Why not CNG?

[Grotius]

I live near the Chesapeake, but I don't have a boat yet. I've noticed, though, that several places at the Chesapeake do exchange CNG tanks, including the marina I'd be most likely to use. If and when I get a boat, I plan day-sailing, then maybe some weekending and coastal cruising, mostly in the Chesapeake area. I don't plan to live aboard the boat, at least not in the foreseeable future. Still, I'll probably want a working galley.

By way of background, I'm looking at boats like the Gemini 105MC, the smaller Seawinds, TomCat 9.7, the smaller Antares or maybe a similarly-sized monohull (but that's less likely). My expectation is that my usual crew would be small -- me and a couple members of my family. Most of my experience has been sailing Lasers, dinghies, and cuddy-cabin cruisers, though I did have a few lessons on a Catalina 36 a few years back. I've never been on an overnight sailing cruise, and so I have almost no experience with the galley.

Anyway, I'm not wild about propane, as I'm the kind of person who always forgets something, and I worry some day I'd forget to vent the bilge or check the sniffers or whatever. Yes, I know these things are largely automated, but still, why take the risk if it's not necessary? CNG sounds safer, as it's lighter than air, and it fuels a stove/oven almost as well as propane, supposedly. (Perhaps I'm wrong about that?)

If CNG is available where I plan to sail, why not use it? If I chose to use it, would it be possible to swap out the propane in a unit like the Gemini's? Or would I have to buy a completely different stove/oven?

Incidentally, I did read some of the recent threads on CNG vs propane, but I wasn't sure whether it was proper etiquette to add my question to them. Hence this new thread.

Thanks in advance.

[Gary1]

CNG is great stuff. The only catch is availability. It's slowly getting better, but that same thing has been said for the last fifteen or so years.

From a safety standpoint, it doesn't get much better, but having a great CNG stove and nowhere to get gas has always slowed me down.

As far as propane is concerned, if you put in a Xantrex system, it's pretty close to fool proof. (Nothing is absolutely foolproof. Fools are so d*****d ingenious.) The Xantrex unit seems to get unhappy with just about any petroleum vapor in the bilge, which is okay, too.

Good luck in your hunt.

[sailingdog]

The problem is that you need a totally different stove for CNG, since CNG is mostly methane, where LPG is usually a combination of Propane and Butane and a "propane" stove can burn either with relative ease.

CNG hasn't really taken off as a marine fuel. The problem is there aren't really any easily accessible CNG vendors in many places, and since there aren't CNG vendors, no one buys CNG-based equipment. Since no one has any CNG-based equipment, there's no motivation to install a CNG fueling station.

Also, CNG is probably less efficient, since methane has a lower heat output value than either propane or butane. The tanks tend to last a much shorter time, since CNG has a lower heat output.

Propane or LPG is perfectly safe if you have a properly installed system and pay a little attention to it. If the system has a propane monitor with a line to the solenoid, the risks is pretty minimal.

[artbyjody]

One other aspect of the difference of Propane versus CNG.

CNG is much more high pressure and a leak will result in the vapors residing in the upper levels of the cabin.

Propane is pressurized as well, but is heavier and will sink to the lowest levels of the boat.

Just one more thing to be aware of.

[Grotius]

Thanks for your replies.

I've read conflicting things on how long CNG lasts. Some people say it lasts longer than LPG. E.g., this post from a Tartan's owner forum in 2005. (Sorry I can't post a link; I don't have enough posts yet.)

I have had CNG in two Tartan boats. The containers will be delivered with 2,000+ psi of pressure and that gives you enough gas to cook all season. The containers are easily removed from the boat and I never had problems refilling at Brewers installations, where you can order and exchange containers.

I'm curious whether you all agree that CNG will last a whole season.

As to availability, people in other threads here have posted links to places that will exchange CNG tanks. Again, I'd post one or two if I could posts links, which I can't do yet. Among others, I found one interesting link at the website of Corp Brothers-Marine Products, which says it has a "network"of CNG exchangers up and down both coasts of the US, including several at my area (and at what I expect will be my home marina). That company says it distributes CNG stoves for Seaward.

Sailingdog, you seem to be right that many stoves can't easily be converted between LPG and CNG. For what it's worth, though, the Seaward stoves advertised by Corp Brothers on their website say they can be set up to work with either LPG or CNG. The website isn't a thing of beauty, though, and I have no idea if it's up to date.

As far as propane is concerned, if you put in a Xantrex system, it's pretty close to fool proof. (Nothing is absolutely foolproof. Fools are so d*****d ingenious.)

(*raises his hand*) I know! Also, part of my concern is that I doubt I'd be the chef, and the current candidates for that job might have less patience with safety checks than I do.

One downside of CNG is that mariners in general have less experience with it. (I imagine there aren't too many CNG users here at SailNet, for example.) If I go LPG, there's a wealth of info on where to store tanks, etc. Moreover, manufacturers like Gemini sometimes carve out niches on the boat explicitly for LPG tanks. Hmm.

[sailingdog]

Well, the rough rule for propane is one pound per person per week. So a 20 lb. tank will generally last 10 weeks for a couple. I doubt the person in that post you quoted is right.

The reason I say this is that methane, the primary component of CNG has a heat of combustion of about 50.009 MJ/kg, while propane and butane are at 46.357 MJ/kg and 45.752 MJ/kg, respectively. However, methane is significantly lighter than propane or butane, having a molecular weight of 16.043 g/mol, versus 58.123 g/mol for butane and 44.096 g/mol for propane.

