BeerSmith IBU calculations

am making a recipe for a beer with a SG of about 1.107 my plan was to do 2 brews at the same time (with a friend) and one would be just base malt and get a 3 hour boil to caramelize the sugars and the other would have the same SG but I would use some darker malts (120L and special B) to get the caramel and roasted flavors with a 60 min boil. We were thinking at setting the IBU’s between 50-60 and I made the 3 hour boil recipe first in BeerSmith set the hop amounts to reach the IBU’s we desired. Once finished with the recipe I copied it and went to change the grains but first I changed the boil time from 180min to 60min and I noted the IBU’s went way down. I played with all 3 bitterness calculations in beer smith and with everything equal only the boil time different (not the amount of time the hops are boiled…see below hop schedule same for both) I am getting huge difference in IBU’s. My question is how can boiling for 2 hours before adding my hops change my IBU’s or is this just a glitch in BeerSmith? Both the 60 min addition and the 15min addition changed the amount of IBU’s it contributed.



Below is my hop schedule from BeerSmith:

1.50 oz Simcoe [11.90 %] (60 min)
1.00 oz Amarillo [8.20 %] (15 min)



Below is the difference in IBU’s per boil time.

60 min Boil | 180 min Boil

Tenseth 52.0 IBU | 60.9 IBU

Rager 74.2 IBU | 80.2 IBU

Garetz 41.5 IBU | 44.5 IBU

Did you keep the boil volume the same for the 3 and 1-hour boils?

No boil volume is set to equipment. 8.5 gal on 3 hour boil and 6.34 gal for 1 hour boil both going to 5 gal. I went in and changed it to the same and it evened it out the IBU’s but that still does not make sense with program having the ability to specify the boil time. If I start with 8.5 and boil down to 6.34 over 2 hours and add the hops isn’t it the same as if I started the boil at 6.34 gal and add the hops? Or is BeerSmith using the boil volume to calculate the SG of the wort for hop utilization? That would make some sense because the utilization factor goes up the lower the gravity. Still I think this is a mistake in the program there should be a linear scale for the hop utilization as the volume goes down and SG goes up. I guess I will just have to fudge my boil volume to get the correct IBU’s on the recipe print out since as far as the hops are concerned this is a 1 hour boil and the SG of the wort will be at the 1 hour boil level. If I am wrong in my assumption let me know otherwise thanks for the help.