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Old 14-09-2011, 10:51   #1
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Sealing an Above-Water Thru Hull

Okay guys and gals. I'm stumped. Help!

On our boat we have thru hull above the water line that was being used as a bilge pump discharge. The problem was that this particular thruhull is that it is in a place that there is no way I could easily get to in an emergency.

There was a fiberglass box built in front of it. It has an fuel tank to one side and a bulkhead to the other. Completely boxed in. You can see it in this photo on the top far left. Notice the mix and match hoses going to it.



Now, this upset me. I can't even hardly get to it to replace the hose going to it and hose clamps. So I want to seal it off. Problem is... You guessed it. I can't get to it.

I was thinking of hammering a tapered plug into the plastic fitting from the outside with some 5200 but I don't know if that will work. I could cut the fitting out from the outside of the boat but that would make for a really ugly repair job. Its along the top of the side of the boat and extremely visible. Realistically to make it look okay I'd have to hire out to have that done and I don't really want to and don't really have the cash on hand for that either.

Any ideas?
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Old 14-09-2011, 11:02   #2
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Re: How to seal an above water thru hull?

As long as you are of a mind to no longer use that thru hull, why not just plug it by plugging the hose on the inside, where you can have access. Leave the fitting and short section of hose where it is.
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Old 14-09-2011, 11:10   #3
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Re: How to seal an above water thru hull?

Aside from a mis-match in the gelcoat, the removal and filling isn't that difficult. I've done a few of them on the bottom of various boats. While the ones I did weren't that large, IE 1 inch instead of 2 inch the process is the same.

First remove the fitting. Then bevel out the hole on a 10:1 ratio. IE 1/4 thick skin would need a dish about 2.5 inches larger on each side. So for a 2 inch hole, you'd be dishing out a 7 inch circle. Take it down to feather thick at the hole.

Next you put a backing plate covered with wax paper up against the back of the hole. Hold it in place with a strut and wedges.

Then start cutting out disks of woven roving and chopped strand matt in alternating layers. CSM/WR/CSM/WR/CSM/WR.... until you have enough to fill the dish completely. each disk will be slight larger than the last one.

Start wetting down the dish, with epoxy, and place the first layer and wet it out. Get as much resin out as you can so its nice tight, then keep adding alternate layers.

Once done, sand it with a block and sand paper in a circular motion so the slight curve of the surface isn't lost. Go slow here and check by eye to make sure you aren't getting a flat spot.

Finally you can finish it off with interprotec 2000, and sand with increasing grits of wet n dry paper.

Once done you should see it blend in with the original glass, and then all you need to do is put on matching gelcoat. That you can either do yourself with a compressor and spray gun or hire a contractor.

It might not be a perfect match but it will be solid.
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Old 14-09-2011, 11:13   #4
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Re: How to seal an above water thru hull?

Any thru-hull valve or thru-hull fitting that you cannot access eventually needs to be permanently removed and the hole glassed over and made like it was never there.

For now as a safety measure I think you would be fine putting a wood plug in there held in with 5200 and coated with 5200 on all sides to eliminate water penetrating into the wood.

I would not leave a valve there forever that you cannot periodically remove for inspection.
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Old 14-09-2011, 11:35   #5
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Re: How to seal an above water thru hull?

If it is a plastic thru hull above the waterline, you can epoxy it in. If it is a bronze you can tap then insert a bronze plug. But I like sabrekai's solution the best, just feather the hole and fiberglass it in. It is a little trouble now, but you will be glad you did it. Fiberglass is a good boat material for this very reason
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Old 14-09-2011, 11:42   #6
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Re: How to seal an above water thru hull?

I like SabreKai's idea too but the truth is I'm too chickensh*t and cheap to haul off grinding a 10:1 or 12:1 hole in a highly visible area. Also I'd have to pull the rub rail off to do it properly.

