 1. less dust inside the boat (where I currently live)

 2. creates a lot of surface area for the first layer of fiberglass to bond to

 3. seem less tricky to get right

I have both a grinder  and a powerfile as well as a tyvek suit and other PPE. For the grinder and the powerfile I have discs/sandpaper all the way down to 40 grit or so.

The weather where I am now is generally 90-95°F. I currently only have 205 Fast Hardener on hand. Should I get the 206 Slow Hardener or go all the way up to the 209 Extra Slow Hardener?

For the fiberglass I am planning to use 1708 stiched mat. That is the stuff which is woven so that it does not have the styrene-soluble binder which can interfere with epoxy. I've heard that the west systems epoxy can work ok with chopped strand mat even though it does not dissolve the binder. But since I do not have any CSM, I might as well just buy the stitched stuff and not worry about it?

One concern that I have is getting the wetted fiberglass to stick to the bottom of the boat. I feel like a square foot of wetted fiberglass is going to be pretty heavy and want to fall off.. Should I wet out the fiberglass and also apply epoxy to the bottom of the hull and then wait for things to get a bit tacky? Or should I use a thickener like 406 Collodidal Silica? Or perhaps a different approach? I'd like to do wet-on-wet -- but as I build up more layers -- it's going to add weight. How do I know when the previous layer is dry enough that it won't delaminate? These are some of the questions that my previous experience doesn't help me answer.

My bottom paint is Interlux Micro Extra. Based on the instructions here,

http://www.yachtpaint.com/LiteratureCentre/micron-extra-info-usa-eng.pdf

It sounds like I do not need to use primer? Just some sanding, cleaning, and 2 coats?

Thanks!

- jeremy

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Single sided with epoxy will probably be fine but double sided is better - think dovetail.

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I'd totally do outside only to keep the dust out. Especially since you are still living in the boat! 

A 12" taper isn't crazy with that thick a hull. 

Start laying up early in the a.m.when it is cool. 

Get a few 24 grit discs to rough remove most of the glass then switch to the 40. Angle grinder not file. 

Yes, gravity is not your friend. So wet out 3-4 layers on a flat surface covered with plastic. Then sprinkle surface with small amounts of colloidal silica to make it the resin thicker so it does not drain away. Then wait a few minutes for the epoxy to get tacky before applying to the hull. Repeat this slowly, letting enough time to get the resin just starting to set. Otherwise it WILL fall on your head. 

Or find someone with a vacuum pump and do it in one or two batches. 

Or get a boatstand with a big enough soft pad to hold the entire patch in place. 

 

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You'll probably find this easier than you think. I did mine with about 8 layers of fiberglass cloth, and the only issue was that too much wet and slimy cloth will slide off the repair. So just do maybe three at a time until the epoxy kicks and becomes gooey. Do the majority of the layers on the outside, and put one or two on the inside with a modest taper/scuff.

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When you say '24 grit disc' what type of disc are you talking about?

From what I can tell there are at least three types of discs that are measured in grit.

 1) sanding discs like this, https://www.amazon.com/dp/B01HXD6PD2/ref=twister_B01M68EIXZ?_encoding=UTF8&psc=1

 2) and flap discs like this: https://www.amazon.com/Mercer-Industries-262024-Zirconia-Density/dp/B01GHY5LYW/ref=sr_1_3_sspa?keywords=24+grit+flap+disc+type+27&qid=1567986577&s=hi&sr=1-3-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUFJSlhZVklGR1kwNUomZW5jcnlwdGVkSWQ9QTA3NDQ0MzRFSjFaWVRKTlc4M1QmZW5jcnlwdGVkQWRJZD1BMDQzMTk0NzIxOTc5WjEyMlRPOTkmd2lkZ2V0TmFtZT1zcF9hdGYmYWN0aW9uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVl

