A boat-wants-to-sink episode happened recently to a well-maintained, two-year-old Catalina 400. While at the dock, the owner's wife opened the seacock to the thru-hull fitting for the galley sink. The valve plug blew out and water rushed in. The threads for the retaining screw had stripped out of the plastic valve. To stop the inflow of water, a diver plugged the thru-hull from the outside. The integral flange valve thru-hull fitting could not be plugged from inside the boat. The boat had to be hauled to replace the valve and the thru-hull fitting. This poses the question of whether plastic thru-hull valves and valve flanges, which can't be plugged from inside, are safe?
John Kretschmer responds:
You ask if plastic valves are safe on thru-hull fittings below the waterline? Well, yes and no. Marelon is usually the material used for valves and it is tough, fiberglass-reinforced stuff. It is an ideal choice for metal hulls where corrosion is really an issue. The problem is that it is possible to overload the handle when opening or closing these valves, and possibly break it, and of course, this is most likely to happen in an emergency. I have two below-the-water Marelon valves on my steel boat and I open and close them with care. I can't see any advantages of a plastic valve for a fiberglass boat other than a bit of weight savings and this is really not a factor for a Catalina 400. Ironically, there is really not much cost saving to the manufacturer; Marelon valves are not cheap. When it comes to below-the-waterline valves, a stout bronze seacock is still the way to go.
You also ask if integral valve flanges are safe on thru-hull fittings when a tapered wooden plug cannot be driven in from inside the hull? The answer to this one is easy, NO! Why would any manufacturer do this is the real question, especially on a fitting below the waterline? In my way of thinking this is simply stupid and dangerous.