Been hoping for a bit more information but lacking that, best guess.
High ABV beer is always going to be a a lot less efficient and more expensive to make. Single vessel systems are really at their weakest at high ABV, much as I love my Braumeister this isn't what its best at.
There is a very handy little equation that helps a lot when designing a recipe. It is designed to give the Plato (oP) of the first running's, if you are full volume mashing its all first running's.
oP = CGAI/(L:G+CGAI
Where
CGAI is the potential yield of the malt, for this lets say its 75% or 0.75 in equation. Yield% or Y% for short
L:G being the mass of water /mass malt, lets say 5 water to 1 malt (5:1)
oP = 0.75/(5+0.75) = 0.13 or 13oP, an SG of (SG=(4*oP)/1000+1 = 1.052 Before the boil.
Pretty obviously the less water you use the higher will be the OG, the limit is about 2:1 any less water and the malt wont be covered
Using the same numbers as above except the L:G gives 0.75/2.75=27.3oP or 1.109
You are looking for an OG of 1.092 (23oP) 0.23 for use in equation
We can rearrange the equation to tell us the L:G
oP=Y%/(L:G+Y%) > L:G=Y%/oP
L:G=0.75/0.23 = 3.26, 3.26 times the mass of malt or 12*3.26=39.13L
We need to take a look at a couple of other factors to do with water. About 0.9L/Kg will stay in the grist and you should be getting at least 10% evaporation. You could go for a larger starting volume and a longer boil.
Boiling for more than three hours isn't recommended, you start causing measurable harm. I would be boiling a big beer for 90-120 minutes and aim for 15% evaporation, just because there is going to be a lot more high molecular weight proteins that we want to precipitate.
We cant do much about absorption by the grain but we need to add the loss to evaporation (10-15%) and its normal to allow another 2% for losses during mashing, say 12-17%. We need to add this to the water bill.
Say we aim for 15% evaporation, add the 2% and we need 17%, means the start of boil gravity should be 17% lower than the 1.092 or 23oP we want at the end of the boil. OG should be 23oP/1.17 = 19.66oP.
If we use the new target to calculate the L:G we get
L:G=0.75/0.1966 = 3.815, for 12kg of malt that's 45.78L
If you mashed in with 45.78L, 12*0.9 = 10.8L will stay in the grain, leaving 34.95 (well 35L) at the start of the boil, boil of 15% and you should have 29.73L that's very close to your target OG.
There are a couple of important caveats.
Make sure you have enough Calcium available, I would want at least 150-200ppm.
Make sure your pH is in the 5.3-5.5 range. If you haven't got a pH meter, use brew n water or one of the other programs to calculate your pH, you wont get down low enough with just salts you will need Acid as Acid Malt or an acid addition.
I would also look at your mash temperatures, you really don't want a protein rest, and mashing for much over 90 minutes isn't going to gain you much.
I would be tempted to mash in at around 62oC for 20 minutes (B-Amylase peak), raise to 66oC for 20 minutes, raise to 72 for 20 minutes (A-Amylase peak), mash out at 78 probably for another 20 minutes. Add another 16 minutes for ramp time and you are just over 90 minutes total.
There are at least three protease enzymes that work in the mashing range, there is at least one that wont fully denature under 65oC, it will do all the work you need early in the mash.
Lift the malt pipe and let it drain for the 20 minutes or so it takes to reach the boil.
Suck up the low efficiency, knowing you are making a mind blowing beer.
Mark
