And, if so, how?
It is my understanding that in standard plumping you use pipe dope on NPT threads to make them watertight, but not on NPS threads. For NPS threads, the connection is watertight because there is an o-ring or gasket which gets compressed to create the watertight seal.
However, with a thru-hull and seacock, there is no o-ring or gasket on the NPS side of things. So no o-ring and no sealant sounds like a recipe for leaky threads. What is the preferred solution?
1) A bit of 4200/Sikaflex 291 on the threads.
This is the method shown in one of Maine Sail's tutorials. However, the description makes it sound like it is not entirely necessary,
On the other hand, perhaps because the thru-hull is pulling hard against the seacock does actually create a watertight seal -- as long as I have the ABYC recommended minimum of 5 threads of engagement?
My primary concerns with sealant on the threads are (i) it could make removal more difficult later. Though perhaps with just a light coating of Sikaflex 291 it will be fine. (ii) the sealant could squish out and get on the ball in the seacock making it impossible to operate. But, once again, perhaps with just a light coating, that is not going to happen.
2) A bit of pipe dope on the NPS threads
This has the advantage that it will not make removal more difficult and will not interfere with the seacock operation if it gets somewhere it should not. But, will it actually create a watertight seal on NPS threads?
3) Fill the entire area between the thru-hull and the hull+backing block with sealant but put nothing on the threads.
This is what someone did on one of the thru-hulls I removed. I guess the idea is that if the water leaks down the NPS threads it has nowhere to go? It doesn't seem like the right answer to me.
Some people argue that if your thru-hulls are supposed to be electrically bonded, you should not use teflon tape on the threads because it will electrically isolate them. Some people suggest using an electrically conductive sealant, but others suggest bare threads. But those that advocate bare threads do not address how to make them watertight.
Summary
I am currently favoring option #1. Is using a sealant, like Sikaflex 291, on the threads the only reasonable way to ensure that the NPS threads are watertight? Or should I use some sort of thread sealant which only seals and has very little adhesion?
Thanks!
It is my understanding that in standard plumping you use pipe dope on NPT threads to make them watertight, but not on NPS threads. For NPS threads, the connection is watertight because there is an o-ring or gasket which gets compressed to create the watertight seal.
However, with a thru-hull and seacock, there is no o-ring or gasket on the NPS side of things. So no o-ring and no sealant sounds like a recipe for leaky threads. What is the preferred solution?
1) A bit of 4200/Sikaflex 291 on the threads.
This is the method shown in one of Maine Sail's tutorials. However, the description makes it sound like it is not entirely necessary,
It seems to me that, while it is less important than the sealant on the mushroom head bevel, the sealant on the threads is still very important. Without it, the water will come up the center of the thru-hull, but then seep down the threads and end up between the hull and the thru-hull. This is exactly what happened on my boat where the PO put no sealant on the threads and only had 2 threads of engagement!
On the other hand, perhaps because the thru-hull is pulling hard against the seacock does actually create a watertight seal -- as long as I have the ABYC recommended minimum of 5 threads of engagement?
My primary concerns with sealant on the threads are (i) it could make removal more difficult later. Though perhaps with just a light coating of Sikaflex 291 it will be fine. (ii) the sealant could squish out and get on the ball in the seacock making it impossible to operate. But, once again, perhaps with just a light coating, that is not going to happen.
2) A bit of pipe dope on the NPS threads
This has the advantage that it will not make removal more difficult and will not interfere with the seacock operation if it gets somewhere it should not. But, will it actually create a watertight seal on NPS threads?
3) Fill the entire area between the thru-hull and the hull+backing block with sealant but put nothing on the threads.
This is what someone did on one of the thru-hulls I removed. I guess the idea is that if the water leaks down the NPS threads it has nowhere to go? It doesn't seem like the right answer to me.
Some people argue that if your thru-hulls are supposed to be electrically bonded, you should not use teflon tape on the threads because it will electrically isolate them. Some people suggest using an electrically conductive sealant, but others suggest bare threads. But those that advocate bare threads do not address how to make them watertight.
Summary
I am currently favoring option #1. Is using a sealant, like Sikaflex 291, on the threads the only reasonable way to ensure that the NPS threads are watertight? Or should I use some sort of thread sealant which only seals and has very little adhesion?
Thanks!
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