Thru-hull Flange Plate and Marelon Flanged Valve Installation.

I just want to say one word to you.

Just one word.

Are you listening?


Plastics!

A detailed description of how we replaced standard bronze seacocks with their flanged Marelon equivalents.

When we last left our heroes [see our blog post titled Thru-Hull Valve Reduction Project] they had removed several thru-hulls (and their associated valves) and plugged the holes left behind in the hull.   They had also decided, wherever they kept a valve in place, to substitute the bronze seacocks and mushrooms with their Marelon equivalents...

We decided to use Marelon valves instead of bronze for a number of reasons, but I should first point out that this is a very controversial decision, and only time will tell if this was the right decision or not.  Many, many people still use bronze or stainless, some use even use titanium, and I do not wish to suggest that our solution is the "right and true" one, it isn't.  There are clear disadvantages to using a composite material in lieu of metal, and we researched this idea extensively and spoke to as many marine professionals, boat builders, sailors and other trouble makers as we could find before deciding on this solution.

Before jumping on our bandwagon, you should be aware that we don't think Marelon is as strong or reliable as bronze under certain conditions, but that no thru-hull/valve combination, regardless of material, should be used unsupported.  Metals like steel, aluminium, titanium and bronze are ductile and tend to handle shock loading better than plastic (which under the right conditions will shatter) and people often do install valves directly off the thru-hull, but (in our very inexpert opinion) that seems like a recipe for disaster.   Any forces placed on the valve, or the hoses leading from it, can exert a significantly magnified force on the thru-hull, which isn't designed for those kinds of loads.

That is especially true for the Marelon thru-hull mushrooms, and we strongly advise you not to use one without a fully supported valve.  One that will not place shearing, twisting, bending or torquing loads on the mushroom thru-hull.  In other words, don't use a valve unless it is either held in place with some sort of bracket, or is internally flanged and mechanically attached to the hull.

On the other hand, Marelon will not corrode when exposed to galvanic currents, which means that you won't need to worry if the metal in your valves is slowly ebbing away, but our real reason for using a flanged valve and creating a backing plate wasn't our worry about the strength of the valve material itself.  We were concerned about the bending force placed on the thru-hull whenever the valve was operated. We'd seen how easy it was to crack off the old bronze valves once they corroded, and we'd seen first hand how easy it was to snap off a plastic mushroom if the valve itself was torqued.   Because we weren't relying on the compression fitting created between the thru-hull mushroom and valve (which would normally be the case) we believed that we had avoided the problem of placing our faith in one particular piece of plastic (what you might call a single point of failure) from keeping the boat from flooding.

Our thinking is that even if the thru-hull broke off completely from the valve, the valve should retain its watertight seal between the flange and the hull.  Of course, this is all probably just wishful thinking on our part, but we felt better knowing that we'd made every attempt possible to prevent a broken thru-hull from flooding the boat. Our only regret is that we didn't take this approach initially with all of the valves in the boat.

We did use this for the 3/4" salt-water (toilet) intake and the 1.5" holding tank output valves located in the bottom of the maintenance cabinets, but didn't for the engine intake valves (in the engine compartments) and sink drain valves (under the sinks on the outer facing hulls), which were both 1" diameter.  At least we did not take this approach initially, but wish we did.

Our approach was to use a flanged Marelon valve, that we mechanically attached to the hull via a backing plate that we chemically bonded to the hull.   Below is a cross sectional view of the  hull (blue) with the flanged valve (black), backing plate (yellow) with embedded bolts (orange), and thru-hull mushroom (green).

The advantage to this approach was two-fold: it eliminated any metallic plumbing parts being exposed to sea water (which meant that we wouldn't need to worry about electrolysis as much) and (if we used the flange plate style valve) it reduced the likelihood that a broken, cracked or leaking thru-hull could result in a complete failure which might leave a large opening in the boat.

One thru-hull removed, one to go...

In fact, if you somehow managed to completely shear off the thru-hull mushroom cap on the outside of the boat (perhaps by driving over a reef), or cracked the threaded mushroom stem (perhaps through a collision, or boat flexing from significant wave action, or just incorrect installation), the valve itself would still remain in place, directly attached to the hull via the bolts and nuts holding the valve flange to the fiberglass backing plate, and keeping the hull intact and water-tight.