3) metal grinding wheels like this, https://www.amazon.com/Walter-Allsteel-Versatile-Grinding-Threaded/dp/B003O9XLT6/ref=sr_1_3?keywords=24+grit+grinder+wheel&qid=1567986512&s=gateway&sr=8-3

Or a thicker version like this:

https://www.amazon.com/Depressed-Center-Metal-Grinding-Wheels/dp/B0791MGN72/ref=sr_1_16?keywords=grinding+disc+type+27&qid=1567986887&s=gateway&sr=8-16

I feel like the sandpaper type discs are just going to get clogged up real fast. And, I think the flap discs are great at first but wear out too quickly? So what I need is a 24-grit metal grinder wheel?

 

7 minutes ago, Zonker said:

I'd totally do outside only to keep the dust out. Especially since you are still living in the boat! 

Angle grinder not file.

 

The idea of using a powerfile came from the following video. Seems like it might be promising for doing a 2" bevel on the inside of the boat while keeping dust to a minimum. I do have a shopvac on board. It is certainly not as fast as a grinder -- but with only a 2" radius and desire to keep dust down, it seems worth a shot. Perhaps I'll start by testing it on the outside of the boat a bit before committing to doing a bevel on the inside. Due to the place of the hole near walls, I am not even sure I could use the angle grinder on the inside. So if the powerfile isn't viable, then I don't have much choice except to do single taper.

Thanks!
 

 

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Reality check:

You had an unreinforced plastic tube in that hole. Or maybe a bronze tube.

this really isn’t a structural repair. It is just a big plug 

still... it can’t ever leak.

 

I would be 100% confident with grinding a nice dish around the hole and applying a five or six layer chop strand mat patch that extends out a few inches. 

but... being the paranoid  guy I am, I would do more 

 

first I would wallow out the hole so it is significantly larger at the top and bottom. 

Then I would tape off the outside ( I live the boat stand under it described above) and I would pour a VERY SLOW CURING fiberglass and resin plug. 

That plug  would be made with some commercially chopped 1/2 inch  fibers  stirred into some resin. I would stir in as much fiber as I could while still having all the fibers soaked. 

 

After that plug had cured I would grind my dish into the outside if the hull 

then I would lay up Fiberglass on the outside until it was too thick everywhere.

after it would be cured I would sand and apply whatever bottom is on the rest of the hull.

 

on the  inside I would be good with a square foot of about five layers of mat.

as you live in there ... I would sand that square foot by hand with a 16 grit disc.

No power tool except a vacuum cleaner

 

even if the inside  is painted with thick gelcoat, you should be able to sand through that and down to the laminated fiberglass in under an hour.

i would make certain EVERYTHING that is  not  the structural fiberglass and resin is gone. Then I would lay on a bunch of layers of glass. . I would taper the pile at the sides and preferably not paint over it. 

Why?? If it ever would start  to peel loose I would see the discoloration.

 

but,,, I am already confident my plug will work well enough by itself. 

I am confident l my outside  patch will work well enough by itself

so... my inside patch, Which probably would he adequate by itself could be simply painted over to match whatever else is in the boat. 

 

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17 minutes ago, Gouvernail said:

Reality check:

You had an unreinforced plastic tube in that hole. Or maybe a bronze tube.

this really isn’t a structural repair. It is just a big plug 

My thought exactly. One is replacing a relatively fragile plastic thruhull. Or a bronze pipe and a bunch of goo. A few layers of epoxy and cloth in a small flair will be plenty. On the underside I'd stick a bunch of cut to size wetted out layers of cloth up there and push it into place with a sheet of PE plastic and thin plywood or whatever will conform to the hull curve...Because I hate grinding and sanding. Then pour in thickened epoxy on the inside later. Must avoid boiling the epoxy with thin layers or very slow hardener. Trivial project. So long as all sealant is ground out to old glass it is never ever going to leak so that is a non-issue.

12" fairing on both sides?!? Heh. That will probably weaken the hull by the time you are done.

