PS Advisor January 1, 2002 Issue

PS Advisor: 01/02

CNG Quandary
I have a 1987 Newport 30 with a CNG-fueled three-burner stove and oven. The stove works very well and the CNG is a good choice for a boat (lighter than air). The problem is finding the fuel. I can get it at a marine store in Rochester, New York, but as far as I know that is the only place on Lake Ontario.

Since we are thinking about taking the boat through the Trent-Severn Waterway to Lake Huron or down the ICW to Florida, I would like a current list of fuel dealers for these areas.

If finding this fuel is going to be a large problem, is there any chance I could convert my stove to run on propane? I realize this would take a vented locker for the fuel, but if I could use the same stove, perhaps by changing orifice sizes, it might be worth it.

-James Owen
Webster, NY


CNG, or compressed natural gas, enjoyed brief popularity in the '80s for the reason you cite: It's lighter than air and so will not tend to settle in the bilge should a leak occur. Propane or LPG (liquid propane gas) is heavier than air and thus must be stored outside the living accommodations in a vented locker also with outside drain. Racers used to install small CNG bottles under the cabin sole, near the boat's center of gravity to minimize pitching, to satisfy rules requiring stoves and fuel.

Other safety advantages of CNG are a higher ignition temperature compared to LPG (1100F vs. 800F) and the percent of gas in the air required for ignition is roughly double that of LPG (5-15% vs. 2 to 9.5%). Neither gas is particularly toxic, but both will displace air in a small area, making the immediate risk one of asphyxiation, not poisoning. Both are highly combustible, of course, and CNG should be accorded the same respect as LPG when installing hoses and operating appliances.

CNG burns more cleanly than LPG, but consumption is greater. Bottle pressure is less than half that of LPG.

So why, with its many advantages, didn't CNG take off? The main reason is the one you cite: lack of dealers. Why Gas Systems in California (the West Coast distributor) and Corp Brothers in Providence, RI (East Coast distributor) weren't more successful in convincing people to make the switch is a bit hard to say. But the bottom line is that dealers are few and far between.

If you plan to cruise beyond known suppliers of CNG, we recommend contacting the maker of your stove regarding a conversion kit to LPG. Most stove makers should be able to supply you with the parts, or arrange to have it done for you.

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Winter Battery Storage
What is the best way to store gel cell batteries over the winter? I have two deep-cycle, closed-cell gel batteries that I have removed from the boat for the winter. I have asked several people about this and they seem to be almost equally split between two approaches.

One camp claims that the best thing to do is to charge the batteries fully in the fall and then periodically "top them off" by re-charging them throughout the winter. This would, of course, only require the most basic automobile battery charger and a watchful eye. However, there is another group that advocates a constant trickle charge throughout the winter, using a much more sophisticated "smart" charger.

What is your opinion regarding the best way to maintain and avoid damaging gel cell marine batteries during an extended lay-up? What are the advantages/disadvantages of each approach. If the best approach is constant trickle charging, what is the best type of charger to use? Do you recommend any specific make or brand?

-James Phyfe
Via e-mail


One of the biggest advantages of gel batteries is their ability to be discharged for long periods with minimal damage. Self-discharge rate is about 3% a month at 68F, less than half the rate for flooded wet cell batteries. You should be able to leave the batteries on board all winter without charging.

An occasional charge certainly won't hurt them, and in fact would be beneficial. Trickle charging, however, can damage the plates. Instead, use a so-called smart charger that incorporates at least the three basic charge cycles: bulk, absorption, and float. A fourth cycle, called equalization, is a controlled overcharge. Equalization can be performed by some chargers; it is done not more than once a month and then only on batteries that are discharged daily. You wouldn't try to equalize batteries during winter storage.

In our tests, we've found a number of good quality "smart" battery chargers, but probably the brand with the most consistent ratings has been Statpower. The size to buy depends on the size of your battery bank. A 20-amp model, good for charging a battery bank up to about 300 amp-hours, sells at discount for about $360.

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