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Old 17-11-2013, 21:05   #1
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Patching Thru-Hulls

I am planning on patching (glassing in) seven thru-hulls. All are about two-two and half inch diameter holes in hull. My question is, better to patch from outside or inside the hull?

I think patching from inside would make the job of fairing outside hull much easier since grinding a taper 12 to one (around 12 inch diameter) on the inside keeping the hole outside 2 inches and also working with gravity.

Down fall to patching inside I believe would be taper of "plug" would be stronger from outside.

Appreciate any thoughts!
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Old 17-11-2013, 21:28   #2
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Re: Patching Thru-Hulls

How thick is the hull, and how easy is access to the inside? Typically I would bevel the outside since internal access is often terrible, but if it was really good, I would bevel both sides half way, and patch from there.

In the middle you premake an epoxy/glass disk that is bonded into place with thickened epoxy to act as a base for the patch work. See #8 at WEST SYSTEM | Projects | Fiberglass Boat Repair & Restoration - Repairing machined holes in fiberglass
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Old 17-11-2013, 23:01   #3
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Re: Patching Thru-Hulls

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Originally Posted by Noreastern View Post
I am planning on patching (glassing in) seven thru-hulls. All are about two-two and half inch diameter holes in hull. My question is, better to patch from outside or inside the hull?

I think patching from inside would make the job of fairing outside hull much easier since grinding a taper 12 to one (around 12 inch diameter) on the inside keeping the hole outside 2 inches and also working with gravity.

Down fall to patching inside I believe would be taper of "plug" would be stronger from outside.

Appreciate any thoughts!

I must have done a thousand of these. You are right, fairing would be easier if done from the inside. But it's a better repair done from the outside. And for me the key issue-this is a very messy process and it's much easier to deal with the grinding dust when outside the boat, not to mention the mess of the layup. Once you've dusted out a few interiors and realize the level of masking required to prevent this, it's a no-brainer. This should be a very quick job.
You say 12-1 gives a 12" diameter. I assume this means the hull is 1" approximately in way of the through hull and you intend to back grind until almost through in the center. I have seen many people do it this way and I think it is overkill. In fact I think making a repair that large for a through hull is potentially making a weak spot by making an unnecessarily large repair. Hole saw a piece of flat stock fiberglass 1/2" thick to the appropriate diameter and bond it in place flush with the inside first, using hot glue and some small wood blocks for perfect alignment to the interior surface. Then back grind the exterior 6-8", grinding until you hit the disc you bonded in and have a scratch on it. Then glass it up and fair it, barrier coat it, and bottom paint it. Usually I slap a glass patch on the inside too. It only needs to be a few matts. I'd do it all in poly. This shouldn't take more than an afternoon, aside from the coating. Which will only take a short time aside from waiting for paint to dry. This method is more than strong enough.
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Old 17-11-2013, 23:19   #4
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Re: Patching Thru-Hulls

We had the local yard fill a couple of thru hulls. They did it as already described above.
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Old 18-11-2013, 19:38   #5
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Re: Patching Thru-Hulls

Thanks, guess it would be better done from the outside in. Still it will be a total mess inside. Six of the thru hulls are deck drains with fiberglass-ed in channels with no sea-cocks. I will have to cut thru those channels before getting to the inside side of the hull of the thru-hulls. What about pouring resin filler into these channels with outside plugged and then grinding outside to patch.
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Old 18-11-2013, 20:13   #6
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Re: Patching Thru-Hulls

Why would you fill deck drains? Most boats suffer from a woefull lack of not enough cockpit drainage, I have never seen one with too much.
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Old 19-11-2013, 20:20   #7
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Re: Patching Thru-Hulls

Deck drains not cockpit, cutting holes thru gunals to drain deck water. Will retain cockpit drain thru hulls for cockpit drains.
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Old 19-11-2013, 20:24   #8
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Re: Patching Thru-Hulls

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Deck drains not cockpit, cutting holes thru gunals to drain deck water. Will retain cockpit drain thru hulls for cockpit drains.



Good call. Thru hulls for deck drains is nuts.
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