Convert from CNG to Propane

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Back to Home ... s/v Stella Blue home ... Plumbing Page ... Projects

So here's my big idea.

This is a one inch through hull,
and is level with the output hose from the locker. I couldn't find a Marelon elbow for the back side, so the whole thing is bronze. What the heck. It's really difficult to reach, so now it's strong and I know I won't break it.

There's a 90 degree elbow on the back, to turn the hose toward the stern. This is actually just a touch lower than the bottom of the access hatch in the stern, so it will serve as a drain if I get three inches of water in the cockpit.

Fortunately, this corner is under the helm seat and behind the propane locker,
so aesthetics aren't critical.
But I'm still going to paint that epoxy plug. Ugly is ugly.

January 2008 --

All the pieces are coming together. After making the new helm seat and fixing the cockpit sole, the new propane locker
has a home. Now to hook it up.

A nearby boat yard exchanges CNG tanks, so CNG availability has never been a problem. CNG is a lot safer than propane, but difficult to find when traveling.

The locker is made by Trident Rubber. This one holds two 10 pound tanks.

There's a serious dip on the inside, as there's plywood above and core below. I didn't feel like grinding the hull clean and laying fiberglass fabric to build up the area, and don't think there's a structural reason for the effort.

So, a piece of plastic film was taped to the hull.

I covered a piece of flat plastic with super thick epoxy and colloidal silica, and set it over the area.

Upon inspection, it was clear that all of the bolts and screws had been installed without any sealant. That's pretty dumb
for a box that's supposed to be installed on a boat.

The output for the drain will be on the opposite (port) side. This will keep the exit for spilled propane 24 inches away from the exhaust, and will prevent water coming in when heeled over. At hull speed, the pressure wave might cover
the drain, and I want to avoid having water come
back into the cockpit. That would be a drag.

Fortunately, C&C did not core the hull in this area, although you can see that a piece of plywood was glassed into the hull to reinforce the original deck drain through hull. I'll put the new fitting in a non-cored area that is
between the plywood and the balsa core.

Hooray. No core here.

Back when I recored the sole, I prepared the corner for the drain. This little corner is solid epoxy.

It's also dirty behind the locker.
That's a lot of atmospheric dirt. I should have cleaned up before taking pictures.

Anyway, for two years I've been thinking that the bottom of the cockpit sole is awfully close to the waterline. It was going to be really tough to make the drain run downhill and exit above the waterline. But I thought I'd have a big idea.

Two years later I have no big ideas. The bottom of a drain fitting is going to be lower than the top of a through hull fitting, and there's no way around it.
I should have painted over the plug with non-skid.

The locker sat under the helm seat, waiting, for seven months.

What the heck is this?

It's mildew, not rot, thank goodness.
In another year it'll be rotten, though.

That's really annoying.

So all the fasteners are now sealed with polysulfide.
The wood strips on the inside were
painted with Rust-O-Leum.

Pulling on the inside end of the rope, while guiding the fitting into place, was a lot easier than I would have believed. Nothing got wet, even though all the work is about eight inches off the water.

Then I injected polysulfide Lifecaulk all around the fitting, making sure that it was
all over the outside flange.

I grabbed a piece of scrap teak and made a bunch of blocks, to be epoxied around the hull. These will support the hose. I could have used marine plywood, but was in a rush. Plywood blocks need to be protected from moisture, using epoxy or paint, but teak blocks can be left natural.

The blocks were rubbed with Acetone to remove surface oil,
so that they'll take epoxy better.

Everyone knows that propane sinks,
and ABYC specs require that tank lockers be sealed and have a drain that vents spilled gas outside the boat.
The vent has to flow downhill, with no loops, and exit the hull at least
24 inches away from the exhaust.

That last part seems silly to me,
since it's a wet exhaust system and there's no chance of sparks.

Because this locker has to be easily moved to gain access to all the stuff in the transom area (like steering) I decided to add a cockpit drain that exits above the waterline. The propane vent hose will feed all the way down through the cockpit drain hose.

When pulled up into place, you can see caulk gooshing out all over the outside.

Good. Caulk needs to goosh all over the place to verify that no spot is missed.


A while back I added a beckson port in each coaming, to gain access. There's too much stuff going on back there, and no other way to reach it.

That big padeye is just a place to clip a harness when you're at the helm. It's held on by 5-16" bolts.

