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post #11 of 46 Old 09-21-2015 Thread Starter
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Re: Thru Hull Fitting Questions

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Originally Posted by overbored View Post
the boat being built in Australia means it would most likely be a british straight pipe thread and not the same as the graco fitting. Vetus makes british thur hull fittings
Interesting- you may be right. I am curious as to why I got one of the Grocco adapters to go on one of the other existing 1-1/2 inch seacocks, but not the current one I am working. The more I clean the threads the father it goes, but at point of diminishing returns on this thru hull. Maybe the threads are too dirty or have some rough spots. I might buy a new 1-1/2 thru hull and see how the threads match up (threads per inch), and maybe used the lock nut off the new through hull to "chase" the threads of the thru hulls now on the boat. With fewer threads on the lock nut, it may chase and clean up the threads (I plan to use some solvent and then oil to see if that helps to clean up the threads while running the lock nut).

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Re: Thru Hull Fitting Questions

I bet a dodgy thru hull mated to a different species of thread will allow you to sleep at night. Enough fiddling will make different threads fit but maybe hiding something (like loss of integrity) I'm all for blending but only in interracial relationships.
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Re: Thru Hull Fitting Questions

the difference between 1 1/2" british pipe and national pipe is that the british pipe is .010" smaller but the thread count is 11 vs 11 1/2 threads per inch for the national. this may be the problem but seeing that they are fitting close close you may have national pipe thread. there is also the issue of the thread angle being different. get your hands on a thread gauge and check. this way you will know if there is a mismatch. A good way to clean up the dirty threads is to use a thread file.

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Re: Thru Hull Fitting Questions

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Originally Posted by overbored View Post
the difference between 1 1/2" british pipe and national pipe is that the british pipe is .010" smaller but the thread count is 11 vs 11 1/2 threads per inch for the national. this may be the problem but seeing that they are fitting close close you may have national pipe thread. there is also the issue of the thread angle being different. get your hands on a thread gauge and check. this way you will know if there is a mismatch. A good way to clean up the dirty threads is to use a thread file.
I think you are on to the problem. Doing some searching I see Vetus (European Company I think) makes thru hulls and valves with the BSP (British Standard Parallel Pipe) thread- go to this link and see the details of say a Grocco fitting vs Vetus for valves:

Valves & Seacocks | Fisheries Supply

And thru hulls:

Threaded Thru-Hull Fittings | Fisheries Supply

Looks like Vetus have BSP and US made have NPS (National Pipe Straight).

I need to find the threads per inch on my thru hulls to determine if that is indeed the problem, or if the problem is built up pipe sealant in the root of the thread.

That brings another question: How do straight pipe threads seal. I know some use a caulk as a pipe sealant- is this correct?

Also from:

https://www.valvesonline.com.au/references/threads

"BSP thread form stands for British Standard Pipe and is common in Australia and the commonwealth countries. It is based on trade size rather than actual diameter which can lead to some confusion when measuring ports."

This could be why when I threaded the 3/4 inch Grocco adapter onto my existing 3/4 inch thru hull, it went on no problem- BSP and NPS for 3/4 inch have same threads per inch where the 1-1/2 inch are different by 1/2 thread per inch.

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Last edited by casey1999; 09-22-2015 at 01:32 PM.
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Re: Thru Hull Fitting Questions

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Yes. I was planning to install new through hulls. After cleaning the existing and seeing that they are in excellent shape, I considered leaving in place. The existing thru hulls are bronze with a very thick wall. The hull side are flush with the hull and sealed well. I sanded down to the thru hull on the outside of boat and the thru hull looked new- bright bronze under the epoxy seal coat. And on inside, absolutely no pink or corrosion once the surface green was cleaned off. The inside also looked excellent. I put a couple hundred pounds force on the end of the thru hull and it is solid.

Considering some new thru hulls now are made in China (even from US based companies), I would trust these 30 year olds over some of the newer available.

It is a big job to get the existing out as they have been glassed in on inside of the boat. Could be done and I am willing to do if necessary, but from working on old boats and old houses- sometimes best to let old dog lay. And as I say, new not necessarily better.

Attached pic is of original install with ball valve (taper thread) to thru hull straight thread. That ball valve is Grocco 1-1/2 inch taper thread- I replaced the original valve with this one myself 5 years ago- Grocco adapter piece was not available and I was told by boat yard "experts" this was ok and met standards, and also marine surveyor did not have a problem with it- live and learn...

As part of the work I am doing I removed and will fill the hole of a 37 year old thru-hull bronze speed log. The thru hull for that was bronze and it was also in excellent shape- looked brand new- even after I cut it to pieces with sawz aw.

