Sealing NP(S) threaded pipe connections

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Nov 23, 2009
17
Pearson 33 Mt Vernon, MD
I've read what thread's I could readily find on sealing threaded NP(T) pipe connections, Tape/dope etc etc, does the same apply to NP(S) connections? IF not what are some suggestions, It's for a raw water intake, 1/2" NPS bronze Perko.
 
Mar 19, 2004
15
Hunter 41 DS Port Clinton, Ohio
NPT is national pipe- taper thread and can be sealed with dope
(best in my opinion) or teflon tape. NPS - national pipe straight is the same thread pitch (threads per inch) but is straight not tapered and usually relies on a soft gasket material to make the seal.
 
Nov 23, 2009
17
Pearson 33 Mt Vernon, MD
Thanks Tim and Winkfish. Yes I had already read the whole pipe dope and tape conversation, great information which is exactly what prompted my question regarding sealing techniques for NPS (straight) threaded connections. I've searched the internet over looking for a flanged seacock or seacock adapter to thread onto the 1/2" NPS threads associated with the already mentioned Perko intake strainer but am unable to find one under 3/4". I really want to replace the NPT threaded ball valve which is currently in place. The only thing I can find is a Marelon 1/2" NPS ball valve. So, back to my original question....

Thanks
 

RichH

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Feb 14, 2005
4,773
Tayana 37 cutter; I20/M20 SCOWS Worton Creek, MD
Go to the groco.net - then select the type/size, etc. Get the part number ... Internet search for the "part number" + groco.

If no luck, call Defender Industries (defender.com) ask for customer service and have them special order what you want (probably the easiest and cheapest).
 
Feb 6, 1998
11,665
Canadian Sailcraft 36T Casco Bay, ME
Thanks Tim and Winkfish. Yes I had already read the whole pipe dope and tape conversation, great information which is exactly what prompted my question regarding sealing techniques for NPS (straight) threaded connections. I've searched the internet over looking for a flanged seacock or seacock adapter to thread onto the 1/2" NPS threads associated with the already mentioned Perko intake strainer but am unable to find one under 3/4". I really want to replace the NPT threaded ball valve which is currently in place. The only thing I can find is a Marelon 1/2" NPS ball valve. So, back to my original question....

Thanks
First of all read this and then consider up sizing your intake to 3/4". The industry has wisely mostly moved away from 1/2 for proper seacocks. You can still find them but they plug quicker and can't often supply enough flow for engine cooling. Groco, Apollo & Buck Algonquin, among others, no longer make proper seacocks in 1/2". You can still find Marelon flanged seacocks in 1/2" but, again, they tend to plug up easier.

As for sealing an external intake strainer:

1) I would never use one of the type that can not be removed from the thru-hull, they are bad news waiting to happen. After you have read the above link and still think you want one PLEASE use the type that opens or can be removed independently from the thru-hull.

2) Thru-hulls going into a proper seacock are sealed with marine sealant like Sikaflex 291 or 3M 4200. The seal comes from the mushroom head to hull interface not from the threads into the seacock.

3) Ball valves / non-proper seacocks, going onto a thru-hull are sealed with dope/tape but the mushroom head to hull is sealed with a marine sealant. One should ALWAYS use at least a "combination threaded" thru-hull if doing this.

4) As I mentioned please consider up-sizing the 1/2" thru-hull to 3/4" as I have seen just ONE barnacle clog a 1/2" intake enough to kill an impeller and over heat an engine..
 
Nov 23, 2009
17
Pearson 33 Mt Vernon, MD
Thanks all for the valuable input. Until now all my sailing experience has been with day sailers and a Bristol 27 weekend cruiser having auxiliary outboards, hand pumped onboard water systems and portable camp style toilets. I'm very new to inboard engines, pressurized water systems and thru hull fittings. I have much to learn about the Do's and Dont's for these systems as I try to get them operable.
The new boat, a "73" Pearson 33 is a recent acquisition and launched after being on the hard for almost 6 years and not very well looked after during that time. The 1/2" strainer and associated ball valve were "professionally?" installed just prior to launch. I do know it was sealed to the hull with 5200.
I very much appreciatte the information provided and will be replacing that 1/2" intake with a proper 3/4" strainer and seacock during the next pullout. Until then, I'll be replacing the brass NPT ball valve with a 1/2" NPS Marelon ball valve. The boat currently has a non functioning Atomic 4 engine with fresh water cooling. I have so much to do this spring and am looking forward to getting some great advice from folks like you during the process.
Thanks again to everyone!
 
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