Knowledge about plumbing standards helps understanding which pieces to order. It further helps separate the signal from the noise in the wide variety of conflicting information received on plumbing. What is described below is mostly a summary of the American Boat and Yacht Council standard (ABYC, 2021). It can be ignored, as some boat manufacturers do, but these standards exists to mitigate risks.
Somewhere between now and the Big Bang, American plumbers figured out it would be useful to have standards regarding threads. They (figuratively) sat down to and came with the American National Standard [for] Pipe Thread. If you ordered two pieces of plumbing that do not fit together, you can be comforted by the fact that it is your fault rather than a lack of standards.
Through-hulls follow the National Pipe Straight (or NPS for short) type (Marine How-To, n.d.). The NPS threads have a constant diameter. The main advantage is that it can be cut at any point and the diameter remains the same. This is ideal for pieces serving different needs in terms of length, as it is the case for through-hulls. After a male NPS thread is cut, it still fits perfectly in its female counterpart, allowing for all the grip that the threads can deliver.
In contrast, the National Pipe Taper (or NPT for short) has an increasing diameter, as a cone. The advantage is to favour the mechanical sealing of the pipes. However, if the male part of an NPT thread is cut, it will no longer fit in its female counterpart. Most pieces of plumbing have NPT threads.
A simple idea, very well understood by insurance companies, is that pieces of different threads standards should not be used in the same connection. In particular, a male NPS through-hull should not be fitted with a female NPT shut-off valve. Because the valve has a conic thread, it will not allow enough turns for the through hull to seal shut (first picture below, borrowd from Marine How-to). This leads to a possible leak or worse, a breakage at the joint.
A through hull should go in an NPS female piece… although the rest of the plumbing is designed with NPT threads. This means that the female piece receiving the through-hull must convert to NPT on its male end. Groco does a sell an NPS to NPT converter exactly for that reason. It is also possible to purchase tri-flanged valves with the proper adapter as well.
Tri-flanged valves or adapters are also a good idea. As argued by Marine How-to, they prevent the valve for turning around the hull, they are fastened to the hull or to a backing plate and are thus stronger (Marine How-to, n.d.).
Practical Installation
The installation of through-hulls, tri-flanged valves and backing plate is very well explained by AtomVoyager (2019; video below). Watching the video gives a good understanding of pretty much everything needed to perform the install. After fiberglassing the backing plates, at least a full day is required for the epoxy to cure enough to be able to continue the install.
Want to Know More?
Have a look at the « boatwork » section of this website for more practical tutorials and techniques.
References
American Boat and Yacht Council, Standards List, retrieved online in October 2023 at this url.
BuyFittingsOnline.com (n.d.). Understanding NPT, NPTF & NPS Thread Connections, retrieved online in October 2023 at this url.
Marine How-To (n.d.). Seacock and Thru-hull: A Primer, retrieved online in October 2023 at this url.