Thru-hull fittings might seem like a fairly insignificant part of your boat, but let’s not forget if they’re at fault, then your boat could sink. If this ever happens it’s guaranteed to ruin your day!
So choosing high quality fittings is an absolute must if you plan to do any remedial work on your boat; for instance: swapping out your old engine for one with a different size cooling water inlet, or installing an air conditioning system.
You’ll notice when you start piecing together a new system that many different material options exist for thru-hull fittings. The main options available are:
- DZR (Dezincification Resistant Brass).
- Stainless Steel
- Acetal (Delrin)
- Glass Fibre Reinforced Acetal
Choosing Metal Fittings? Beware of Galvanic Corrosion
Metallic fitting are the natural choice for many people because they’re perceived as strong and resilient against the corrosive effects of sea water. We like to think that manufacturers wouldn’t be silly enough to produce fittings made from easily corroded metals, so that means all fittings are more or less created equal, right?
Well no not exactly, if we delve a little deeper it doesn’t take long to see that not all metals are created equally in the world of through hull fittings.
If we keep one eye firmly on the below galvanic series chart, let’s look in more detail at each metal and discuss the pros and cons of each.
Brass – a Big No No!
Looking at the chart above we can see that brass sits somewhere in the middle of the chart and seems like a pretty good choice for resisting corrosion. However when you consider that brass is an alloy of copper and zinc you can see a weak link in its chemical composition.
Zinc is highly susceptible to corrosion, and is used to make the sacrificial anodes on boat hulls. As we don’t want our through hull fittings to corrode in this an anode would it’s not such a great idea.
For this reason I wouldn’t recommend ‘standard’ brass at all, certainly below the waterline. For a few bucks extra bucks it’s worth opting for DZR brass instead, which doesn’t suffer from the same corrosion risk.
The clue is very much in the title with this one. DZR brass is brass that has been through a process that resists the leaching of zinc, making it a far better choice for fittings that will stand the test of time.
DZR brass is generally considered one of the two most suitable choices for through hull fittings, with the other being bronze.
It’s a pretty close call determining which is the superior choice between DZR and bronze. Bronze sits ever so slightly lower in the table meaning it is technically more noble (ie less likely to corrode), however it is also usually around twice as expensive.
Obviously in an ideal world you would always plump for bronze because it is the clear winner in terms of corrosion resistance, however cost is usually a big determining factor with any upgrade project, and when need to be careful what you’re spending DZR is a perfectly functional alternative.
316 Grade Stainless Steel
In the past I’ve designed boats where, for one reason or another, stainless steel was stipulated as a requirement, but again it is an expensive option, with stainless fittings typically costing even more than bronze ones.
What’s more whilst stainless steel is indeed resistant to corrosion (well 316 is, 304 is not suitable below the waterline), it certainly isn’t perfect. Given that the corrosion resistance of stainless steel is dependant on a thin oxide layer on its surface; in stagnant, deoxygenated water in particular, 316 stainless is susceptible to crevice corrosion, or corrosion around the edge of the fitting. Therefore in time it could corrode and present a leak risk.
Problems With Dissimilar Metals
The other consideration to keep in mind when selecting metal fittings is whether you’ll be faced with any issues of galvanic corrosion as a result of dissimilar metals being in contact with one another.
In other words the further apart two metals are on the galvanic corrosion chart the more susceptible they are to react with one another and therefore corrode.
One scenario where this could be as issue is when mixing bronze and stainless steel fittings, particularly when they are screwed together. These metals are far enough apart on the galvanic chart that they will start to interfere with one another after a while.
It is good practice either to either keep dissimilar metals out of contact with one another, or ensure there is a barrier between them (ie. one of the surfaces should be painted).
Choosing Plastic Fittings
Whilst metal fittings rule supreme in the strength and durability department, plastic fittings are immune from the effects of galvanic corrosion, making them a pretty compelling alternative for many people.
In fact where plastic fittings can really thrive is below the waterline, where they won’t be affected by ultraviolet radiation from the sun. In this environment the only major concern is mechanical trauma (ie. being struck by rocks, pontoons or other boats). To counter this, there are now glass fibre reinforced plastic fittings, which come pretty close to the strength and resilience of metal fittings.
Acetal is used to make fittings because of its excellent strength, stiffness, low wear, and low coefficient of friction. This makes it easy to machine, and more importantly easy to screw together in the case of threaded parts.
Also crucial is that acetal has low moisture absorbing properties so there are no concerns about leaking by capillary action or otherwise.
As one of the earliest developed and most widely used thermoplastics it’s no surprise that thru-hull fittings are available in nylon.
Once again nylon is strong, durable and waterproof, and is only marginally less hard wearing the acetal.
If it’s good enough for the flag put on the moon by neil armstrong, then it’s good enough for the skin fittings on a boat!
Best Materials For Specific Fitting Types
Whilst all the materials described above have specific merits, it’s fair to say some materials are better suited to certain fittings than others.
Skin fittings don’t tend to stand too proud of the outer hull surface of a boat, so they’re less likely to get clobbered than something that protrudes too much. To that end it doesn’t matter too much which material you opt for below the waterline with the exception of standard brass.
Above the waterline I would avoid plastic fittings because of the risk of degradation from UV light over time.
Sea Cocks and Ball Valves
Sea cocks and ball valves are perhaps the most critical links in systems that exit below the waterline; if they seize up for any reason you’ve got a major headache turning a system off for maintenance, so they need to be as reliable as possible.
I would definitely stick with DZR or bronze for sea cocks/ball valves and associated fittings unless you’ve got a reason not to, and even then I would view stainless steel as the only viable alternative.
The same rules apply for water scoops/strainer inlets as sea cocks and ball valves. For flat profiled grille type inlets I guess you could get away with a plastic fitting if you really wanted to, but I’m not aware of many (if any) manufacturers who offer such products.
Inboard Fittings (Hosetails, Strainers, Pipe Fittings)
On the inboard side of a thru-hull fitting you’ll obviously want to match the material on the outside to prevent galvanic corrosion, although you might be able to use plastic instead. I would only do this on auxiliary systems, and critically, where you can be sure the fittings can’t possibly get accidentally damaged when you’re carrying out maintenance.