Seacocks by Kevin McGoldrick

This article originally appeared in Offshore Magazine.

Seacocks are arguably the most overlooked and under maintained system on a boat.  Other than holing a vessel below the waterline, or losing a propeller shaft, few scenarios will sink a boat faster than a failed seacock.  As a marine surveyor, I often see seacock installations that do not meet American Boat and Yacht Council (ABYC) Standards.  Short comings run the gamut from the use of residential plumbing gate valves to incompatible materials to flimsy installations.  The most common failures are a seized valve (usually in the open position), lost or broken handle, rotted wood backing blocks, and corrosion.



Let’s begin by getting our terminology straight.  According to the ABYC a  Through Hull Fitting is designed to attach to hoses or valves to permit water to enter or exit the hull.  A Seacock is a valve used to control the intake of discharge of water through the hull.  A seacock is operated by a lever type handle usually operating through a 90˚ arc, giving a clear indication of whether it is open or shut.  Seacocks are required on all hose or piping lines penetrating the hull below the maximum heeled waterline (MHWL).  The MHWL is defined as an angle of 7˚ for power boats, and the level of the sheer amidships for sailboats.  The purpose of a seacock is to stop the admission of water should the hose or piping fail.



Seacocks are required to withstand significant force before failing.  The acid test as described in ABYC Standards states that “A seacock shall be securely mounted so that the system will withstand a 500 pound static force applied for 30 seconds to the inboard end of its connecting fitting, at any point in its most vulnerable direction without the system failing to perform as intended”.  To be conservative this test should include any elbows or hose barbs attached to the seacock.  A 500 pound static force is a significant amount of force.  Many times a through hull fitting will be held in place with a narrow threaded ring with a ball valve and hose barb screwed on top.  Although I’ve not personally tried it, I am not convinced that this common installation technique would withstand the 500 pound test.  Add some elbows and reducers and the chance for failure increase dramatically.


Considering the importance of the seacock system to the integrity of the vessels’ hull, only purpose made Marine UL listed seacocks should be installed.  The small increase in the cost of materials and labor is more than offset by the secure installation and piece of mind they provide.


More often than not, the failed seacocks I observe are of gate valve variety.  Gate valves (the kind with the round metal handle) simply do not belong on any through hull fitting on a boat.  Replacing them with purpose made seacocks makes for an excellent owner project that will result in a safer vessel that is easier to use.  Gate valves operate by raising and lowering a metal gate on a threaded rod as you turn the handle.  Most of these valves are not manufactured for marine use and consequently have a number of major short comings.  Although the metal used may look like the bronze fittings on the rest of your boat, non-marine grade fittings are manufactured from alloys that are high in zinc and eventually become brittle in the marine environment.  Some yellow brass alloys can be as high as 41% zinc.  Since you cannot readily tell if a particular valve is made from a marine grade bronze alloy you should only use fittings from reputable manufacturers carrying a Marine UL listing.  Even gate valve bodies that are manufactured from the proper alloy may use mild steel or other corrosion prone metals in the stem fitting, packing nut, or handle.  The result is galvanic corrosion that causes the stem to seize or shear and the handle to crumble in you hand.  


Unlike purpose made seacocks, one cannot immediately tell from look or feel if a gate valve is open or closed.  Even when a gate valve appears to be closed tight, marine growth may be blocking the gate from fully seating.  Being able to tell at a glance, or by feel, if a seacock is open or closed could save precious time if the need should arise in an emergency.


Finally the design of the gate valve itself does not make it a suitable fitting as a seacock.  It has no means to be independently secured to the hull; instead it is screwed into the through hull fitting (usually a bronze mushroom fitting).  Seacock fittings have a substantial flange that allows the seacock to be through bolted to the hull with 3 or 4 bolts.  Through bolting creates a much stronger installation over the rather flimsy ring nut provided with most mushroom fittings.  Perhaps the most insidious problem with gate valves is that they are manufactured with tapered threads known as American Standard Taper Pipe Threads or ‘NPT’ and should only be used with other fittings that are also NPT.  Through hull fitting have straight threads known as American Standard Straight Pipe Threads or ‘NPSM’.  The NPSM through hull fitting is able to catch a few threads in a NPT gate valve fitting before it binds up; the result is that only 10 to 20 of the available thread area is used creating a very weak link.


Generally available Marine UL listed seacocks are manufactured from bronze or a reinforced plastic called Marelon® which is manufactured by Forespar Products Corp. (  The bronze alloy used to manufacture seacocks has a low zinc content of 5% or less making it well suited to the marine environment.  Seacocks often have other fittings such as a bonding wire connection point, drain plug, and quick release plugs that make winterization easier or allow engine intake seacocks to do double duty as emergency bilge pumps.  Whether you are installing a bronze or plastic seacock, it is advisable to use the same material throughout the installation, for example the through hull fitting and any elbows or hose barbs.  One of the major benefits of plastic seacocks is its corrosion resistance, adding a bronze elbow or hose barb would defeat this purpose.  In the case of bronze seacocks using elbows or through hull fittings of black iron or stainless steel is a recipe for galvanic corrosion.  Even the mounting screws and nuts should be bronze and not stainless steel.  Bronze seacocks that are expected to come into contact with bilge water should be connected to the vessels’ common ground point to minimize the possibility of stray current corrosion and to help protect persons onboard or in the water from a short in the AC shorepower system.  The bonding wire should also be connected to the vessels’ cathodic bonding system which, in turn, is connected to the vessels common ground point.  The cathodic bonding system is typically connected to a single zinc anode outside the hull thus protecting all submerged metal from galvanic corrosion simultaneously.  Plastic seacocks are immune to galvanic and stray current corrosion and thus do not require anode (e.g. zinc) protection or a bonding wire thus making installation a little easier.



