Straight Thread Thru Hull to NPT Valve

[s/v Jedi]

I’m pretty busy with some boat projects myself so thanks to all picking this up with info on the Groco flanged adapter. I agree that the MarineHowTo site of Rod Collins is the best source of info. It’s what I use. I recently found that the fiberglass backing plates as described by RC are now being sold ready made and I think by Groco as well.

For those unfamiliar with the flanged adapters: they have linear thread on the bottom to accept the thru-hull fitting. After tightening the flange can be bolted onto the hull (ideally onto the tapped backing plate which is epoxied onto the hull). Then what is left is tapered NPT thread on top to accept a ball valve. Groco has the perfect valves for this as well.

All this is available in bronze as well as in stainless steel

[a64pilot]

Assumption is that you remove the locking nut from the existing thru hull so that the adapter can be screwed all the way down flush with the backing plate.
Further assumption is that the intent is to then drill through both the backing plate and hull to mount counter sunk screws from the outside, thus essentially making it the same as a seacock?
My backing plate is only 1/4” thick, not thick enough for screws to do much in it.

I’m in the Bahamas, Spanish Wells to be exact, and things like taps etc. are non existent, although I would not think that retapping from NPT to straight would not be a good idea anyway.

Assuming I can get one here my intent will be to first dry fit it, screw it down until it bottoms out on the lock nut, Mark it, then remove and assemble the valve and T etc. then install with Teflon tape and pipe dope.

Where it is, it can’t be hit by anything substantial, it’s in the compartment with the waste tank, and Lord forbid even if it were broken off, it’s easily accessible to bang in a soft wooden plug that I have several of, and even then the compartment is not connected to the bilge and the top of it is above waterline, so worst case is that compartment is flooded, but it won’t sink the boat.

[Hillbilly]

Faced same problem and got same advice, to force the two threads together, not OK with me. Cut through-hull to desired length and had machine shop put NPT on end. Perfect. Threads compatible because pitch same and larger end of NPT taper has same diameter a straight.

[Q Xopa]

Originally Posted by s/v Jedi

I’m pretty busy with some boat projects myself so thanks to all picking this up with info on the Groco flanged adapter. I agree that the MarineHowTo site of Rod Collins is the best source of info. It’s what I use. I recently found that the fiberglass backing plates as described by RC are now being sold ready made and I think by Groco as well.

For those unfamiliar with the flanged adapters: they have linear thread on the bottom to accept the thru-hull fitting. After tightening the flange can be bolted onto the hull (ideally onto the tapped backing plate which is epoxied onto the hull). Then what is left is tapered NPT thread on top to accept a ball valve. Groco has the perfect valves for this as well.

All this is available in bronze as well as in stainless steel

Groco and Dreamgreen.org both sell them. Havent used either yet but just measuring and listing mine to buy the flanges, backing plates and thru hulls myself ready to get started.

Speaking of measuring anyone know of the easist way to measure the existing Thru hull sizes? I know there is measurements listed for the outer mushroom head diameters, ie '1250' 1.25 inch has 2.62 in diameter head etc as listed in the catalogue.

Step sizes on the wrench would be easy. Anyone know if or where this info is listed?
Im probably just geing to get all of them from Groco.

Talking about threads, Ive noticed they do the traditional NPT thread, but are also now offering BSPP thread as well.

Anyone got some comments to add about the pros and cons of each flavor of threads?

[Q Xopa]

Originally Posted by s/v Jedi

I’m pretty busy with some boat projects myself so thanks to all picking this up with info on the Groco flanged adapter. I agree that the MarineHowTo site of Rod Collins is the best source of info. It’s what I use. I recently found that the fiberglass backing plates as described by RC are now being sold ready made and I think by Groco as well.

For those unfamiliar with the flanged adapters: they have linear thread on the bottom to accept the thru-hull fitting. After tightening the flange can be bolted onto the hull (ideally onto the tapped backing plate which is epoxied onto the hull). Then what is left is tapered NPT thread on top to accept a ball valve. Groco has the perfect valves for this as well.

All this is available in bronze as well as in stainless steel

Groco and Dreamgreen.org both sell them. Havent used either yet but just measuring and listing mine to buy the flanges, backing plates and thru hulls myself ready to get started.

Speaking of measuring anyone know of the easist way to measure the existing Thru hull sizes? I know there is measurements listed for the outer mushroom head diameters, ie '1250' 1.25 inch has 2.62 in diameter head etc as listed in the catalogue.

Step sizes on the wrench would be easy. Anyone know if or where this info is listed?

Im probably just geing to get all of Bronze ones from direct from Groco.

Talking about threads, Ive noticed they do the traditional NPT thread, but are also now offering BSPP thread as well.

Anyone got some comments to add about the pros and cons of each flavor of threads?

[Cheechako]

Sometimes the taper to straight only results in 1.5-2 threads of attachment. That makes me a bit nervous. I have found if you search through fittings that some tapered ones are different starting diameter than others and will go on/in further. Just a thought.

[Q Xopa]

Originally Posted by a64pilot

Assumption is that you remove the locking nut from the existing thru hull so that the adapter can be screwed all the way down flush with the backing plate.
Further assumption is that the intent is to then drill through both the backing plate and hull to mount counter sunk screws from the outside, thus essentially making it the same as a seacock?
My backing plate is only 1/4” thick, not thick enough for screws to do much in it.

