I always do 5 gallon batches, 60 minute boils and boil off 2 gallons in the hour. Beersmith says that is about a 25.5% evaporation rate.
I'm getting ready to do a 9 gallon batch and using the same rate Beersmith says I need to collect 12.6 gallons.
I am not a physics major but this number seems like BS to me:
--First I assume that because I'm starting out with almost twice the preboil amount that the boil will be less vigorous due to limitations on my burner, resulting in less boil off. I wouldn't expect Beersmith to adjust for this, but it's in the back of my head.
--Second, assuming that I had the exact same boil vigor (temperature), why would more liquid evaporate in the same period of time just because there were more liquid available? The same amount of the liquid's surface area is exposed to the air, is it not?
I have nothing backing me up on this, but it seems to me that boil off should be linear, as in "I boil off 2 gallons per hour", not "I lose 25% per hour". By that math I would collect 11 gallons instead of 12.6....
Maybe I'll split the difference and collect 11.8 gallons just so my OG isn't too far off either way....