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.

The title says it all! Simply threading a ball valve, or in-line valve as pictured above, onto a thru-hull (top photo on this page) is not a proper seacock. Sure, many sailors refer to any thru-hull mounted valve as a seacock but in the proper sense threading a ball valve onto a thru-hull or mushroom head is not a "proper" seacock.

This ball valve shows what can happen, in less than a year, using cheap "yellow brass" valves purchased from Home Depot. This valve was literally a week or two away from catastrophic failure of the boat sinking type!

As I've said, and will continue to say, USE ONLY marine rated valves of either bronze, stainless steel or Marelon and DO NOT buy valves for your boat at hardware stores or home centers like Home Depot!

This is a relatively new item in seacock technology. It allows the use of an NPT threaded ball valve on the top and a NPS threaded thru-hull then threads into the bottom.

As far as I know these are currently only made by Groco. The benefit is that even if the valve wears out you simply thread a new one onto the flange. This avoids a very time consuming job of removing the thru-hull, thru-bolts and seacock and then having to re-install these parts and fair over the bolt holes.

This picture shows the complete assembly minus the boats hull. Note that the handle clearly states Marine Rated. Use only Marine UL rated valves and seacocks for below water line applications. Many insurance companies demand this for coverage and insist on marine rated valves so don't cut corners here.

I've had a lot of emails and questions regarding the "mismatching" of different thread types such as NPS (NPSM) and NPT.

Someone challenged the idea that mismatching threads was OK because "so many do it" so last night I made a cut away view to show why using two different thread types is really not such a good idea even if "so many do it". While many boat owners screw NPT threaded valves directly onto NPS threaded through-hull fittings it's clearly not a logical idea. NPSM or NPS, as it's referred to, is the actual sub class of straight threads used in the marine industry. The M means "mechanical" seal they are not intended to seal or take presure like NPT. Manufacturers such as Groco warn against doing. Mismatching threads is done by lots of people but as you can see you don't get a lot of purchase compared to using matching threads. All threading machines have slightly different tolerances and some straight thread might screw in more and some may screw in less. The parts in the photos were purchased right off the shelf at my local chandlery.

To make this photo possible I basically used a bronze nipple, or threaded pipe, with standard plumbing threads of NPT (National Pipe Tapered) and a bronze coupling also the industry standard of NPT thread.

I cut the bronze coupling almost in half, for a cut away view, so I could thread the NPT bronze nipple into one side and an NPS through-hull fiting into the other. I then sprayed each with a McLube, to reduce friction but save the picture quality, and threaded both the NPT nipple and the NPS through hull into the cut-a-way bronze coupling by hand and until I had an equal resistance.

The results even surprised me! As you can clearly see the NPT nipple threaded into the NPT coupling a quite a bit further than did the NPS through-hull. One would expect this because NPT and NPT threads match. If you were to take a wrench to both you might get one more turn at best out of the NPS through hull but you may still get two or three full turns out of the NPT nipple.

If you look very closely at the picture you can also see the outer-most threads of the through hull are already NOT fitting tightly against the female threads of the coupling and the inner-most threads are quite tight or virtually bottomed out...!!! The square peg evidently does NOT fit a round hole..!

This coupling represents the threads of an in-line valve. Most all available ball valves or gate valves have NPT or tapered threads and most all commercially available through hulls have NPS or straight threads which is a clear & potentially dangerous mismatch.

Sticking a ball valve directly onto a through hull gives you about three or four threads between sinking and floating so I can't, with a good conscience, recommend you do it. For safety's sake I suggest using proper seacocks with flanges..!!

I would have cut away an actual ball valve but I don't have a machine shop. The coupling represents and has the same exact NPT female threads as an in-line valve of either the gate or ball type...

To accurately measure the nominal wall thickness of this 1" thru-hull I first needed to measure the thread depth that had been cut into it. I used the depth gauge of my calipers to figure out the depth of the thread cut. In this photo you'll notice a little tab of metal sticking out the right side of the caliper. That tab of metal was the actual thread depth from the pointed outer surface of the threads to the bottom of the thread valleys.