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Old 12-25-2001
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Converting CNG to LPG

I have recently purchased an older sailing vessel that uses CNG (Compressed Natural Gas) for the stove supply. Because I live in an area where it is impossible to obtain CNG, what do I need to do to convert my stove to LPG?

Tom Wood responds:
The actual sheet-metal body of a CNG stove is the same as an LPG, or propane, unit. In fact, there’s a good chance that your stove was manufactured by Seward in California, who made most of the CNG stoves for a company named Gas Systems. If this is the case, you’re in luck as Seward is also a major builder of LPG marine stovetops and ranges.

CNG use for cooking on board sailboats had a huge burst of popularity in the early 1980s. This was mostly in response to the fuel crises of the '70s, but also had some roots in environmental laws and many on-road vehicles were converted to CNG at the time. The idea was to have CNG filling stations on every corner much like gas stations now exist.

In fact, CNG has major safety advantages for use on boats. Because it is lighter than air, a leak from a CNG source does not tend to flow down into the bilge and form an explosive pool lying in wait for the first available spark to blow up the boat. CNG tanks do not require a vapor-tight locker drained over the side. The tanks themselves initially appeared to be little more than modified SCUBA tanks and were designed originally for exchange—you brought in an empty and picked up a full one. Since the gas was not as hazardous, the tanks could be shipped by truck without the hazardous materials charges and hassles of other fuels.

The bad news is that CNG systems burn a lot of fuel. Boats that used five pounds of propane a month often used four times as much CNG. And since the system never took off the way the designers envisioned, CNG became a hard-to-find commodity.

What’s involved in the changeover? Since the stove body, piping, and accessories are virtually identical, you may be able to change the burners and air orifices and have the stove converted. Contact the manufacturer of your range and see if this is the case.

The bad news is that the conversion of the stove may prove to be the easiest part of the process. You’ve just started. You will now need to install that vapor-proof LPG box in an above-the-waterline location and drain it over the side. New propane bottle(s) will need to be added along with a new LPG regulator. Depending on the location of the locker, you will probably need to run new tubing or hose from the tanks to the stove, and if the lines are old, this is probably not a bad idea anyway. While you’re at it, install a new LPG shut off solenoid with a remote switch in the galley—and you might as well get one of the new ones with the gas-sniffer alarm built in.

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