First you need to install a backer plate and then you need to figure out how you will bolt the flange of the seacock down.
Yes, I know that many, perhaps most, boats do not have their seacocks bolted, but that is, in our opinion, a very poor practice since any force coming in contact with the top of the seacock or the attached hose, such as a clumsy foot—a problem I’m famous for—can exert an incredible sheer force on the through hull threads. And if those threads strip, things are going to get very damp very quickly.
Add an aluminum hull and a seacock that is made of bronze, like most of those in Morgan’s Cloud, and you need to figure out a way to keep the two electrically isolated. (See the slideshow at the bottom of the post for how the bronze seacocks on Morgan’s Cloud are installed.)
Oh yes, and you need to make sure the seacock that you are about to install really is bronze and not some form of brass. This is particularly a problem in Europe where valves are sold for marine use as “tonval bronze”, which is not bronze at all, but brass.
Brass is really bad news in salt water since the zinc leaches out of it leaving a spongy and weak potential boat-sinker behind. One more thing: if you tee something off the seacock you will use a nipple and all nipples, at least as far as I know, are made of brass, not bronze, so you really should have a custom nipple turned from bronze by a good machine shop. While you are at it, have your custom plumbing fittings made to schedule 80, which is beefier than schedule 40.
With all of the above metal seacock hassles, when we needed to install a new seacock for the drain required for our new separator exhaust system, we decided to give a Forespar seacock made from Marelon, a tough plastic, a try. An experience that can only be described as “out of the frying pan and into the fire”.
The installation manual said:
If it is desired to additionally fasten the king-nut to the backing block with screws, there is provision for such, but it is completely unnecessary.
Not only do we believe the above instruction to be just plain wrong, particularly with a plastic through hull, we don’t believe fastening with screws is adequate either. We through-bolt our flanges (king-nut) to the backing plate.
Wait, it gets worse: Despite the above assurance, there is no practical way to actually accomplish the above since one of the four holes provided is obstructed by the handle and the flange is so small in the way of two others that there is no way to get a screw head, never mind a washer, to bed in the space allowed. Despite this the manual states:
On the backside of the king-nut there are four (4) blind ¼” holes. These may be drilled through to the front side before installation to allow for round-headed screw fasteners.
I suspect that the non-functional holes and the associated text in the manual are only there to serve as a fig leaf—you were wondering how I was going to work that in, huh?—so that the company can say “Well, you could have bolted it down”.
This otherwise excellent product has been let down to save a few pennies by not providing a decent flange for proper fastening–a real pity. See the photos below for how we solved this problem.