Originally Posted by hukdizzle
This is exactly what I am talking about. I am thinking that the boil off rate will be slightly lower due to the fact that the liquid will not boil as vigorously being larger. This is the same exact kettle and setup I use for 5.5 gal batches.
Not sure, but it sounds like you're missing the point that is being made here. The boil off rate in BeerSmith is useless. Boil off has to be measured in gallons per hour, not a percentage of the volume. If you boil off 1-1/2 gallon with a five gallon batch, you'll boil off 1-1/2 gallons with a ten gallon batch. That means that the percentage you use in BeerSmith will be about half. Adjust that percentage number until it gives you the same volume boiled off as you get with a five gallon batch.
This is assuming a constant intensity of boil and you'll have to apply more heat to the larger batch to achieve that. But from the sound of it you may be boiling a bit too hard as is. You're not just going full bore with your burner, are you?