I would only use plastic above the water line.

I installed Tru Design and they are great.

DO NOT use seacocks bought at Bunnings. The Tru Design are a specific hardened plastic designed for below water applications in boats. Bunnings are designed for irrigation systems.

Hence the Tru Design are more expensive

I recall reading positive things in PBO about "Marelon" seacocks. Might be worth doing a search for them.

Cheers, Graeme

Wander66 said..

Seebreasy73 said..
I was browsing Bunnings the other day and seen some plastic seacocks. I have one or two seacocks soon needing replacement and I was wondering if anyone has any experience with these instead of bronze or stainless? I could not think of a reason, why wouldn't these work on a boat.

Don't use the irrigation valves as seacocks. Even my slightly saline bore water kills them after a couple of years, it would be much worse in salt water.

Is this the point where I say I have used them for about 18 years? Toilet inlet and outlet, no degradation, all fine. Nice and epoxiable. Toilet outlet just above the waterline but the inlet is about 200mm down. A bit hard to turn but no change in all the time I have owned my boat.

just found this:

Applications
PVC valves are used intensively in water management and irrigation. PVC is also suitable for corrosive media, such as sea water. Moreover, the material is resistant to most acids and bases, salt solutions and organic solvents. In applications where corrosive chemicals and acids are used, PVC is therefore often chosen above stainless steel. PVC has also some disadvantages. The most important drawback is that regular PVC cannot be used for media temperatures above 60?C (140?F). PVC is not resistant to aromatic and chlorinated hydrocarbons. PVC has a lower mechanical strength than brass or stainless steel, and therefore PVC valves have often a lower pressure rating (PN16 is normal for valves up to DN50). A list of typical markets where PVC valves are used:
Domestic / Professional Irrigation
Water treatment
Water features and fountains
Aquariums
Landfills
Swimming pools
Chemical processing
Food processing

So I guess it should be ok to use on the cold water side even a Bunnings one. I would still leave the original thru hull bronze fitting in place, just use the plastic valves on top.

Yes the Bunnings ones. I actually like the idea of PVC because I epoxied the holding tank tap into the bulkhead. PVC loves epoxy whereas the nylon valves don't. So it works well for me.

Had some pvc cocks on ag spray rig they handled the chemical but the handles mostly broke. On my boat I would be happy with pvc ( from a plumbing supplier) above the waterline, fresh water tank etc but below the waterline I'm inclined to use the trudesign rated seacocks quality hose and clamps, the price difference is cheap insurance.
ps. If you change out your bronze skin fitting as well there's one less cause of electrolyses