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Old 15-09-2013, 19:53   #1
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Fitting thru hull to uneven hull

Hi,

We have been installing new thru hulls with flanges. However, we have come to a thru hull in a location with significant concave hull curvature. Our hull is about 1 inch in thickness at this location. The curvature is so dramatic that when screwing the thru hull to the flange there is gap that grows from nothing to a 1/2 inch gap between the hull and the flange.

Obviously, adding a backing plate will not solve this problem. I need someway of creating a mold of the space using epoxy or fiberglass and I was hoping that someone would have some advise in this regard.

As a point of note, the thru hull itself fits pretty flat to the outside of the hull when inserted fully.

- z
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Old 15-09-2013, 21:35   #2
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Re: Fitting thru hull to uneven hull

Sand the hole contour area in question, aply some mold release or wax to the truhull, mix some epoxy with silica and Microballons, truhull in place, tight, fill the gap well with the mixture, let it dry, release the truhull, could be need some more in the voids, sand the mold smooth , now you can keep the epoxy like that or if you feel in doubt aply a layer or 2 of FG in the epoxy... i have the discharge bilge pump with a mold in epoxy, same problem, inside i got a curvature , imposible to lay flat the nut,...
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Old 15-09-2013, 21:41   #3
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Re: Fitting thru hull to uneven hull

Make epoxy puddy bedding (by mixing silica, microfibers and whatever or buy). Slightly sand the the hull surface, set a pvc tape on the thru hull and screws and set in place loosely. Fill the gap with the puddy, let set and take the thru hull of. sand and paint the the bedding and place the thru hull..
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Old 15-09-2013, 21:55   #4
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Re: Fitting thru hull to uneven hull

I had something similar on my Hydrovane installation. I have a canoe stern with lots of curves. I had to make backing plates and custom "timber pads" that allowed the flat brackets of the wind vane to mount flush to the hull, and then the same for huge interior plates.

I did the "reverse hollystone" technique.

Tape sandpaper down (double sided tape is the best but not required), and start sanding a piece of teak. Eventually it will get perfectly smooth. It takes a little bit of practice to get the stroke exactly so that you're sanding it the right way. Up and down is different than side to side, etc.

Epoxy the thing, then epoxy it to the hull. In my case fasteners went through it.
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Old 15-09-2013, 22:22   #5
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Re: Fitting thru hull to uneven hull

Quote:
Originally Posted by neilpride View Post
Sand the hole contour area in question, aply some mold release or wax to the truhull, mix some epoxy with silica and Microballons, truhull in place, tight, fill the gap well with the mixture, let it dry, release the truhull, could be need some more in the voids, sand the mold smooth , now you can keep the epoxy like that or if you feel in doubt aply a layer or 2 of FG in the epoxy... i have the discharge bilge pump with a mold in epoxy, same problem, inside i got a curvature , imposible to lay flat the nut,...


I buy a cheap crappy SS thru hull in the proper dimension and mold release it for use on all of the boats skin fittings. The reason being, once I've mold released the threads I'm not comfortable bedding it in, no matter how many solvent wipes happen. Only costs $25 or so for the sacrificial fitting, less than the solvent you'd use cleaning up a couple of fittings. Then you get to install clean new fittings with no wax or epoxy on them. After a while you end up with a box of different size fittings.
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Old 15-09-2013, 22:32   #6
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Re: Fitting thru hull to uneven hull

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I buy a cheap crappy SS thru hull in the proper dimension and mold release it for use on all of the boats skin fittings. The reason being, once I've mold released the threads I'm not comfortable bedding it in, no matter how many solvent wipes happen. Only costs $25 or so for the sacrificial fitting, less than the solvent you'd use cleaning up a couple of fittings. Then you get to install clean new fittings with no wax or epoxy on them.

Good advice, but not my case, sometimes i just use a regular carwax, geting ridd off wax traces can be tough, well i wash the fitting several times and pass a wire brush to the threads,so far so good good adesion after all this years, last time to spray paint the waterline this fitting gime a hard time to move it out of the hole... but i take note about the spare truhull for the next time...
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Old 15-09-2013, 22:36   #7
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Re: Fitting thru hull to uneven hull

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Good advice, but not my case, sometimes i just use a regular carwax, geting ridd off wax traces can be tough, well i wash the fitting several times and pass a wire brush to the threads,so far so good good adesion after all this years, last time to spray paint the waterline this fitting gime a hard time to move it out of the hole... but i take note about the spare truhull for the next time...


