Thru-Hull 2

Preferred & Non-Preferred Installations
The example on the left side of the photo is the preferred method for installing a proper seacock. The preferred method includes a wide base flange for strength and the ability to through-bolt or screw the flange to the boat.
The example on the right side of the photo is the non-preferred method of installation and has a number of down falls.
The most obvious weakness is the fact that there is nothing to keep the thru-hull from possibly twisting when turning the handle to open of close the valve. The only thing holding this thru-hull to the boat is some marine sealant and that very small lock nut. The second issue, is not so obvious, but, still just as scary. A thru-hull fitting is NPS or straight thread and the bronze ball valve is NPT or tapered thread. This installation can cause an improper mismatching of thread types which you will read about bellow.
The third major issue is strength. Simply threading a ball valve onto a thru-hull fitting, without a flange, creates a weak point in which the thru-hull could possibly snap off or break if anything substantial were to hit it in rough seas. A mishap like this happened to me when a spare alternator fell off a shelf in rough weather and cracked my thru-hull fitting. This is part of the reason I wrote the blog about installing proper seacocks and this primer to show why to do it the right way.

Thru-Hull Basics

This item is called a thru-hull or sometimes a "mushroom head". They are straight threaded (NPS thread) so they can be cut to the correct length for your hulls thickness and then be threaded into a female straight threaded seacock or a flanged adapter. The one on the right has already been cut to length. Most all thru-huls installed into proper seacocks will need to be cut to the proper length.
When installing them with a proper seacock the nuts are not used.