Mon Mar 22, 2010 7:22 pm
You definitely want to use a bigger one or you will lose a gallon or so to blow off. A six gallon is the absolute minimum size to use and 6.5 gallon is better. Even at that be sure to use a big bore blowoff tube or use some Fermcap S in the fermenter.
Also remember that when fermentation takes off the temperature of the wort will rise. For a big beer like this the temperature rise can be as high as 10° so you need to watch your fermentation temperature closely.
As far as how long? The answer is simple: Until it is done. Don't get in a rush on a big beer. It takes a while for the yeast to eat all that sugar. The yeast is going to be under some osmotic stress from being pitched into a high gravity wort at the beginning. Towards the end it is going to be feeling the toxic effects of being in all that alcohol. For this reason you will need a big pitch of health yeast. Try brewing another beer first in the 1.040-1.055 range first and use a big chunk of that yeast cake for the imperial IPA.
Don't worry about trying to get the beer off the yeast to avoid off flavors. Leave the beer on the primary yeast cake for at least 3 or 4 weeks. Up to 6 weeks shouldn't be much of a problem. Even if you hit your predicted final gravity sooner than that, don't mess with it for a while. Even though the gravity is no longer dropping, the yeast is still working to clean up unwanted esters that were created during the fermentation process. It can take another 5-7 days for that to take place after the gravity drop stops.
I would allow for 5-6 weeks for total fermentation and cleanup time. If I were bottling, I would let it sit in a bright tank for another week or so just to make sure I wouldn't get bottle bombs.
That's kind of a long answer, but fermentation time is the least of your worries on a big beer.
Have fun with it. I love big beers like that.
Wayne