[editor's note: This is all supposedly true, at least in theory.  The problem with theory is that in theory, theory always works, where-as in practice, theory often turns out to be less reliable. ]

The obvious downsides are that it was more expensive than bronze (about twice as much) and a relatively newer technology with less of a track record.  Another downside is that we had to make and install custom flange plates on which to attach the valves, which took time (and money), and was a bit tricky to do. Originally, we wanted to somehow embed nuts into the plates, but it was easier (and we believe a bit stronger & more secure) to have bolts sticking up out of the plates instead. We used two pieces of stock 1/4" thick fiberglass plates just slightly larger than the footprint of the flange, and glassed them together, making a triangular plate slightly thicker than 1/2", with three bolts sticking up out of it, and a large hole in the center.  

The two thru-hull backing plates installed.

To do this, we first drilled matching 5/16" holes in the plates for the three flange bolts and a slightly larger than 1.5" hole in the center for the thru-hull opening. On the bottom piece we extended the holes on each side, creating a slightly larger slotted opening, and then ground stainless steel bolt heads to fit into it (which would mechanically prevent the bolts from spinning in place should they break free from the fiberglass) and then glassed the the two plates together, and glassed the bolts in as well.

[editor's note: this is actually harder than it sounds, because you need to make sure that the three bolts are perfectly aligned and square to the plate.  We recommend making a 1/2" thick template flange (out of some other material that won't stick to fiberglass) that matches the hole pattern of the valve flange.  Using this, instead of the valve, means you'll be able to install the plates without having to readjust the bolts later on.]

This allowed us to have a backing plate with threads sticking up out of it, that would eventually be glassed (chemically bonded) onto the hull itself, making a relatively secure means of keeping the valve in place, even in the event of a catastrophic failure of the mushroom.  In order to get a strong bond between the fiberglass backing plate and the hull, we had to glass the plate to the hull before the valve was installed.

Kelly's ingenious solution of running a pipe clamp
through the opening and clamping the backing plate
to the hull as the fiberglass hardened.  It helps to protect
the bolts with tape (shown above) and to make sure you
get a complete bond between the plates and the hull.

To clamp the plate to the hull Kelly (the fiberglass specialist at Napa Valley Marina) came up with the brilliant idea of using long pipe clamps, with the pipe running through the thru-hole.   That allowed us to put a large amount of clamping pressure onto the backing plate as the epoxy hardened, creating a strong chemical bond between the backing plate and the hull.  Once hardened, we had a means of attaching the flanged valve to the boat.  We used BoatLife caulk to create the seal between both the Marelon mushroom and the hull, as well as between the valve flange and the backing plate.  


[editor's note: One of the best pieces of advice we got from the Kelly was this: if you are going to use any kind of sealing caulk, get the white color instead of black.  The reason being that, despite your best attempts, it will get on everything, and all over you, and the white doesn't show up as bad, especially if you have to go to work the next day.   Sounds silly but that really did turn out to be great advice, because this stuff is horrible to get off your skin, and if it weren't for exfoliation, I'd still be stuck to the hull.]

We could only find Marelon flanged valves in the 3/4" and 1.5" sizes. Rather than enlarge the openings, we decided to instead use their 1" valve for the sink drain and engine intake.  That was, in retrospect, a big mistake for which we paid dearly.   Perhaps it would have gone better if we'd build a support bracket for the valves themselves, but given our experiences (see below) with the Marelon thru-hulls cracking, we don't believe that is the proper solution either.  At least for us, your mileage may vary.

First pass at cutting an opening.  We later
cleaned this up and added a door panel.

But we digress...back to eliminating the holes in our boat.  In attempting all these changes, we realized that the maintenance closet door, which was at least two feet above floor level, meant that any work we needed to do on the thru-hull valves (or bilge pumps) at the hull level meant hanging upside down, arms outstretched, inside a deep hole.  Being that I've already suffered an IRS tax audit, a Barium enema, and have been divorced twice, I figured I'd already had my share of suffering and could skip that step.  And since the forward toilets would no longer be in the way, there was no reason we couldn't just cut the door opening down further, and add a larger door.

But we couldn't find a 1" Marelon thru-hull valve with a flanged base plate.  Which meant that we used the thru-hull "mushroom" screwed on to the standard valve to create the seal (along with caulking) between the hull and the ocean.