The result, after some trimming with my Handy Dremel Tool, is this hard plastic shim. It will fit between the backing plate and the hull, and be glued in with caulk.

I could have ground the hull down to glass and built the area up with fiberglass laminate and epoxy. However, not only did I not have a weekend free to do the work,
but I don't think it's structurally necessary.
The backing plate is fiberglass, and over sized.

I did not use a sea cock. This is above the waterline,
and a sea cock would have made the fitting
higher than the drain on the locker.
That would defeat the entire purpose.

I'll use really good 1" hose.

All the pipe fittings are set using polysulfide caulk.
The backing plate is 3/8" thick vinylester fiberglass, from McMaster-Carr.

All right.
With the drain in place, I can put propane in the locker and start installing the plumbing to the galley and barbeque.

Here's the hole on the outside. It's well above the waterline. The waterline itself is pretty low down on the bottom paint.

After I repowered, and recored the water soaked cockpit sole, the waterline in the stern is back to the designed line, despite all the stuff I've installed in the stern.

These fiberglass propane tanks have recently become available.
I bought them on line from the chandlery at

They're really cool, as they don't rust or corrode,
and you can actually see the propane level.

The nearby propane supplier had never seen them before,
and didn't want to fill them up.

So, I'm going to have to make sure all the stickers never fall off, to prove that they've been tested and certified.

I ran a rope
through the hull,
and fixed the through hull
to the end.

Oops, autofocus screwed up.

The epoxy shim was threaded over, and then
a good wide, thick backing plate of prefabricated fiberglass from McMaster-Carr. Finally the nut was threaded on. Now I can take tension off the rope without having the thing fall into the water.

Caulk gooshing everywhere, of course.

They fit in the locker just fine, but the hardware that holds the tanks in place was designed for traditional metal tanks, and the brace sits about four inches higher.

Rather than disassemble the whole thing,
I just lengthened the steel rod
using some coupler nuts.

Fortunately, I'd saved the
extra threaded rods that Edson supplied with the new pedestal.
A couple of those worked just fine.

Here's a picture of the existing CNG line, as it passes under the port quarterberth and through the bulkhead into the space behind the ice box.

The boat was built with a copper gas line.
I think hard copper gas lines are no longer allowed.

This hunk of protective hose is just fine.
The new propane hose will fit through there,
and I can follow the existing path.

There's no point learning how to make swaged propane hose fittings, or buying a bunch of new special purpose tools.

I also don't want to blow my boat up.

It's about 15 feet from the galley range to the transom, so I bought a 25 foot hose with fittings attached.
It's been pressure tested.
The extra 10 feet will be used up
making the hose fit around corners.

I almost decided to go for it and pull the old stove out. (The new one should be here in a month.)

But I was hungry. Sitting on the stove was a
vacuum frozen lunch of Wally's Red Beans and Rice,
in a Foodsaver boil-in-bag.
(I've really come to love that Foodsaver Vacuum Toy.)

So I'll put off the last four feet as long as possible.

Back under the port quarterberth, the hose is attached to the liner using Ancor 5/8" steel braces. These braces have nice rubber protection, so they won't chafe or cut the hose. I used a hunk of old sanitation hose to protect the propane line as it passes through bulkheads.

The propane hose is firmly supported every 12 to 18 inches. Once the lid for the quarterberth is back on, it will be tucked way back up in the corner, and should be protected from damage.

Then I started thinking about what might happen when the boat is heeled over and locker contents shift.

Now the entire hose run
is protected by some
extra bilge pump hose.

As the hose enters the engine room, it goes vertical to stay
dry and protected.

Because it passes close to that light fixture, I put a section of white water hose around it.

It doesn't actually get hot there, and in fact that light is only on when I'm working back by the engine control panel.

But what the heck. It also marks the hose as "propane" for safety. Anyone working back here should have no confusion about what that hose run is for.

I didn't go out and buy a bunch of hose for this. These are leftover scraps from my
"hose bag." Thus the different sizes and colors.


I have the ice box ripped out,
as part of the Freezer/Fridge project.

That's convenient.

The new hose won't fit without removing the old gas line, so the final four feet into the galley can't be run until
I'm completely ready to replace the stove and oven.

Any place where the hose could potentially chafe is also protected by
scraps of hose.

