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Compass Marine | all galleries >> Compass Marine "How To" Articles >> Replacing Thru-Hulls and Seacocks > A Poor Idea
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A Poor Idea
22-FEB-2008

A Poor Idea

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The picture on the left is what not to do if starting from scratch and installing new seacocks. If your thru-hulls and valves, on your boat, are installed & look similar to this configuration you're not alone. This photo was taken on a brand new 2011 production sailboat.


Unfortunately, for many owners of production boats, the factories cut many corners to save money, proper flanged seacocks being one of them. This is understandable as the cost savings is huge to the manufacturer, and they can be relatively safe, though no where near as strong as a flanged seacock.


A proper seacock installation involves more than just a thru-hull or mushroom-head, as some refer to them, and a valve. The problem becomes more complicated when you learn why this is not necessarily the safest way to install a seacock.


The vast majority of all available thru-hulls or, mushroom-heads, have what's referred to as a straight thread or NPS (National Pipe Straight Thread), and the vast majority of all ball valves are a tapered thread or NPT (National Pipe Tapered Thread). NPS thread and NPT thread are NOT intended to be used together! Many unknowing boaters though know nothing about NPT or NPS threads and as such many times use the two together quite incorrectly.


In short, a proper seacock must have a female NPS flange that is through bolted to the hull to which the NPS straight thread thru-hull or mushroom head is then threaded into.


Another major problem with this installation is the mismatching of metals below the waterline. That hose barb is yellow brass and not bronze! This particular builder "claims" to build to ABYC specs but the ABYC P-06 standard / UL 1121 standards specifically states that all metals must be galvanically compatible. Yellow brass and marine bronze bronze are not galvanically compatible and never have been.


Please remember this is a 2011 vessel. The picture below of the broken thru-hull comes from this same builder..

Nikon D200
1/60s f/7.1 at 70.0mm iso800 full exif

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RamblinRod 02-May-2012 20:16
I am not sure what the concern is of threading an NPT male into a straight female thread. This is common practice and perfectly acceptable. You just need some form of thread sealant whereas with mated NPT threads one can theoretically not use sealant (though most everyone will regardless, for a belt and suspenders approach.

RE: Disimilar metals. Yellow brass and marine grade bronze are both copper alloys and are so close on the galvanic chart there is no issue around galvanic corrosion. The problem is that standard brass will suffer dezincification when seawater flows past or through it. Standard brass should never be used below the waterline or on critical fittings in a boat.
Weasle Baxter 30-Jun-2008 13:30
Hi,
I agree that the threads aren't right. Can't believe it had never occured to me. But I can honestly tell you that after 34 years of running boatyards and businesses that install through hulls (thousands) I've never seen or heard of a failure or leak caused from this.
I teach now, and I 'spose I have to discuss this with my students.