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Sanding disc. You might use one or three depending on quality. It will eat through glass very quickly.

I agree that you might not need a 12:1 taper - but I sure wouldn't pour a plug with some chopped fibers. It's just not as strong. 

Thru-hulls are a weak point but why not take the hour or so to do it right?

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The Gougeon Bros book is a good resource for this kind of think and Appendix D example 8 covers this kind of repair. They use some filler/epoxy puck in the middle then 12:1 tapers on both sides, which lets you get away without tapering out for the full hull thickness.

Book is free online at https://www.westsystem.com/wp-content/uploads/Fiberglass-Manual-2015.pdf

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There is also common sense to most repairs. The industry standard is 12:1 taper. Keep in mind it doesn’t matter if the hole is 3/4” or 34”. Likewise it’s 12:1 if the laminate is 1/8” or 1/2”.

The location of the hole is non structural. This is proven by the fact there was a 3/4” hole drilled in it. Therefore you could simply fill it with solid epoxy filler if you wanted. It has less chance of leaking than the thru hull fitting.     I am not recommending this idea.

The hull never needed to be 1/2” in the first place. Complete 1960’s overkill.

Personally I would grind the edge so it has a flat of about 2”. Then fill it up with glass and resin. That would be a 5” hole. Only work from the outside. You will hate yourself if you do both. It is not a better repair.

Keep in mind that when building a new hull that you only need to lap the cloth 2” to develop full strength. 

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Thanks everyone. Here is what I have learned so far and my updated plan:

This is a machined hole, not a puncture, crack, etc. So I am not concerned about damage to the surrounding laminate. The only purpose of this project is to permanently plug up the hole.

One option is to just fill up the hole with epoxy. That would probably work but doesn't fill me confidence.

Another option is to create a 1/2" thick, 12:1 bevel and then add lots of fiberglass to rebuild the laminate back to the original shape. That option involves a lot of grinding, a lot of fiberglassing, a lot of fairing,  and removes a lot of perfectly fine laminate.

A more pragmatic approach is to split the difference. Create a 3/16" thick puck that goes in the middle of the hole. Now when creating a 12:1 bevel it only has to go deep enough to get to the puck. Accordingly the diameter of the beveled area can be more like 5"  on each side, instead of 12" on a singleside. This is the process described in the Gougeon Brothers pdf from the west systems website.

Creating the puck and installing it can be a bit fiddly (I've seen people do it an youtube). So another similar approach is something like what Gouvernail suggests. First fill the hole entirely with epoxy and fiberglass. Perhaps even make the hole hourglass shaped before filling it, so that even if the plug came completely delaminated from the hull, it would leak, but couldn't possibly slip out. By itself, that would probably work.

But to be extra sure, grind out 1/8" to 3/16" laminate with a 12:1 taper on the inside and outside, and layup some fiberglass circles going from largest to smallest. Fair the outside, and then paint according to manufacture's instructions. Now it should be strong & leak free, while removing a minimal amount of the existing laminate.

The disadvantage of filling the hole entirely instead of making a puck is that you are just adding more epoxy and fiberglass that you are going to immediately grind away. However, given that my hole is only 3/4" in diameter, it is probably easier to add extra material and grind it away, than to try to create and fit a puck.

Due to the mess that grinding can make inside the boat, I might only bevel on the outside. In that case, I'd at least want to hand sand (or maybe powerfile) the inside surface down to bare fiberglass and then add a couple layers of fiberglass. The inside of the hole is inside a storage locker, so looks are not important.

Since I do not want to stress out about this patch when I am offshore, I am willing to do some extra work now to be 100% confident in the work.

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Sounds like a good compromise. But go smallest to largest. Otherwise you just add steps in the laminate that are not as nice.

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If I had simply done it rather than read this, I would be finished. 

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Maybe it's just me but films where better when they had to pay for film.