That white hose doesn't really protect from chafe, but is just to label the hose.

The blocks are solid teak, epoxied onto the boat. I didn't want to use plywood scraps for this. In this picture one can see an old, original scrap of plywood, that had fallen apart after 20 years of humidity.

The hoses exit the interior under the helm seat.

The black fitting is vapor (and water) tight, and is made by Trident. The wiring for the solenoid also exits here, through a Blue Seas clam fitting. I really like those.

The fittings are caulked with LifeSeal,
which is safe for polycarbonate plastics.

On the propane locker, there's another Trident hose fitting, and a cheap little West Marine deck fitting for the wire.

The nice thing about the Trident fitting is that it can be easily loosened. Hand tight will be enough to hold the hose, but when I need to move the locker a couple of turns will loosen it enough to allow the extra hose inside the locker to slide out.

For the same reason, the West Marine deck fitting will also allow extra wire to slide out of the locker.

This is important.

Here's an extreme example.

The slack hose and wire slides out of the locker, and the locker set up on the coaming.

This will allow it to be moved out of the way, so that one can work on all the stuff in the transom. There are lots of things back there, like steering cables, that need to be
accessed for maintenance or repair.

The backside of one of the bolts came in really handy to hold another clamp, and secure the propane hose
through this section.

I've never actually seen this before. The pic was taken by holding the camera through the Beckson port and
snapping blind.

On the other side, you can see the locker drain hose. It goes down into the new cockpit drain, and actually extends all the way
to the through hull fitting.
I'll cover it with UV protection as well.

Any gas that leaks in the locker will drain all the way down and out the hull. It would have to leave the locker drain hose, and come back up the cockpit drain hose to get back
into the boat. That's not very likely.

Here is the hose and wire
with the locker in place.

They must be protected from UV exposure, and I think I have some black plastic sheathing in the garage. Finding it may be a challenge.

Note the turnbuckle. On both sides, the locker is secured to the boat using small 316 stainless turnbuckles from McMaster-Carr.

Inside the locker, the low pressure
output is protected by another bit of hose,
so it won't chafe against the threaded steel
rod that secures the tanks.

The wire for the solenoid is tightly secured.

It was very satisfying to flip the switch inside,
and hear a nice big "click." The wire and breaker
were installed years ago.

The low pressure hose feeds into this "Tee" fitting,
with a separate hose for the gas barbeque.

I haven't figured out what to do with that hose yet.
A permanent installation, with a hose run to the barbeque, would require more holes in the boat. and leave a hose permanently exposed and hanging over the transom.
Yet connecting and disconnecting it every time I want to use the barbeque will be a real hassle.
They make "quick connect" fittings, but I'm a bit worried about corrosion after long term exposure to salt water, that could eventually allow a leak.

This Tee will be bolted down to a teak block,
epoxied to the inside of the locker.

All of the NPT fittings that came from Trident
were sealed with Red Loc-Tite.

There's a special Yellow Loc-Tite for propane,
but I don't see it on the shelf anywhere.
If the Red stuff is good enough for the manufacturer, it should be good enough for me.

I was very careful to keep it on the threads only.
One doesn't need sealant on flared gas fittings,
just on NPT fittings. However, I wanted to keep the nuts secure, so they can't possibly
vibrate loose over time.


All the hoses and wires are tucked back out of the way,
and can't be seen once
the helm seat is back in place.

Naturally, the old hose uses an NPT fitting, but the new one has a 1/4" reverse flared fitting.

And, naturally, I couldn't find plumbing fittings to make the new hose fit into the old NPT female fitting on the regulator.

Finally, I called Trident, and special ordered a little brass fitting,
part number 6401414.
Only two bucks, and
eight bucks shipping.
I wish I'd known, and ordered it
with the rest of the stuff.

There are a couple of details
to wrap up.

The Trident locker came with the old style Left Handed POL tank connector, so I had to get a new hose with the
new OPD connector.

Finally, the old CNG stove was pulled out,
and the last few feet of hose run completed.

The stove area needs a good cleaning and a coat of paint,
and then the new stove goes in. That'll be nice.

This teak block is epoxied to the hull, behind the new fridge/freezer, keeping the hose tightly supported
every 12 to 18 inches.

A few weeks later I added the Gas Barbeque and a
new Galley Stove.