I’m in the Bahamas, Spanish Wells to be exact, and things like taps etc. are non existent, although I would not think that retapping from NPT to straight would not be a good idea anyway.

Assuming I can get one here my intent will be to first dry fit it, screw it down until it bottoms out on the lock nut, Mark it, then remove and assemble the valve and T etc. then install with Teflon tape and pipe dope.

Where it is, it can’t be hit by anything substantial, it’s in the compartment with the waste tank, and Lord forbid even if it were broken off, it’s easily accessible to bang in a soft wooden plug that I have several of, and even then the compartment is not connected to the bilge and the top of it is above waterline, so worst case is that compartment is flooded, but it won’t sink the boat.

Yes the countersunk screws right thru backing plate and the hull is the intended way.

But Mainsails alternate method using bronze studs epoxied into the backing plates sounds good to me and how I am going with.
3 less holes in the hull per throu hull fitting sounds good to me.

[Minggat]

[QUOTE=a64pilot;2870175]I know it’s not optimal. In fact if it weren’t for the fact that I know it’s done, I wouldn’t have believed that it is in fact done, seems pretty stupid
[ QUOTE]

I wonder how many millions of situations your thinking are stupid.

Both threads ARE in fact NPT. A bulkhead fitting (which is essentially what you have)will be missing the taper.

Unless you really can’t get enough turns to get a good engagement, it seems that you’re turn a non-issue into a big problem.

44 years as a glorified plumber/water treatment engineer.

[a64pilot]

I guess I’m arguing again, but you can’t screw a nut down an NPT thread all the way with no resistance.
An NPT thread is tapered for lack of a better word, I’ve cut too many of them on pipe myself.
Embedding bronze studs into a G10 plate is fine, but assumes of course that you have not mounted the plate and can access the backside to counter sink it for the screw heads.
Unless your talking actually tapping the G10 and using actual studs, which I guess would be OK, but I’d want the plate to be at least .500” thick myself.

[Empty Pockets]

Threads really only need to be as deep as the bolt diameter. 1/4” plate, 1/4” bolts...

[s/v Jedi]

I used 1/4” G10, drilled and tapped it for 1/4” thread, then cut machine screws to length and fastened it. Remember that the thru-hull fitting itself also fastens it so it’s plenty strong pretty quickly.

The nice thing is that you can align the fiberglass backing plate to be parallel with the outside surface so that everything sits flush when tightened. I put some toothpicks underneath during the dry fitting, glue them in place with superglue, then take it apart and cover with a thick epoxy filled with microfiber and cabosil, then assemble using the toothpicks as stops, wipe excess epoxy off, then fit the thru-hull fitting with 5200 and let everything cure.

For OP being at anchor, I would just cut the inside part of the fitting to length and teflon tape and screw the flanged adapter on instead of the nut. Prevent the outside fitting from turning. Next teflon tape and thread the valve on and that’s it until next haulout at which time a new thru hull fitting and G10 backing plate can be installed.

[a64pilot]

Originally Posted by Empty Pockets

Threads really only need to be as deep as the bolt diameter. 1/4” plate, 1/4” bolts...

Assuming the plate and the studs are the same material.
G10 is strong, but maybe not as strong as the studs?

[Bill Seal]

What about running a NPT die onto the end threads of the NPS thru hull so you can get a few more threads for a better mechanical connection and be leak free also?
What worries me more is the mickey-mouse little nuts on the inside of the thru hull or the fact the thru hulls are sched 40 instead of sched 80.

[a64pilot]

Originally Posted by s/v Jedi

For OP being at anchor, I would just cut the inside part of the fitting to length and teflon tape and screw the flanged adapter on instead of the nut. Prevent the outside fitting from turning. Next teflon tape and thread the valve on and that’s it until next haulout at which time a new thru hull fitting and G10 backing plate can be installed.

I had thought about doing that, but using 5200 as opposed to Teflon tape. Issue is preventing the thru hull from rotating and breaking the 5200 seal that is there now.

About the only way I can cut the thru hull as it’s already installed is a die grinder and a cut off wheel. That may bugger the threads up enough to be an issue.

I may go with simply installing the adapter and leaving the nut on, that means no cutting and the nut should hold the thru hull from rotating, and it has to be at least as strong as just putting a valve on a thru hull as is common practice.
Then next haul out redoing the whole thing.

I had to cut off the 32 yr old valve by cutting the thru hull, and was surprised at how it wasn’t corroded and how thick the walls were.

[s/v Jedi]

Originally Posted by a64pilot

I had thought about doing that, but using 5200 as opposed to Teflon tape. Issue is preventing the thru hull from rotating and breaking the 5200 seal that is there now.

About the only way I can cut the thru hull as it’s already installed is a die grinder and a cut off wheel. That may bugger the threads up enough to be an issue.

I may go with simply installing the adapter and leaving the nut on, that means no cutting and the nut should hold the thru hull from rotating, and it has to be at least as strong as just putting a valve on a thru hull as is common practice.
Then next haul out redoing the whole thing.

I had to cut off the 32 yr old valve by cutting the thru hull, and was surprised at how it wasn’t corroded and how thick the walls were.

I agree, that would work just fine. The important thing is that you get the correct threads and then make it pretty next haul out