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Old 04-08-2020, 12:09   #1
er9
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plugging / reducing thru hull size.

i want to install a 3/4 thru hull to replace and old 1" thru hull.

i already have the 3/4 thru hull and fittings so am thinking of ways i can reduce the 1" hole in the hull to accomodate the new thru hull.

McMasterr sells 1" o.d. x 3/4 i. d. g-10 and FRP tubes. i was thinking of buying and cutting a piece of FRP tube and epoxying it into the old hole to make up the difference in diameters.

the thru hull will have a 5" diameter g-10 backing plate epoxied to the hull on the inside and then i would epoxy some 6oz glass around the top of the g-10 backing plate.

to me this would seem sufficiently strong and watertight so i dont have to grind out the old thru hull hole, re-glass and re-drill.

this seem like a sound plan? experienced opinions?
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Old 04-08-2020, 12:13   #2
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Re: plugging / reducing thru hull size.

It would work, the "donut" is supported inside and out. You could just use 5200. Another option is to just fill the old hole with glass/resin, then hole saw the correct size. A bit of taper outside would be nice on a layer or two of glass, but again, with a thru hull and seacock either side, it's not going anywhere.
SO... it's an 1/8" gap. All 5200 is probably ok.
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Old 04-08-2020, 12:22   #3
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Re: plugging / reducing thru hull size.

Quote:
Originally Posted by Cheechako View Post
It would work, the "donut" is supported inside and out. You could just use 5200. Another option is to just fill the old hole with glass/resin, then hole saw the correct size. A bit of taper outside would be nice on a layer or two of glass, but again, with a thru hull and seacock either side, it's not going anywhere.
SO... it's an 1/8" gap. All 5200 is probably ok.
thanks, i thought of that first but the FRP tube is pretty inexpensive and really good quality stuff. a lot stronger than i could make.
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Old 04-08-2020, 12:39   #4
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Re: plugging / reducing thru hull size.

If you do that, know that you are taking shortcuts on one of the primary systems aboard.

Without shortcuts, you either install a 1” flanged adapter and valve followed by a 3/4” hose adapter, or you close the hole by grinding bevels outside and inside and glass it closed using 1708 and epoxy, then drill for a 3/4” skin fitting and install that with a 3/4” flanged adapter and valve.

See https://marinehowto.com/replacing-th...-and-seacocks/
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Old 04-08-2020, 12:56   #5
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Re: plugging / reducing thru hull size.

You really should not shortcut this. Glass in the hole properly with a good 12:1 bevelled patch inside and out and then drill a new smaller hole. Don't take short cuts on items where failure can result in a sinking.
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Old 04-08-2020, 13:19   #6
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Re: plugging / reducing thru hull size.

Why don’t you keep the 1” thruhull and seacock and use a 1” to 3/4” reducer on the inside?
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Old 04-08-2020, 14:06   #7
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Re: plugging / reducing thru hull size.

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Why don’t you keep the 1” thruhull and seacock and use a 1” to 3/4” reducer on the inside?

that would require buying a complete new thruhull, backing plate, seacock, etc....$250. it might be what i end up doing if i go this route because re-drilling/re glassing would be more than double that. however....a piece of fiberglass tube is $18.
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Old 04-08-2020, 14:08   #8
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Re: plugging / reducing thru hull size.

Quote:
Originally Posted by er9 View Post
that would require buying a complete new thruhull, backing plate, seacock, etc....$250. it might be what i end up doing if i go this route because re-drilling/re glassing would be more than double that. however....a piece of fiberglass tube is $18.
My bad, I thought you already had 1” seacock in place.
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Old 04-08-2020, 14:18   #9
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Re: plugging / reducing thru hull size.

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Originally Posted by Knotical View Post
My bad, I thought you already had 1” seacock in place.
no old seacock and thru hull are getting removed and replaced. they are in pretty rough condition. i estimated the size of the old thruhull based on the fittings i could see. when i was finally able to remove my sea strainer and put a caliper on the thru hull it measured 1". what can ya do? guess it might be a $250 mistake. could be worse...at least i can re-use the 3/4 thru hull for my toilet intake.
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Old 04-08-2020, 18:49   #10
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Re: plugging / reducing thru hull size.

Wait a minute... you made a mistake measuring and are now trying to install the wrong, smaller size?! it doesn’t work like that, the diameter is not just a size someone had at hand but is calculated to be required for a certain flow rate. You must replace with same size.

The cost you’re calling is way off. Groco 1” thru hull fitting is $18 and the matching flanged adapter is $38. Then the valve is $47 so it’s only $100, not $250. These are all the Groco components that are supposed to go together and I just checked pricing on Defender.

Also, closing the hole with fiberglass does not cost more than $20 in materials.
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Old 04-08-2020, 20:50   #11
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Re: plugging / reducing thru hull size.

no its all good i figured it out. just an error/ misunderstanding on my part.
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Old 05-08-2020, 03:56   #12
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Re: plugging / reducing thru hull size.

Quote:
Originally Posted by s/v Jedi View Post
Wait a minute... you made a mistake measuring and are now trying to install the wrong, smaller size?! it doesn’t work like that, the diameter is not just a size someone had at hand but is calculated to be required for a certain flow rate. You must replace with same size.

The cost you’re calling is way off. Groco 1” thru hull fitting is $18 and the matching flanged adapter is $38. Then the valve is $47 so it’s only $100, not $250. These are all the Groco components that are supposed to go together and I just checked pricing on Defender.

Also, closing the hole with fiberglass does not cost more than $20 in materials.
Don't think you read the original post. The OP wants to replace an old 1" thruhull that's in rough shape with a 3/4" that fits their purpose.
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Old 05-08-2020, 05:07   #13
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Re: plugging / reducing thru hull size.

Quote:
Originally Posted by pcmm View Post
You really should not shortcut this. Glass in the hole properly with a good 12:1 bevelled patch inside and out and then drill a new smaller hole. Don't take short cuts on items where failure can result in a sinking.
How exactly do you think the OPs plan would fail? Grinding a 12:1 scarf in and out for something like this would be major trauma and quite ridiculous. What you are suggesting is appropriate for major structural repairs, not what the OP is doing, its not necessary even if you are abandoning a fitting.
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Old 05-08-2020, 08:54   #14
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Re: plugging / reducing thru hull size.

There are 'through hull plug fittings' sold at Boat Owners Warehouse in the US. This is a through hull fitting minus the through hole. Use this to fill the existing 1" hole. Then drill a new hole for the through hull fittings you want to use.

I've seen people taper and layer fiberglass to plug an unused through hull hole. Personally, this practice doesn't make sense to me on any level.
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Old 05-08-2020, 09:48   #15
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Re: plugging / reducing thru hull size.

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Originally Posted by vpbarkley View Post
There are 'through hull plug fittings' sold at Boat Owners Warehouse in the US. This is a through hull fitting minus the through hole. Use this to fill the existing 1" hole. Then drill a new hole for the through hull fittings you want to use.

I've seen people taper and layer fiberglass to plug an unused through hull hole. Personally, this practice doesn't make sense to me on any level.
much thanks.
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