Capn_bill, I didn't know if epoxy would bond well to plastic. If I filled the hole with epoxy what would you suggest? Just sand and rough up the inside of the hole then use a thickened epoxy to fill?
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Old 14-09-2011, 12:05   #7
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Re: How to seal an above water thru hull?

How about a plumbing test plug? You can get either the mechanical kind, which will probably stick out from the hull about an inch, or the inflatable kind, where you might, over a long period of time, have to worry about deflation. Some are all plastic (Titan plastic plug), others you would have to replace the cheapo bolt with something more corrosion resistant. Whether you can find a standard one that will fit the thru hull diameter is the hard part.

Or how about spray in place expanding foam? Get the good stuff that is rated as fire-blocking and water resistant. Wouldn't ever want to take it back out though.
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Old 15-09-2011, 20:42   #8
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Re: How to Seal an Above-Water Thru Hull ?

If your arm is long enough to reach the end of the plastic barb with the hose removed, then get a plug with a slight taper, and cut it so it is flush on the outside and not reaching the end of the barb on the inside. Drill a 1/4 inch hole through the plug, countersink the end and insert a long 1/4-20 SS bolt. Goop it up with 5200 around the bolt. Then cut a disk from either SS or mild steel (epoxy coat it) and drill a hole in it. When ready, hose off and someone inside with long arms, insert the plug with more 5200. Put the disk over the bolt and add a lock washer and nut. Tighten as needed to snug up the plug.

You might make up a waldo (tool) for holding the disk while you slip it over the bolt. A long extension and socket will take care of the nut with the washer taped to it. You'd be surprised at the weird stuff we came up with while building DC-9s.
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Old 15-09-2011, 20:50   #9
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Re: How to Seal an Above-Water Thru Hull ?

eeewww-- what is that brownish colored translucent fluid dripping from your overhead in there......
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Old 16-09-2011, 00:23   #10
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Re: How to Seal an Above-Water Thru Hull ?

Can you cut the hose barb part of the thru-hull off and screw a pipe cap on the threaded part?
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Old 16-09-2011, 08:17   #11
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Re: How to Seal an Above-Water Thru Hull ?

Quote:
Originally Posted by zeehag View Post
eeewww-- what is that brownish colored translucent fluid dripping from your overhead in there......
Degreaser. :P
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Old 16-09-2011, 08:29   #12
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Re: How to Seal an Above-Water Thru Hull ?

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Can you cut the hose barb part of the thru-hull off and screw a pipe cap on the threaded part?
Hop,

This is a really great suggestion and I'm ashamed I didn't think of it. Only problem is that I probably can't get that far in. I can't reach the barb so cutting it off could be a problem using some sort of saw on a stick.
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Old 16-09-2011, 08:31   #13
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Re: How to Seal an Above-Water Thru Hull ?

Quote:
Originally Posted by SabreKai View Post
If your arm is long enough to reach the end of the plastic barb with the hose removed, then get a plug with a slight taper, and cut it so it is flush on the outside and not reaching the end of the barb on the inside. Drill a 1/4 inch hole through the plug, countersink the end and insert a long 1/4-20 SS bolt. Goop it up with 5200 around the bolt. Then cut a disk from either SS or mild steel (epoxy coat it) and drill a hole in it. When ready, hose off and someone inside with long arms, insert the plug with more 5200. Put the disk over the bolt and add a lock washer and nut. Tighten as needed to snug up the plug.

You might make up a waldo (tool) for holding the disk while you slip it over the bolt. A long extension and socket will take care of the nut with the washer taped to it. You'd be surprised at the weird stuff we came up with while building DC-9s.

Interesting. Hmmm.. I like it.
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Old 16-09-2011, 13:39   #14
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Re: Sealing an Above-Water Thru Hull

Cut and plug from the inside, however close you can get, then use thickened epoxy paste from the outside and fill the void, make a nice smooth surface in the opening of the fitting when viewed from the outside. When the tank comes out someday, remove and repair the hole.
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