Yeah, I use PartAll Paste #2, which guarantees an excellent trouble free release. But it's a bear to remove. Wire brush and a bucket of acetone, followed by rags and more clean 'tone. Cheaper and faster to have a series of sacrificials. Then you just leave 'em waxy, let the epoxy mess kick on it and peel it off later, no worry about getting wax or epoxy too far up in the threads, etc. etc.
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Old 15-09-2013, 22:44   #8
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Re: Fitting thru hull to uneven hull

For the flanged thru hulls such as the OP is using, I also often use a disc of flat stock like a giant washer on the inside. 5" disc is good for the 1 1/2" Groco. I like 1/8" flat stock or G10. You apply your epoxy for potting in to the hull, and then install the thru hull assembly using the disc like a big washer. Clean up the squeeze out around the disc, make a nice fillet, scratch it all up, and glass a few matts over it. Scratch again, and gelcoat to match. This gives a perfect circular "pad" that looks like it was molded into the boat. It's my preferred method professionally, and it's how I did my own boat.
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Old 15-09-2013, 22:49   #9
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Re: Fitting thru hull to uneven hull

Question Minaret, i have in the engine room a huge thruhull for the cokpit drains, the hole is oversized, seems to me the previous idiot use a wrong hole shaw and the truhull got a big play in the hole, all this years i dont found nothing suspicious until i got some seepage from the base of the thruhull, is the only one we dont replace it in the last haulout because is in super condition, How can i macht the hole with the truhull, i think in use epoxy again but i dont feel confident in just epoxy , my other idea is to fill flush the hole with FG and make the hole true this time... Cheers.
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Old 15-09-2013, 22:57   #10
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Re: Fitting thru hull to uneven hull

Quote:
Originally Posted by neilpride View Post
Question Minaret, i have in the engine room a huge thruhull for the cokpit drains, the hole is oversized, seems to me the previous idiot use a wrong hole shaw and the truhull got a big play in the hole, all this years i dont found nothing suspicious until i got some seepage from the base of the thruhull, is the only one we dont replace it in the last haulout because is in super condition, How can i macht the hole with the truhull, i think in use epoxy again but i dont feel confident in just epoxy , my other idea is to fill flush the hole with FG and make the hole true this time... Cheers.


Cut a glass flat stock disc a little bigger than the nut on the inside and bond it in place over the existing hole, then fillet and glass the inside much as I explained above. Then re drill from outside with the correct size hole saw and install. You just need a thru hull that is longer by the thickness of the flat stock plate + the glass and gel you put on it, ie fit a new skin fitting.
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Old 15-09-2013, 23:02   #11
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Re: Fitting thru hull to uneven hull

That can be work to for the Op problem with the curvature problem...'? Get it , thx for the advice...
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Old 16-09-2013, 07:48   #12
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Re: Fitting thru hull to uneven hull

The problem is actually even worse than the curvature:

The hull not only curves but has ridges and valleys in the space to be taken up by the flange that were not a problem in the original installation. The original thru hulls used a locking ring, not a flange, so the curvature and and inconsistencies in the hull form on the inside were relatively isolated from the small area that the ring made contact.

Here is what I am thinking based on all of your feedback.

1) Get a "thow away" through hull of the correct size, maybe a cheap plastic one, and wrap it in teflon tape and wax it well.

2) Pass this thru hull up through the hull and put a round backing plate made of 1/8" fiberglass on the thru hull.

3) Follow the backing plate with the lock nut to set the backing plate at the right location.

4) Mix up epoxy and kitty hair to the right consistency and fill in the gap between the backing plate and the hull.

5) Let it kick off most of the way then remove the locking nut and thru hull.

Obviously, this is preceded by make sure the area is clean and scruffed up for adhesion.
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Old 16-09-2013, 07:54   #13
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Re: Fitting thru hull to uneven hull

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Originally Posted by zboss View Post
The problem is actually even worse than the curvature:

The hull not only curves but has ridges and valleys in the space to be taken up by the flange that were not a problem in the original installation. The original thru hulls used a locking ring, not a flange, so the curvature and and inconsistencies in the hull form on the inside were relatively isolated from the small area that the ring made contact.

Here is what I am thinking based on all of your feedback.

1) Get a "thow away" through hull of the correct size, maybe a cheap plastic one, and wrap it in teflon tape and wax it well.

2) Pass this thru hull up through the hull and put a round backing plate made of 1/8" fiberglass on the thru hull.

3) Follow the backing plate with the lock nut to set the backing plate at the right location.

4) Mix up epoxy and kitty hair to the right consistency and fill in the gap between the backing plate and the hull.

5) Let it kick off most of the way then remove the locking nut and thru hull.

Obviously, this is preceded by make sure the area is clean and scruffed up for adhesion.

Yeah, that's pretty much it. I'd just use wax, no tape. And put the epoxy down first instead of trying to trowel it under the disc after its screwed down, you're less likely to leave a void. Don't forget to dry fit first, and if the gap between the disc and hull is really big due to curvature make sure to mix your filler peanut butter thick so you don't have ooze out. Good luck!
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