I am fairly new to all-grain brewing having done about 5 now. I use a large cooler plumbed with a copper valve for my mash and it holds temp pretty well. I generally have been making 3 gallon batches because my brew kettle has a 5 gallon capacity. Now to my problem, today I brewed an Imperial stout with the intention of the SG being 1.091 and the FG 1.023 and an ABV of ~8.9% a week from now. I mashed 11.75lbs of grain for 60 minutes in 3.75 gallons of water at 154 F. My strike water was 168 bringing the temp of the mash to ~155, reason being I generally lose 2-3 degrees throughout the mash. When I stopped the mash the temp was 152 after 60 min. I sparged once with 3 gallons, then collected the wort for the boil. I took an initial SG reading after cooling the wort to 60 degrees and it came out at just over 1.052 far below the targeted 1.091 I was hoping for. Should I have mashed for 75min? Anyone have any thoughts on what went wrong.....