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View Poll Results: Is it worth it to replace factory thru-hulls with Proper Flanged Thru-hulls
Yes, I've done it. 23 35.94%
Yes, the boat I have came with Proper Flanged Thru-hulls 13 20.31%
Yes you should, but I haven't done it on my boat yet. 5 7.81%
Yes, in my non-boat-owning opinion. 0 0%
No, don't waste your time and money 23 35.94%
Voters: 64. You may not vote on this poll

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Old 16-08-2022, 07:41   #1
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The cult of the Proper Flanged Thru-hull

Actual practice on most boats built in the last 30 years, for thru-hulls, is to install a straight-threaded thruhull with a locknut inside, and then attach a ball valve, or maybe a street elbow and a ball valve, and then a hose barb and so on. This has been the case with boats I have looked at made by Tartan, Catalina, Hunter, Morgan.


There is a case to be made that this arrangement isn't very strong (because it depends on the thin wall of the threaded thruhull for strength) and prone to leaks (because of the connection between tapered and straight threads). Flanged ball valves on the inside, aka Proper Flanged Thru-Hulls, address both concerns, and are available in brass and nylon.


On my Morgan I went through the laborious process of replacing the thru-hulls with Proper Flanged Thru-Hulls, in the prescribed fashion, with the flange bolted to studs bedded in the fiberglass and 5200 on all contact points between the fiberglass and the brass. It worked great and was very secure.


Was it necessary? I'm not sure.
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Old 16-08-2022, 07:55   #2
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Re: The cult of the Proper Flanged Thru-hull

The logic for the flange is to make the entire fitting mechanically strong enough that it will not break off if somebody or something falls against it, or uses it as a step, or puts a 3 foot wrench on a stuck valve handle, or any number of other unforeseeable mechanical stresses are imposed on the fitting.

The standard for good practice also says that all seacocks should be easily accessible.

Most sailboats built in the last 30 years grossly violate the accessibility requirement with many of their seacocks, which--in a perverted way--makes the requirement for physical strength less important.

Does two wrongs make a right?

P.S.: Hopefully you didn't really use "brass" for your seacocks, but are using that as a generic term for any kind of vaguely yellowish metal.
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Old 16-08-2022, 07:57   #3
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Re: The cult of the Proper Flanged Thru-hull

I have 15 thru-hulls. Every single one of them is a thru-hull with a ball valve. The ball valves are all bronze. You should not use brass if you are ever going to go to salt water because the salter water will dissolve the brass.


I have had zero problems. The thru-hulls in the boat when I bought it were the same and lasted 30 yrs.


The only issue with the system is that the thru-hulls are not a tapered thread and the valves are. The solution was use Sikaflex 291 as thread dope. I have never had a leak.

I would not try to put in flanged thru-hulls. It would be difficult and in some cases impossible.

IMHO the flanged thru-hulls are over kill but that is just an opinion and most people have 1 or 2.
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Old 16-08-2022, 08:43   #4
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Re: The cult of the Proper Flanged Thru-hull

Just installing seacocks now; guess I owe some membership dues to the cult of the proper flanged seacock & thru hull arrangement.

IMHO Thruhulls are literally holes in the boat & seacocks are keeping the boat afloat; this is not a place to cut corners

Bronze ones in the engine room where all the below static waterline thru hulls are arranged for easy access (and thus more easily kept turned off until needed) and plastic ones for above static-waterline and sanitary outlet. Apart from proper install, its important to make them easily accessible location-wise or else they tend to be ignored & not exercised

The ABYC recommends flanged bolted-on seacocks on all below-waterline thru-hulls (ideally up to the toe rail) which can take a side-hit in case something comes loose in a seaway such as a battery. Some of the Marelon sea cocks can be epoxied in place. Given the option I would not rely on the mushroom head alone though this is done widely.

The bolts for the flanged seacocks do not penetrate the hull; their heads are buried beneath the backing plate that holds the seacock, with the threaded ends pointing out (so yes a backing plate is required too for a proper install, I use pieces of G4 fiberglass sheet epoxied in place for all seacocks and other fixtures and bronze threaded bolts for the studs to mount the bronze seacocks. Bronze can be epoxied over without crevice corrosion issues unlike SS steel)

One cult requirement I see missing often even in proper flanged installs is the necessary rubber hose immediately after the seacock -- too many examples of heavy "christmas tree" arrangement multiple outlets attached directly to the seacock is not a good idea.