Unfortunately, we also used Marelon thru-hull mushrooms, which we realize now, are not strong enough for this application, at least not in our opinion, and apparently some other netizens think so as well.  Unfortunately, the sink drain holes and the engine cooling water intake holes were both 1" diameter, and we ended up using the standard valve & thru-hull mushroom fitting without a flange plate.  Although we made sure we had as good a fit as possible, the mushroom cracked as we were trying to install it.  We weren't comfortable with the possibility that the mushroom could shatter and expose a 1" hole to the sea, but we were assured by the folks at the marine supply store that the Marelon could handle it.   Perhaps this is true for some other installations, but it wasn't the case on our boat, and we should have listened to our gut.


Unless you have an absolutely orthogonal [at perfect right angles] fit between the mushroom head, the outside skin of the hull, the inside skin of the hull, the valve base, and the sides of the hole going through the boat, you will ultimately place slightly more pressure on one side of the mushroom's lip than the other, putting significant bending force on the part and transmitting those stresses into the threaded shaft as well.   Worse still, any bending force introduced at the valve (by operating the valve lever for instance) would be magnified and transmitted into the mushroom, which isn't really designed to withstand those kinds of pressures.

The problem was that all of the holes in our boat are in curved surfaces with uneven side dimensions. For bronze or titanium thru-hulls this probably wouldn't be an issue at all, as the ductile strength is adequate, but for plastic [even Marelon], where the material has a tendency to shatter, this is a horse of a different color.  More to the point, it is entirely possible, just from the operation of the valve handle, to theoretically place enough force on the mushroom to potentially cause it to crack, if not shatter or fail complelety.


[editor's note:  we learned this "the hard way" by trying this very approach.  It ended with us noticing that one side of our boat was a bit lower than the other, mostly due to water continuously leaking in one night while we slept.  Apparently the thru-hull mushroom in the starboard side had cracked, and it slowly but surely flooded the bilge.  We hope you learn from our mistake.]


We're not exactly sure when this happened, but the leak was discovered only a few weeks after installation.  Given that we had not installed any plumbing yet (the far end of the valve was plugged) it could only have been caused by improper installation and/or hull flex.  The crack probably occurred when we installed it, but went unnoticed because the caulk kept enough of the seal intact.  That probably worsened while we were sailing about on the bay, perhaps even on returning back from the yard, but it didn't leak at all for the first few days.  Eventually it did, and at some point it became fast enough to significantly flood the starboard bilge before we noticed the problem.

Worse still, the valve was in a cramped area with little elbow room.  When I tried to fix it (and because the mushroom stem was so badly damaged already) the valve broke off completely from the mushroom, leaving a gapping hole in the boat and a fire-hose of  water streaming in.  This is one of those moments where being your own mechanic leaves you wishing you had someone to fire besides yourself.

We eventually removed this valve and replaced
 it with a flanged version with backing plate.

So, our advice (for whatever it is worth) is that no matter how good a fit you think you can make, we'd recommend NOT using a Marelon (or any non-metal) thru-hull mushroom unless the valve it is attached to has an integrated  flange plate AND you have added a supporting base plate to the hull to which it can be attached, effectively eliminating the structural requirements (and stresses) on the thru-hull mushroom itself.   That may sound like a bit of a religious statement, and to be honest it probably is, but I sleep better at night knowing this is how the boat is configured.

The picture to the right is of our engine compartment's seacock.  Eventually we removed these 1" valves and replaced the engine valves with their 1.5" flanged counterparts, which required enlarging the openings and adding a backing plate.  To see the description of how we sealed the thru-hulls under the sinks, go to the blog entry titled Thru-Hull Valve Reduction Project.  We also removed and sealed up the sink drain thru-hulls completely, eliminating two more holes in the boat.

We did learn a valuable lesson or two.  First, always assume that every thru-hull in your boat is going to break open, and be prepared to have everything you need to stop the leak at hand, preferably lashed right to the valve.  To that end, you should have an appropriately sized wooden plug fastened somewhere close by, several rolls of emergency tape, and a bucket of StayAFloat readily available.  We also think a few Nerf footballs are good to have lying around, as they can be stuffed into just about any oddly shaped hole that you need to plug.