BTW- none of my thru hulls are bonded- I think this is one reason they have lasted so long.
My point was not at all about reusing an old thru-hull, provided threads are correct, but rather a bit of shock that you would even attempt this job by leaving it in place. What a PITA.....


Remove it, clean it with a grinder and wire wheel and install the backing plate and thru-hull properly all with new sealant. Now you can cut the thru-hull to the proper insert depth and not Mickey mouse around with hopeful gap filling and depending upon 30+ year old sealant...

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Re: Thru Hull Fitting Questions

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My point was not at all about reusing an old thru-hull, provided threads are correct, but rather a bit of shock that you would even attempt this job by leaving it in place. What a PITA.....


Remove it, clean it with a grinder and wire wheel and install the backing plate and thru-hull properly all with new sealant. Now you can cut the thru-hull to the proper insert depth and not Mickey mouse around with hopeful gap filling and depending upon 30+ year old sealant...
The existing thru hull does not have any sealant- it is glassed into the hull. The exterior of the thru hull is flush and was glassed in as I found when I sanded down to inspect the bronze. The hull is solid glass about 1-1/2 inch thick and where the seacokk is, there is a backing plate glassed into the hull with an addition of more glass on top of that. Maybe not the "proper" way to install a thru hull, but it is solid and the bronze has not suffered any corrosion damage. I would say this install is probably more solid than any production boat out there. The removal of this thru hull would be difficult and require me to hole saw out the existing thru hull, grind away a lot of existing glass and imbedded backing plate, do a proper hole fill with a 12:1 taper and then drill out the new hole for new thru hull. I would say the final product would be no better than existing, and a good chance it would generate a weaker hull structure in area of the thru hull. I got no problem with doing the work, just don't want to create an inferior product. Seeing existing is in fine shape, and has lasted many years, I see now reason why it should not last another 30 years. I got plenty of space to micky in. As far as gap filling, Grocco sells the backing plates I am using and suggest using them with no addition of epoxy to bed them against the hull. Now I don't agree with that, but that is what a material supplier is recommending. The way I plan to do it is bed the Grocco epoxy plate onto the hull and run bronze bolt thru the grocco adapter, into the grocco inserts (located in the Grocco plate) and then into drilled and tapped holes located in my glass fill. This will be way stronger than the Grocco suggestion for install.

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Last edited by casey1999; 09-22-2015 at 04:29 PM.
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Re: Thru Hull Fitting Questions

Maine sail correct. Get the old thru hulls out and start fresh.
You cannot see the condition of those thru hulls under the glass. Which is concerning. Also you won't have to use those adapters you can use the real flange type sea **** with backing plates.
Changing the valves without doing the thru hull is just doing 1/2 a job.


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Re: Thru Hull Fitting Questions

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Maine sail correct. Get the old thru hulls out and start fresh.
You cannot see the condition of those thru hulls under the glass. Which is concerning. Also you won't have to use those adapters you can use the real flange type sea **** with backing plates.
Changing the valves without doing the thru hull is just doing 1/2 a job.
Not so sure one can just use blank statements like: "You cannot see the condition of those thru hulls under the glass." As an engineer I make many decisions based on only 1/2 the data. I have worked on boats that travel many feet below the surface- sea chest much more critical than the 10 psig my thru halls will see. If as an engineer I asked to visually inspect every item I made a go not go decision on- I would be out of a job. In any case, as I said, I sanded down the hull side epoxy on the flush thru hull and inspected- nothing but bright bronze. Cleaning away the green patina on the ID and OD of the exposed thru hull indicates nothing but solid bright bronze. I would trust these thru hulls over many new ones now days. Unfortunately, many of the US based companies are manufacturing their bronze fittings overseas and with questionable QC (see the recent Gun Boat post seen here on Sailnet), so as I say, new not necessarily better.

Some good reading:
http://michel-christen.com/2T-H.pdf

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Re: Thru Hull Fitting Questions

If the through hull walls are really thick, maybe it would be possible to re-thread them to a closely matching NPT thread. Sometimes these thread adapters form a weak link - if they crack under stress (I have seen it happen, but not on a through hull), you will have a big problem.

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Re: Thru Hull Fitting Questions

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If the through hull walls are really thick, maybe it would be possible to re-thread them to a closely matching NPT thread. Sometimes these thread adapters form a weak link - if they crack under stress (I have seen it happen, but not on a through hull), you will have a big problem.
The thru hull wall is thick- but towards the ID, my understanding is the od of a pipe is the same no matter if BSP (British Standard) or the American standard NPS. I spoke with Grocco and they do make a seacock, but with the British Standard BSP threads on the hull side and NPT threads on the ball valve side- problem is they have a 6 month delivery time.

However, my thru hull fitting threads may still be NPS- maybe I just need to clean the threads more. On the first one I worked I got the fitting on without too much difficulty.

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