Boat owners tend to get a little crazy at the prospect of drilling holes in the bottom of their boat.  The reality is that installing a seacock is well within the skills of the average do-it-yourselfer and need not be an exercise in fear management.  The only caveat here is that if your hull was built using cored construction additional precautions are necessary.  Cored construction refers to a hull manufactured with a core material such as balsa, foam, or other product sandwiched between inner and outer layers of fiberglass.  This type of construction results in a very ridged and lightweight hull and adds some level of sound and thermal insulation.  These desirable attributes require that bond between the core material and the fiberglass remains intact.  If water is permitted to enter the core material it could break this bond resulting in a weakened hull structure.  Some hulls are cored above the waterline only, while others are fully cored above and below the waterline.  Unless you already know your boat is cored below the waterline, there is no easy way to determine if it is cored or solid fiberglass reinforced plastic (FRP).  Sometimes you can determine this by looking at the hull to deck joint because the core material will stop short of the joint and the transition will be evident.  However the only sure way to determine the construction method is to contact the manufacturer or to pull an existing seacock and examine the existing hole.  Cored hulls usually require extra work to ensure that water does not enter the core material causing serious hull problems in the future.  Some of the better builders of cored hulls remove the core material at locations where seacocks will be installed, in effect creating a small patch of solid FRP hull and protecting the core from moisture intrusion.  Some manufacturers bond aluminum plate into the hull in place of the core material thus protecting the core while creating an immensely strong area to mount the seacock.  Sometimes these seacock mounting locations are built into the vessel to permit additional seacocks to be added at a later date.  If you have a cored hull, look for these areas.  If you must install in a cored location you will need to create your own patch of solid FRP hull by removing an area of inner laminate and core material leaving only the outer laminate.  Additional fiberglass material must then be bonded over the area from inside the hull sealing the core and creating a reinforced area of solid FRP.  If you are not familiar with the use of epoxy resin and structural fiberglass cloth, this is a job best left to a professional.  However once the fiberglass work is done, the rest of the seacock installation is straight forward.  Solid FRP hulls are generally easier to locate and install seacocks because one can install them just about anywhere that is convenient. 


Seacocks should be installed in a location that allows the handle to move fully from opened to closed without interfering with nearby equipment or seacock handles.  They should be placed where access is quick and easy in case of an emergency.


Once you have settled on a location for your seacock select a small diameter drill bit and drill a pilot hole from inside the hull outward at the center of the through hull location.  Now switch to the appropriate sized hole saw, locate the pilot hole outside the hull, and drill half way through the hull thickness.  Finish up the hole by drilling from the inside out.  Drilling halfway through the hull will ensure that you end up with a smooth edge around the hole both inside and out.


Next fabricate a backing block for the seacock.  The backing block is located between the hull and seacock and its purpose is to distribute the load of the seacock over a larger area of hull.  The easiest material to work with is marine grade plywood.  Using the same hole saw that you used on the hull, drill a hole in the center of a large piece of plywood.  Next scribe a circle around this hole that is approximately one to two inches larger than the seacock flange and cut it out with a jig saw.  Drilling the center hole first, then cutting the larger circle, is easier than trying to drill the center hole in a small circular piece of plywood. Waterproof the backing block by coating it with two or three coats of unthickened epoxy.


Test fit the installation by screwing the through hull fitting through the hull and backing block and into the seacock.  Through hull fittings are often much longer than necessary and need to be trimmed.  The goal is to allow the through hull to screw tightly into the seacock just before it bottoms out in the seacock.  Examine the backing block; it should rest evenly on the interior of the hull without any gaps.  If the hull is curved or uneven it must be leveled out prior to final fitting.  This may be accomplished by grinding the area if it is slightly uneven, or by making a pad of thickened epoxy to level a curved hull section.  Once the fit between the backing block and hull is flat and smooth, drill the  holes for the seacock mounting bolts.  To drill the holes temporally re-install the through hull fitting, backing block, and seacock.  From inside the hull, drill the mounting holes using the seacock mounting flange as a guide.  Use an extra long drill bit if necessary to allow the holes to be drilled straight.  Next, from outside the hull use a countersink bit to allow the mounting screw heads to sit flush with the outer hull. 


Final installation involves coating the backing block with two or three concentric rings of bedding compound such as 3M 4200® or equivalent around the center hole.  Add additional bedding around each screw hole.  Attach the seacock and backing block to the hull by inserting the mounting bolts from the outside (use more bedding at the screw heads) and attaching with locking nuts from the inside.  Add a liberal amount of bedding compound to the through hull fitting where it mates with the hull.  Do not put bedding compound on the threads of the through hull fitting or seacock unless you want the next person (who may be you) to remove by cutting it out.  Screw the through hull fitting into the seacock securely and clean up the excess bedding compound before it has a chance to dry.


The Two Hose Clamp Myth:

Contrary to popular belief two hose clamps on all seacocks is not required by ABYC Standards.  In fact, some hose barbs are simply not long enough to accept two properly spaced hose clamps and may result in premature failure of the hose by pinching the hose against the end of the hose barb.  Good practice is to secure below the waterline hoses with two clamps only if the hose barb is long enough to do so.  Hose connections that use smooth walled pipe (as opposed to a serrated type hose barb) are a different situation and should always use two clamps or be replaced if it is not long enough to do so.


As extra insurance, all seacocks should have a tapered soft wood plug attached to each seacock with light fishing line.  In an emergency, the plug can be jammed into the seacock which will swell and seal the fitting.


The use of gate valves, especially in older boats is common.  This seacock is obviously leaking and is clearly beyond its useful life.  Note the corrosion where the bonding wire attaches.


 A bronze seacock.  Note the screw on the mounting flange for connection of the boding wire.


 Reinforced plastic seacock.



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