About Fig Leaves And Installing Seacocks

Installing a seacock properly is not a trivial project.

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 slideshow below for how we solved this problem.

Slideshow requires a reasonably up to date copy of the Adobe Flash plug-in or iPhone/iPad or Android and that java script be enabled.

{ 14 comments… add one }

  • David Hayward May 24, 2010, 6:16 am

    Here’s a good description and pictures of an install.

  • David Hayward May 24, 2010, 6:17 am

    Here’s a link to his other projects

  • Andrew Fennymore-White May 24, 2010, 4:34 pm

    I appreciate your problems, I have used Marlon seacocks on our aluminium hull and welded a heavy wall (1/4″) 1 1/2″ threaded tube stub straight into the hull, keeping it very short so as to reduce the bending moment you rightly worry about. Then you fit the seacock with a backing nut so that you can position the handle in the correct orientation. You can weld a heavier disc of aluminium over in an over size hole if you wish to spread the load even more. This way you have none of the problems you are experiencing with isolating dissimilar metals, numerous holes for screws etc. and no possibility of leaks around the spreader/ backing plate. A Forespar seacock is able to be serviced as well. The cry that plastic seacocks melt in a fire are fine but, to be fair, if the fire is so intense that it melts a water filled seacock then you have a larger problem than the seacock failing. My personal solution was to have no seacock in the direct engine space.

    • Peter Bateman June 30, 2011, 6:24 am

      On Aluminium and steel boats we have added gusssets between the pipe and hull along half the length of the pipe that brace the short length of Pipe to reduce the bending issue. We also use Schedule 80 PVC ball valves that are above the waterline on Aluminum hulls. if it melts it may put out the fire. Hooray! then we stop and worry about sinking!

  • John May 25, 2010, 6:34 am

    Hi Andrew,

    I like the idea of the welded tube stub.

    However, I guess that, even with your very fine system, I would like to see the seacock bolted down, as ours are. The bolts are not a problem in that they are only into the backer plate, not through the hull.

    By the way, we really have not had any “problems isolating dissimilar metals”. In fact, after our experience of dealing with the challenges of plastic, if I were doing it again, I would go back to bronze.

    I think the ultimate system might be stand pipes bringing the seacock above the waterline, like on the Boreal.

    Like you, I don’t worry about fire and plastic seacocks much. Even with metal seacocks, like most of those on MC, the hoses are going to burn through anyway. And it is likely that the seacock will be both open and inaccessible in a fire.

  • Peter Bateman May 25, 2010, 12:28 pm

    A few questions please? After your seacock installation on Morgan’s Cloud will you leave them open to the sea or will you fit an external strainer? I have an aluminum boat in the yard and feel that the strainer is not a good idea if we have the stand pipe fitted as we can rod the stand pipe clean. Why two different brands of sea cock, The Marelon and the Bronze? Steve Dashew has a lot to say on the subject on his new FPB.

  • John May 25, 2010, 3:52 pm

    Hi Peter,

    The bronze was to replace an old seacock (24 years) of the same make that was worn where the handle engages the ball. We decided to try the Marelon for a new seacock required by our new exhaust system, as a supposedly easier installation option since no isolation would be required. However, due to the poor design of the flange, it was actually harder to install it properly than a bronze one would have been.

    No we do not have strainers on the outside. Just another set of complications: how do you fasten them? What material? etc.

    Occasionally we will get a lump of weed blocking an inlet, but not often. When this happens I just pull the hose off and poke it out and shut the valve quick. I also have a wood plug at hand, but have never had to use it. Sure, you get a bit wet, but it is a lot less dramatic than you would think. However, I would not try this with a seacock over say 1-1/4″.

    • Arthur Sommer July 5, 2013, 10:26 pm

      Honestly the only way to go is seachests welded to the hull if possible coming up above the water line with access at the top so you can check and or clean you strainers then you can get all your raw water for engines, a/c ,saltwater wash down ,head ,gen,ect. Directly from the chests without worry.

      • John July 6, 2013, 8:37 am

        Hi Arthur,

        I agree, and if I were building from scratch I would certainly consider a sea chest.

      • RDE July 6, 2013, 10:18 am

        Right you are Arther,
        And then you add a blow-down port on top– air if you have a big compressor, or large volume water from your emergency bilge pump. Now you can blow the seaweed off your hull surface mounted pre-strainer without getting wet!

        • John July 6, 2013, 10:57 am

          Now there’s a great idea!

          • RDE (Richard Elder) July 6, 2013, 6:14 pm

            Hi John,
            Not my great idea! I just had the benefit of working on a project where the owner always hired three or four different designers to solve every problem he could conceive of—.

  • Ed Arnold May 26, 2010, 5:15 pm

    I remain a strong believer in standpipes such as I had on my aluminum hulled Nomad. All discharges went down one standpipe, each with a valve and vented loop. The various intakes were also in a standpipe with dip legs below the water level. I had vented loops in all discharges and intakes. I had no problems with poor engine cooling or other sea water uses, but this could be a factor in other situations. With the standpipes near the centerline, I had no worry about hose breakages or back siphoning at any angle of heel. The standpipes were anchored to the aluminum hull framing, so there could be no high torques.


  • Dan Hansen June 30, 2011, 6:02 am

    I have found an interesting seacock / valve solution in my aluminum boat. Short aluminum tubes (7 to 10 cm) are welded to the hull, somehow PVC fitting and GF ball valves are attached to this tube. I have not dismantled the valves yet as the boat is still in the water and I just bought it half year ago. But this configuration lasted 30 years and is still tight and dry.


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