Sea Cocks, something to think about.

SEA COCKS, are a "Door".

A "Door to a HOLE".... in YOUR boat.

Who, wants a hole in their boat?

Right, no one!

.........but, 'some' are needed.

Most CSY's are now in 2020, approximately 40 years old. Metals below waterline have been and are, subjected to 'Crevice corrosion", electrolysis, galvanic action. Our 'bronze doors' might need a good check, or replacement.

Bronze Thru-Hulls are great. It's a great metal for underwater fittings but can deteriorate it to the point of failure especially if ignored.

so..... Keep the door in good shape & closeable!
Sea Cocks should have a rotating ball inside, or, a tapered plug. Gate Valves, which is a sliding door arrangement are to be replaced and considered trash. Of the true sea cocks, Groco and other makes have time tested bronze units, some with a base that can be screwed or bolted to the hull or pad backing up the sea cock; some do not have this extra flange. Going to Home Depot and buying a 'BRASS' Ball Valve for $5. is not cool. Brass will not survive and not an acceptable replacement for bronze..there is a big difference. Remember, if your 'cheapie' fails, your boat might sink-even with you aboard-and your insurance company will not pay.

Forespar sells a reinforced nylon valve that is somewhat different from traditional bronze sea cocks and I do recommend them and as of 2006, when this post was originally written, had them installed for 12 years, but do your own homework.

Backing plates made from wood can and will deteriorate and fail their purpose if allowed to get wet. If you find your backing plates made of wood is soft and worthless, do not blame the wood. Blame the last owners or yourself. Wood does not disintegrate until something gets it wet "Over and Over"! More than likely, the simple 'Thru Hull" was bedded properly. More than likely too is that the Hose, connected to the top of the sea cock was not secured properly and the slight weeping of moisture has finally 'rotted' your wood backing plate.
OK. How could this be prevented for the next round? Well, coat your "Marine Plywood" backing plate with several coats of epoxy resin AFTER you have shaved it to perfection and it fits the hull like a glove. Then paint it nicely.. Backing plates for sea cocks are to strengthen the mount for THIN or weak hulls. Thin hulls whether steel, aluminum or fiberglass Need a backing plate. They can be weaked under stress from side loading or fatigue to the point of failure with hull flexing. Seacocks need to be capable of sideways pressure of 500 lbs according to ABYC.
So to get them more inflexible, make the hull 'Thicker' with a backing block. On CSY's, this might not be a necessity, but another consideration is that the inner and outer layers of the hull might not be perfectly parallel. If not, the 'head' of the Through Hull and the 'base' of the Sea Cock might not be parallel. This could apply side loading to the threads on the 'tight' side, a no no.

In 1994, while the boat was on the hard with projects underway, I layed up a section of fiberglass 1'x2' properly wetting out the cloth and squeegying the excess resin from it, then when cured, made parts from it. The finished fiberglass material was 5/16-3/8 thickness. For Thru Hulls, I used a hole saw for the proper hole diameter, then used a sabre saw to surround that hole for a total diameter of 4" or so. Ending up with a round backing plate made of an indestructible material and bedded with the Thru Hull and Valve until firmly snug then, let things set up. Twelve years and not a drop of moisture and, with the non metalic Forespar valves, no deterioration.