Groco makes a thru hull adaptor plate that lets you swap the valve assembly without having to remove the thru hull, in case of unlikely chance there's ever a problem with a proper UL-rated marine bronze seacock
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Old 16-08-2022, 09:08   #5
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Re: The cult of the Proper Flanged Thru-hull

Quote:
Originally Posted by Sailorman97 View Post
The only issue with the system is that the thru-hulls are not a tapered thread and the valves are. The solution was use Sikaflex 291 as thread dope. .
Are you putting NPT threaded valves directly onto NPS threaded through-hull fittings? This is a no-no specifically prohibited by ABYC.
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Old 16-08-2022, 09:17   #6
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Re: The cult of the Proper Flanged Thru-hull

I found these wirh a quick Google search
https://www.boatoutfitters.com/stand...room-thru-hull
Note they terminate the NPS thread with NPT.
I guess the gods at ABYC missed this possibility.
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Old 16-08-2022, 09:22   #7
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Re: The cult of the Proper Flanged Thru-hull

I’ve seen a lot of boats sink, none due to thru hulls. But if peace of mind come with doing it right, why not?

When we got the current boat one thru hull had a slight leak. It was just below the water line, so in general wouldn’t have filled past water level. I gooped and tightened it.
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Old 16-08-2022, 09:38   #8
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Re: The cult of the Proper Flanged Thru-hull

Apologies for imprecise prose, yes, I should have written bronze rather than brass in two places up thread.
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Old 16-08-2022, 09:50   #9
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Re: The cult of the Proper Flanged Thru-hull

Quote:
Originally Posted by Tetepare View Post
I’ve seen a lot of boats sink, none due to thru hulls. But if peace of mind come with doing it right, why not?

When we got the current boat one thru hull had a slight leak. It was just below the water line, so in general wouldn’t have filled past water level. I gooped and tightened it.
No actually since your boat weights a few tons and the leak was already below waterline it would continue to sink lower as water trickled in at a faster rate and higher pressure, possibly making the hole bigger ...
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Old 16-08-2022, 09:55   #10
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Re: The cult of the Proper Flanged Thru-hull

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Originally Posted by Bill Seal View Post
I found these wirh a quick Google search
https://www.boatoutfitters.com/stand...room-thru-hull
Note they terminate the NPS thread with NPT.
I guess the gods at ABYC missed this possibility.

The link is for a "standard" thru hull fitting
The combination NPT-NPS thread thing is not new, the problem is that it results in a weaker fitting arrangement. Some people resort to re-machining the threads too, and this only further weakens the thru hull. In fact there are already some less expensive bronze thru hulls (Apollo) that have much thinner walls than the Groco or better yet Spartan Marine thru hulls and seacocks.

Honestly, why not just do it right instead of bandaids?
These are literal holes in your boat; is this the place to cut corners? I'd eat a month's worth of ramen if it meant saving money for doing a proper job of protecting literal holes in my boat.
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Old 16-08-2022, 10:00   #11
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Re: The cult of the Proper Flanged Thru-hull

I replaced my 42 year old Marelon with new Marelon seacocks. It was easy, and I sleep better.
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Old 16-08-2022, 10:15   #12
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Re: The cult of the Proper Flanged Thru-hull

CW602N brass from good quality manufacturers is entirely suitable for seawater and will remain suitable for decades . This is behind the DNZ marking ( CR)

There’s no field evidence that flanges are needed
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Old 16-08-2022, 10:41   #13
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Re: The cult of the Proper Flanged Thru-hull

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Originally Posted by Cyrus Safdari View Post
No actually since your boat weights a few tons and the leak was already below waterline it would continue to sink lower as water trickled in at a faster rate and higher pressure, possibly making the hole bigger ...
Not really. Even considering that under high pressure around the thru-hull input would be limited, in this case it would to at least top over a half bulkhead, which couldn't happen because the rest of the boat is floating and it would never get that far down into the water.
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Old 16-08-2022, 11:03   #14
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Re: The cult of the Proper Flanged Thru-hull

The rest of your boat includes a lead keel weighing several tons pulling your boat down glug glug glug
As more water enters the boat, it sinks lower. As it sinks lower, the hole is lower underwater too so the water pressure increases so more water gets in faster even if the hole size remains constant
At no point will a sailboat stop sinking unless it has special foamed or watertight compartments built in of sufficient size to hold afloat a flooded boat which do exist

Take a pail, put a brick in it just heavy enough to keep the pail bottom slightly under water. , and poke hole just below waterline on the side of the pail then try to float it in a pool or bathtub
What happens? Does the water at any time stop flowing in and equalize? Of course not, with a brick in it.. The more water enters, the deeper the pail sinks as it gets even heavier, the faster more water enters, until the pail sinks.

Boats with leaks below the waterline whether from a seacock leak or whatever don't just magically stay afloat unless they have built-in "inherent buoyancy"
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Old 16-08-2022, 12:50   #15
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Re: The cult of the Proper Flanged Thru-hull

Quote:
Originally Posted by Bill Seal View Post
I found these wirh a quick Google search
https://www.boatoutfitters.com/stand...room-thru-hull
Note they terminate the NPS thread with NPT.
I guess the gods at ABYC missed this possibility.


Thatís not the prohibited connection, thatís a workaround.
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