What this means is that the heating value you get out of one mole of each gas is very different, with methane being by far the lowest. One mole of gas takes up about the same volume for each gas....and the values are:

Butane 2659.2435 MJ/mole
Propane 2044.1583 MJ/mole
Methane 802.2944 MJ/mole

So, butane gives you 3.314 times as much heat as butane for the same volume of methane. Propane gives you 2.548 times as much heat for the same volume of methane. Given tanks of equivalent volume, butane should last longer than propane, which will last considerably longer than methane.

However, if you're planning on going to higher latitudes, CNG is probably the best fuel for you, since it freezes at the lowest temperature.

[Grotius]

Sailingdog, thanks for that detailed and helpful reply. Um, I might want to sail as far north as Maine, but I'm not sure that's what you meant by "high latitudes." Anyway, some further thoughts on your points:

Propane gives you 2.548 times as much heat for the same volume of methane.

Fair enough: methane is lighter. But do your numbers account for variations in the degree of compression of the natural gas? After all, we're not comparing the volume of propane with the volume of methane; we're comparing propane to *compressed* methane, no?

From what I can tell from inexpert Googling, the compression of CNG varies from 2000 pounds psi to 3600 psi. The Tartan owner I mentioned earlier said he was using 2000 psi. Here's a link to their conversation; one owner says he has to buy a LNG tank once per season, and another owner says he has to buy a tank twice per season. Maybe the tanks are huge?

Variation in the degree of compression might explain why I keep reading posts like this:

CNG Pros -
Compressed so CNG is equal to much larger quantity of LPG. CNG is more expensive per lb/gal but lasts longer than LPG.

I found that quotation at this page. (Yay, I finally have enough posts to include links!)

Do you or anyone else have a picture of a typical CNG cannister? If they're the size of a small rhinoceros, that might finally persuade me to cave and go with propane.

[sailingdog]

Grotius-

Think of it this way... when you really compress CNG, it liquefies... so the volume of liquid it can have is basically limited to that of the tank....Propane is also stored in liquid form in the tank. Methane, the primary component of CNG has a liquid density of 422.62 kg/m³. Propane has a liquid density of 582 kg/m³. Given the same volume of liquid, the propane is significantly heavier. That would be the optimal storage of CNG...but IIRC, it is actually not compressed to the point where it liquefies, so that would be a fairly high number for CNG's actual heating value. However, using that figure, we'll proceed.

If the tanks are the same size, the heat value of the propane will be 2.548 * 582/422.62 or

3.50891108 TIMES THE HEAT VALUE.

To get the same effective amount of heating, you'll need to carry four tanks, versus the one of propane. If you really want to go with CNG...go ahead... you'll have to refuel more often, and the fuel is much harder to come by... especially if you're cruising in more remote locations. You're acting like Forest Gump.

BTW, that would be a very conservative estimate since CNG has to be compressed at much higher pressures than does propane. A propane tank only has to withstand 150 PSI or so at most. A CNG tank has to withstand 2500 PSI...so which tank do you think is heavier given the same size tank? Which tank do you think has more internal volume given the same size tank?

[catamount]

I have CNG on my boat.

The typical CNG cylinder is about 28 inches long, and about 7 inches in diameter, and weighs around 30 pounds. It has a capacity of 84 cubic feet of compressed gas at around 2000 psi, which supposedly will give you about 20 hours of burner time.

http://www.corpbrothers.com/catalog/cng.htm

One of the differences between CNG (methane) and LPG (propane) is that, while it burns as a gas, LPG is stored as a LIQUID, while CNG is always a gas. Liquids are much denser than gases, so you can pack quite a few more BTU's into an LPG tank than you can into a compressed gas cylinder.

In my limited experience, CNG exchanges are available up and down the east coast more or less per the directory on the Corp Brothers directory (I've gotten gas at Bert Jabin's in Annapolis, Great Bay Marine in New Hampshire, and the Carousel Marina in Boothbay Harbor, Maine, and I know that it is available at Robinhood Marine, Georgetown, Maine; http://www.corpbrothers.com/marine.htm). The cost is relatively high (I just paid $73.50 for my most recent exchange). Part of that cost is for the testing and maintenance of the high pressure gas cylinders that happens with the exchange, something that's not an issue (or least much less of one) with LPG tanks.

My father had CNG on his boat, but switched over to LPG before a big cruise up to Newfoundland due to concerns about the availability of exchange. As it turns out, we used less than 10 pounds of LPG for the whole 5-week cruise with a crew of four.

I like the CNG well enough, and appreciate the safety aspect of the gas itself (just remember that high pressure compressed gases introduce a different set of safety concerns). Still, if you're going to be cruising outside of the US, and/or are living (and cooking) aboard for long periods and not just weekends, you are probably better off with LPG or even diesel or kerosene as a cooking fuel.

[sailingdog]

Catamount's point about the additional dangers due to the high pressure that CNG is stored under isn't one to take lightly. I've seen what happens when a gas cylinder, in this case it was helium, falls over and damages the valve and regulator. The tank ends up shooting off like a heavy metal rocket. Imagine that happening on the limited space of a boat with a flammable gas as propellant.

BTW, some CNG tanks are about 9.25" in diameter 23.5 cm to be exact, which look like this: