Boil time and OG

Changing the boil time seems to alter the amount of water required while the OG remains the same , while I would expect it to result in a higher OG with a lower volume. Is there a setting somewhere to change this? I have looked but cannot seem to find anything related to this. It makes it more difficult to design a beer with long boiling time.

BeerSmith calculating the sugars extracted using the volumes and the Brew house efficiency.  If you change the volumes in a recipe, the program will adjust the amount of sugars extracted from the mash to keep the same brew house efficiency.  So, as you increase the boil time, BeerSmith calculates the same sugar extraction to accommodate the added water volume needed for the additional boil off which means a lower gravity from the mash but the same gravity into the fermenter since brew house efficiency is based upon the sugar content and volume into the fermenter for the same grain bill.

You can see this in the recipe by noting the post mash gravity at your present boil off rate, then changing the boil off rate within the recipe.  You will see the post mash gravity decrease as a result of the additional water.

So, to increase the gravity into the fermenter for a recipe, you either need to add more grain or change your brew house efficiency to raise the mash efficiency to accommodate the added mash water.  This is not necessarily a linear adjustment as there is a limit to how much sugar you can get from the grains.

Thanks.