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Slight hijack, but, this thread got me thinking about something.   As many gallons of West as I've used over the past 25 years, one guideline that I've adhered to even though it seems counter-intuitive, is that when building up a seam, you use correspondingly smaller layers of glass tape, rather than increasing the overlap w/each outer layer.  

It seems that if you increased the overlap, that you'd have more surface area for each layer of your seam to adhere to, and that that additional adhere surface area would result in a stronger seam than the current wisdom of adhering each layer to the underlying layer w/no overlap to the "main surface".    

Can someone help me understand what I'm not getting? 

 

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There are two schools of thought, both are right when you fair out the patch.  Stress flows like water, it concentrates at steps[ beware of notches].  You can get a dryer lamination by going large to small and sanding the taper down.  You can get a faster fairer lamination by going small to large and using the final layer a cheap pleel ply substitute.

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I am using epoxy because

  1. I already have epoxy resin from previous projects
  2. I have some experience using epoxy but none using polyester
  3. According to resources I have read, epoxy creates a 20% strong bond than polyester when making a repair like this where it will be a mechanical bond, not a chemical one.
  4. I can use 406 filler to help make the overhead layup easier

For someone experienced with polyester, it might be done in an afternoon. But since I do not have polyester resin or experience using polyester resin, I do not see a compelling reason to choose it over epoxy. I suspect it would actually take me longer to use polyester -- though I would learn a new skill. But I only need to learn so many new things per repair. So I'll save the polyester for another day.

 

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As has been pointed out previously this is not a repair, its just filling a small hole in a thick hull with presumably no compromised laminate so it makes no sense to grind away a relatively large amount of sound laminate for this. I have done many of these types of projects by all kinds of procedures over the years depending on access from the inside and have settled on a very simple method that creates minimal fairing and general disruption. Remember, its just a small hole. First, sand away bottom paint around the area then take a hole saw with an oops arbor and open out the hole a little to clean out any caulk to clean glass, then use a router with a rabbet cutter or fly cutter and rabbet around the hole a bit over 1/8" deep and then epoxy bond in a 1/8" G10 disc. I will then bond in a G10 disc the size of the hole to make up the remaining thickness to the inside and either laminate a couple of plies of double bias on the inside if its readily accessible  or bond another 1/8" disc. no need for any heavy grinding, degreasing and hand sanding prep is fine. On the outside you can just dish it out a little and lay a ply or two of double bias but not really necessary, just filler is fine to fair it in. G10 is a much better quality laminate than any of the surrounding laminate or anything any of us is capable of. It would be great if someone would make a through hull out of glass filled epoxy with a beveled flange and no hole in the center that you could just bond in with epoxy and be done with.

Steve.

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On 9/10/2019 at 10:40 PM, A guy in the Chesapeake said:

Slight hijack, but, this thread got me thinking about something.   As many gallons of West as I've used over the past 25 years, one guideline that I've adhered to even though it seems counter-intuitive, is that when building up a seam, you use correspondingly smaller layers of glass tape, rather than increasing the overlap w/each outer layer.  

It seems that if you increased the overlap, that you'd have more surface area for each layer of your seam to adhere to, and that that additional adhere surface area would result in a stronger seam than the current wisdom of adhering each layer to the underlying layer w/no overlap to the "main surface".    

Can someone help me understand what I'm not getting? 

 

The way it was explained to me, the resin going off is an exothermic reaction (gives off heat).  This WILL result in shrinkage.  If you go large initially and small later, shrinkage can cause the smaller layers to "cup" the initial layer, tending to pull it off the substrate.  If you start small and increase the size, the tendency is instead to pull down, increasing the strength of the bond with the substrate.

I have read explanations favouring the large to small approach also, on the basis of largest bond area initially and later layers are adding mechanical strength to the structure rather than bonding strength to the substrate, so it is probably moot for most of us who aren't working to aerospace standards. 

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