Nylon thru-hull fittings like this are culprits in more than a few sinkings, when used as cockpit drains that ended up underwater and were cracked due to age and UV damage. Nylon is most definitely not for below-waterline use.
The difference between bronze and some types of brass in the world of plumbing fittings concerns their relative potentials for galvanic corrosion. Dissimilar metals in contact can set up an electric current in an electrically conductive fluid (an electrolyte, such as saltwater). One of the metals will be “eaten away” or “sacrificed” in the process. Sacrificial anodes are made of zinc because it is one of the least “noble” (i.e., highly active) metals and is thus sacrificed to protect your engine, rudder, propeller shaft, refrigeration condenser, etc. from damage due to galvanic corrosion. However, with thru-hulls, seacocks, etc. dissolving fittings are not an option. You want all the fittings to remain intact.
To avoid this potentially hazardous situation, make sure you are not setting up the conditions for galvanic corrosion by connecting dissimilar brass fittings to bronze fittings. Generally, you can tell them apart just by looking at them. Bronze fittings have an outer surface that is rough in feel and appearance, almost like it’s covered with sand. That’s because bronze fittings are sand cast, and then machined.
So what should you do?
You should continue with what has always been sound marine practice: use bronze fittings for the thru-hull mushrooms, seacocks, strainer bodies and valves. If one of the pipes must be turned through an angle, use a bronze Street Ell, either 90° or 45°. Once the bronze fittings are in place and aligned, thread a bronze hose barb into each, and join them with sturdy hoses and stainless steel clamps.
Don’t confuse bronze hose-to-pipe fittings like those made by Groco and Perko with small brass hose-to-pipe adapters. Bronze fittings used for seacocks are fine for use in saltwater. But what if the application absolutely demands other fittings? Try fittings of some other, nonmetal material such as nylon or marelon, both of which we carry. These galvanically inert materials present no problems when used in conjunction with any marine metals.
This shiny brass pipe-to-hose fitting is fine for a fuel line, but brass should never be used in raw-water plumbing that is at risk of galvanic corrosion.
Marelon is a high strength, polymer composite, developed as a solution to electrolysis and corrosion problems associated with bronze/brass plumbing fittings. Marelon will not corrode or deteriorate; no electrolysis. UL/ABYC approved; ISO certified. Use below and above waterline.
Acetal plastic (nylon)
Inexpensive plumbing alternative. Only for above waterline use.
For your education, we’re reprinting this excellent bit of advice from the pros at Professional Boatbuilder magazine:
Silicon bronze and leaded red brass make good seacocks. Leaded red brass, sometimes referred to as “85-5-5-5” due to its constituent alloys of 85% copper, 5% zinc, 5% lead and 5% silicon, should not be confused with ordinary brass, whose zinc content is much higher. High-zinc-content alloys like ordinary brass or Tobin bronze must never be used for seacocks or any raw below-the-waterline (BTW) plumbing. This prohibition includes even the smallest seacock or raw-water strainer drain plugs, which are often inadvertently replaced with brass rather than bronze hardware.
Approved composite materials include glass-reinforced, UV-inhibited nylon such as acetyl and polybutylene terephthalate. Marelon, manufactured by Forespar (Rancho Santa Margarita, California) is an example of an excellent non-metallic, UL-approved material for BTW seacocks. All approved seacocks and through-hull fittings, metallic or composite, should carry the UL approval number 1121 or 618C, for “Marine Through-Hull Fittings and Sea-Valves.”
Steve D’Antonio, Professional Boatbuilder, August/ September, 2008