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How to fix a thruhull hole

Old 11-18-2020, 08:14 AM
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Default How to fix a thruhull hole

I am working on fixing all the holes in the hull of my SC and am at a thruhull on the bottom. It is up front where the head was so I am guessing the discharge. It is 3" in diameter and 12" from the keel between stringers. Hull is about 1/4-3/8" thick here. Access from the inside is easy since the whole deck is out. So, my question.

Should I grind down at a 12:1 ratio on both the inside and outside to make the repair or only grind down on the inside? From experience in other areas of the boat I am figuring around 12 layers of 17 oz biax and epoxy will get me back to full thickness. 6 outside and 6 inside starting about 24" in diameter and working down to the 3" hole size or 12 layers from the inside only?

I guess my only concern is working from the outside is I will have a lot of hull to get the correct shape and ensure a good connection. Working from the inside only will mean I will only have that 3" hole to make sure matches the contour of the hull bottom. I could make a mold of the hull bottom before grinding I guess and use that pressed up against the wet glass to ensure the proper patch. What do you guys think? If pictures will help I can go take a couple.
Old 11-18-2020, 11:35 AM
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I think you are going to want to address this from both sides with the outside as the primary patch and the interior as a backup. Epoxy a solid glass slug in the hole and get a few layers on the inside then attack it from the outside 6" around the plug working big to small to achieve full thickness. I am not an expert but have been studying hard on here! I'm on the last stages of one of my projects as well. Nest of luck
Old 11-18-2020, 12:10 PM
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I'm a noob, so some of the pros will chime in I'm sure.

I just did a few of these and I beveled the inside and outside (more bevel on inside for my first step. Then, I placed packing tape across the hole on the outside (these were smaller machine holes than you're talking about though (figure 1 inch or so). Then I made a mix of epoxy, cabosil, milled fibers and chopped strand and filled the hole, leveling it to the dip in the middle of my inside bevel. Then, while that was still nice and tacky, I ran a quick pass of thickened epoxy over the plug and wet out a few layers of 1708 across my bevel. It seems nice and strong. When I do the bottom, I plan to sand the plug repair again, add a little more bevel and glass over it on the outside.

If it's a much wider hole, (you mention 3 inches) you might do better just using glass layers from each side since you'll need a bigger bevel and the middle section might be thin. I'd still make a backer for the outside bottom with packing tape (and cardboard etc) and glass against that from the inside. Then glass the outside layers. That's how I did the larger holes on my transom skin (outdrive keyhole, exhaust etc).
Old 11-19-2020, 05:35 AM
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I like the idea of tape for making the mold. Stick tape to hull, glass over it, pull off hull and peel tape off. Voila, perfect mold. That solves my major concern. Thanks.
Old 11-19-2020, 08:37 PM
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I would make the repair from the inside then bevel the outside only enough to lay about 2 layers of glass on it.
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Old 11-20-2020, 01:59 AM
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Hey my Seacraft brother, I had 2 holes to fix there too. The PO did 1 layer of 6oz cloth and that was it. Not good. Mine was 10ish layers of 1708.

Here do this:


most people can’t even see my repairs when done. Just need a little practice. And a lot of fairing

Last edited by Xcomunic8d; 11-20-2020 at 02:04 AM.
Old 11-20-2020, 06:58 AM
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Thank for the help. I may need the distraction working on the boat this weekend and will try.
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Old 11-23-2020, 03:04 PM
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I ground down the outside, cleaned it up and added two layers of glass. The contour was not as bad as I thought it was going to be after I really got down in there. Laying on your back and sanding/grinding does a number on your shoulders and triceps. The heat lamp right on it worked well and the layers came out great. I went to add two more layers yesterday, but it was too cold. I did not like the consistency of the resin after I mixed it up so I scrapped it. I may start keeping the resin and hardener inside so it stays warm and then bring it out to mix up only when needed.
Old 11-24-2020, 03:31 AM
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Yup I’d keep it warm and dry and try to avoid direct sunlight. Especially in summer time. Post some pics. Why’d you need 2 more layers?

Did you do all your layers at once not just two but all 10 on the patch on a prep table then apply at once? I grind it back and cut patches out and compare to boat. Then after I have my layers, I’ll layup the resin and stack them. Roll the excess out. Apply some resin to hull, And put the patch on the hull and roll out. Leave cure depends on conditions. Sand/grind it back. Fair it smooth. Gelcoat. Prime and paint (in my case) or whatever top coating you prefer.

that video has served me very well.
Old 11-24-2020, 06:48 AM
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Just a personal preference, but I like to do no more than two layers at a time. I want ensure a good bond with the existing hull and have no air bubbles. I am not good enough to get air bubbles out of 6 or more layers of glass in the time it takes before the resin starts to kick. So, I take it slow. Grind, clean, wipe with wax/grease remover, apply a layer, get out all the air, apply another layer, get out all the air, let it cure, clean off the blush with soapy water, rinse, wipe with wax/grease remover, apply two more layers, rinse and repeat until filled. It may be slower, but I am confident in my layup.

The hull sides are surprisingly thin. There is one hole where the little kitchenette sink drained from. I ground it down and it looks like one layer of glass inside and out is all it will take. I know the sides only job is to keep water out, but I was expecting thicker. It is about 1/16th of an inch thick. I am thinking about a whole layer of glass added to the insides just for peace of mind.

I have the before pics, but no after. Next time I am out there I will snap some more.


Angle of camera is causing distortion. It is an equal 5" all around the hole.
Old 11-29-2020, 04:06 AM
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Looking good brother. You are right the sides can be surprisingly thin and flexible in certain places. Particularly when the cap is removed. Work at your pace we all have our own thing. Not right not wrong as long as results are good. Took me 30-40 gallons of resin to get to techniques I’m describing. Until then produce quality and speed will come.

but I’ll give you a freebie I learned. Try laying up your patch layers all together and using a bondo spreader first and go over with fin roller. apply Resin to both sides of layer 1, then just add resin as you go. Squeeze out extra, get on it fin rolling. Essentially you have a 1 layer patch made of 6+ layers. If you don’t have at least 20 mins you may need to use less catalyst. Don’t let this harden. Should only take 10mins. Apply resin to hull. Apply patch squeeze and fin roll. Let set.

if it works great if not you’re doing great work.

Old 11-30-2020, 06:51 AM
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Thanks for the advice. My only concern is getting the air bubbles out between the new glass and existing hull. I put one layer on and get the bubbles out then do the next layer. If you have 6 layers of glass can you still get the bubbles out? And a pic of the patch ready for fairing and paint. I think.

Old 11-30-2020, 09:11 AM
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I don't like the idea lifting six layers off of a piece of cardboard and trying to apply them all at the same time either, you did good.
A lot of professionals prefer the small first going to the large, my preference is large first then the smaller.
Gerald
Old 11-30-2020, 09:43 AM
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Originally Posted by commuter boats View Post
I don't like the idea lifting six layers off of a piece of cardboard and trying to apply them all at the same time either, you did good.
A lot of professionals prefer the small first going to the large, my preference is large first then the smaller.
Gerald
It made sense to me to start with large and work to smaller too. Lots of coverage with that first layer to get a good bond.
Old 11-30-2020, 05:49 PM
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Nice work, looks like you know what your doing.
Old 11-30-2020, 08:27 PM
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Originally Posted by surv View Post
Nice work, looks like you know what your doing.
Fake it till you make it.

Thanks though. I am getting there. Every day is more experience. Only minor mistakes requiring do-overs so far.

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