CNG Capacity

[Frank P]

All,
I will eventually be converting from CNG to propane and would like to know what the capacity of the standard (exchange type) CNG tank is. I am trying to get a feel for what size propane tanks I will need. We typically burn through one CNG tank on a 3 week cruise.

I guess there is a couple of ways to approach this, by pounds and by BTU. It this point, it would be great help if someone could tell me how many pounds of CNG are in the cylinders.

Thanks
Frank

 

[Rick I]

Don't know about CNG but we go through a 10# tank of propane in about 22 - 25 days. We have three meals a day, use the oven a lot (bread etc). From my experience we seem to go through more propane than the average cruiser. I carry two 10 pounders, wish I had room for more. I would think 10 pounds of propane would last the average cruiser (if there is such a thing) a month.

 

[Ross]

I use twenty pound propane tanks and get 40-50 days from one. There is more heat in a cubic foot of propane than in a cubic foot of natural gas.

 

[Rick D]

Locker?

Where are you thinking of putting the propane locker, Frank? Isn't the stock propane 40.5 in the port aft locker?

 

[Steve Dion]

The CNG bottles are charged at about 2000 psi. I think that they can be charged much higher, but that is what the dealers do.

 

[Stu Jackson]

To answer your question

try www.safgas.com

 

[Tom Spohn]

As a follow-on to Steve Dion, a shorthand way to calculate time left on a bottle of CNG is to take the pressure and divide by 100 to get hours on one burner remaining. So a new bottle at 2000 # will last about 20 houirs. I have all the other data such as BTUs, weight, etc. on my boat and will write it down next time I go there.

 

[Stu Jackson]

You could do that, or

consider this, Frank. The propane canisters come in at least two sizes and just about everyone agrees that having two installed canisters, even if only one is being used at a time, is the way to go to avoid "running on empty" just when you're starting to cook. So, instead of trying to figure out comparisons between the two fuels, just figure out how to maximize the installation of a locker that'll hold two canisters. Your other fallback is one (larger?) canister and using the green bottles for a fallback. This approach is essentially "the use of the available" which you will end up getting to regardless of any comparisons you may make.

 

[Bill Roosa]

Why convert????

CNG is safer and cheaper, why are you converting?
At typical (scuba tank size) is what they call a 20 hour bottle. One burner for 20 hours. You can charge them to 3000 PSI and mine lasts for 3 weeks of steady use. The GF drinks lots of coffee and bakes a lot so our use is probably higher than normal.

 

[Rick I]

CNG is safer and cheaper, why are you converting?

Probably because CNG, hard to find in US & Canada, is nearly impossible to get anywhere else. If you're going cruising propane's a lot more available.

 

[Frank P]

All ,
Thanks for all your input. I will try to answer your questions here.

I agree that CNG is safer than LPG and it alleviates one of the many various safety issues the skipper contends with onboard boats. However, for me, the tanks are too heavy, too much of a hassle to get the bottles exchanged and too expensive (when comparing actual cooking time).

I carry two CNG cylinders on board which is mandatory for my type of cruising for the exact reasons Stu mentioned plus that CNG is not available in all ports. This methodology will be maintained with my propane system. Limited space is my primary issue otherwise I would just go forward with two 20lb tanks which I know would be sufficient.

The current CNG location on my 40.5 is in the port combing locker which is not “air-tight” from the main cabin. Future plans will have a small genset and hopefully the propane tanks in this location. However, I am sure I cannot fit two 20 lb tanks and the genset but I might be able to squeeze two 10 lb tanks in with a custom propane locker. Alternatively, I could store the tanks on the stern rails feeding to a small vented enclosure in the combing locker to keep solenoid and valve out of the weather.

So just a re-cap, it appears that a good rule of thumb for propane usage is about 2 lbs/day and one CNG cylinder is equivalent to a 10 lb propane tank

Thanks Guys, your input helps greatly. Based on this info I will be going forward with 2 10 lb tanks.

Frank

 

[Frank P]

ooops, I mean a 1/2 lb propane per day!

 

[Tom Spohn]

Frank
Here are the specs. on the CNG tank. 2200# psig @70 deg. when full, Capacity is 84 CF which is 100,000 BTU's. Remaining time on tank is Guage Reading/110 Each tank holds about 20 hours on one burner. I do not have the weight but they are heavy. I can carry one at a time, but for two I use a cart.