Boil Off ExExperiment
I will be brewing on Satuday....and I had lost all of my data in Beersmith due to a computer crash...so I ran an experiment tonite to determine my boil off rate.
10 Gallon SS Pot, diameter 15.5 inches Start volume = 6.5 gallons End volume = 5.0 gallons Time = 60 minutes..... Time does not include heat up.... So....1.5 gallons per hour....in this case 20.1%......seems aweful high but that is what I got. Now, I did have a pretty rigorous boil....maybe it was excessive....but I did mark my gas nozzle location on my propane heater so I can repeat it on my brew day.....so I guess I will go with it. I am WONDERING what others have for a boil off rate on their equipment. 
Typically 15, but I've seen 20 before. If you have a refractometer, you can monitor SG while you're boiling and adjust the speed or duration of the boil to hit your numbers

I use Blichmann 20G kettles and get 2 gallons of boil off in an hour. I normally do 10 gallon batches. For most recipes, I start at 13 gallons preboil and end at 11 gallons in a 60 minute boil. I end up with about 10.5 into the fermenter after cooling.

My pot is 13.5" in diameter, and I boil off about 1.2 gallons/hour, so I think your numbers are definitely in line.
Just don't think of it in that percentage number, and don't enter it as a percentage into BeerSmith. The boiloff/hour is a constant variable for your particular pot no matter how much volume is in the pot, i.e., if you had 9 gallons in there, it would still be 1.5 gallons/hour, so the percentage is irrelevant. You are good rolling with that 1.5 gallons/hour number for your pot. 
Sounds like your in the ballpark. In my 8 gallon pot I start with 6.5 gallons and end up with 5 gallons after a 60 minute boil.

Quote:

I use a Blichmann 10 gallon kettle and I get 1.25 gallons of boil off on average in an hour, and that's with a nice rolling boil. That's what I always count at and use as my basis. If I do a really aggressive rolling boil then 1.5 gallons in one hour but I don't usually boil that aggressively as it just wastes more propane and boils off more, only done that one time.
Rev. 
20% is certainly in the ballpark with a vigorous boil in a keggle, in my experience.

I have a 10 gallon kettle and I lose about 2 gallons per hour.

Just FYI for all you guys posting your kettle volume, boil off is a factor of the liquid surface area, so the overall volume of the kettle is irrelivant. It's the diameter of the kettle that matters. A 3 gallon kettle with a 16" diameter will boil off more than a 15 gallon kettle with a 14" diameter. Larger volume kettles tend to be bigger in diameter than smaller volume kettles, but that isn't always the case. The guy I brew with has a short and fat 8 gallon kettle that is quite a bit larger in diameter than my tall and thin 11 gallon kettle, thus he has a higher boil off rate.
Also, as far as how vigorous the boil is, that is a variable you should keep constant if you want to hit your numbers. I boil as vigorously as possible right below the boilover point, which keeps that variable constant and therefore keeps that boil off rate constant. Controlling as many variables as possible = consistent beer! Just some tips...seeing a few false assumptions in this thread :mug: 
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