Thanks for all the responses. Sounds doable, but maybe more trouble than it's worth.
bob
I was going to do just this, with my boat, 15+ years ago. When we had kids onboard coastal sailing in season, we went through quite a bit of CNG. I can remember going through 1 tank a week during a two week cruise.
What that means: The smaller CNG bottle is called a 20 hour tank: A 4,000 btu typical stove burner would burn for 20 hours.
Back then, the local CNG exchange would give me a new bottle and take the old, for about $60 bucks. CNG, btu per btu is ridiculously expensive compared to propane.
The conversion would have been easy compared to creating a propane locker. That is a project on our boat.
We've never had a problem finding CNG exchanges in New England. Plus we keep two tanks onboard(in a nice arrangement where the tanks slide back into an aft unused area of a cockpit locker), so you have plenty of time to get a back up.
Today, I think there are more options for CNG exchanges in our area.
These days, with mostly just two of us - and we cook quite a bit onboard for coastal sailors - a tank lasts us almost all season. We exchange one bottle a year, easily, a few miles away. Price has gone up, exchange is about $70. It's definitely not worth converting for us.
Find out if CNG is hard to get where you plan to cruise, and figure out how much gas you use cooking, then look at what you have to do to convert?