Thanks for all the comments. Plenty to think about. I'll be upping my game with this one, with multiple drill-aeration, water adjustments, and staggered sugar feeding.
I am not an expert but brewed a few strong beers recently. Highest FG was 1020 which is quite sweet. I would imagine 4 packs of US05 is too much and will ferment most of the sugars out even if you have lactose in it. Is brown malt neccessary along with the black malt? You could always try cold steeping the roasted malts for a smoother flavour. I do that with a lot of my Stouts as you can up the percentage and still get a smooth taste.
Cheers, I'm basing the number of packets on the Brewer's Friend pitch rate calculator, and basing the FG on their recipe builder which thinks 81% attenuation is reasonable. I'm erring on the side of caution, and wouldn't want to stress the yeast towards the end. Using both black and brown malt came from the The Kernel recipe. I've never used black malt before, but I assume that using both would give more complexity than one or the other. I haven't cold steeped before either, so I'll definitely look into that.
Two things I'd make sure of when brewing high gravity are aeration and an adequate starter.
I've got an air pump and a sterile air filter and use that for 30 minutes on any high gravity beers. Some high gravity beers benefit from a second aeration after the initial few hours after pitching.
At the very least give it a good thrashing with a paddle, preferably one shoved into a drill!
Regarding the starter, look into stepping it up a bit, the yeast will need a lot of help getting through all that sugar. Also check your water profile to make sure you have the right level of minerals and use some yeast nutrient in the last few minutes of the boil.
*Edit* just saw you're using dried yeast, so skip this bit! Except maybe buy 6 packs and keep a few as backup in case you get a stuck fermentation.
Lastly, I imagine your fg will be a fair bit higher than 1.025 maybe more like 1.035+.
I was thinking about doing a starter, but I only have a 2L Erlenmeyer so probably won't be able to build up one that is big enough, hence the packets. That said, I'll be spending ã9 on yeast, perhaps I should just get a liquid yeast and do an initial 2L step in a flask, then do a 4L one without stir bar. Now I just need to think about what yeast would work.
For aeration, I'll be sure to beat the hell out of it. Reminds me, I need to get a new charger for my drill! Never heard of a second aeration, but I should be back home 12h after pitching so I'll give that a try. I was going to use my tap water mostly, as it has high Ca levels, but Mg is only ~7.5mg/L, and sulphate 20-50mg/L, so I should probably take my first foray into water adjustment with an Epsom salt addition. What makes you think I'll end up with a higher FG? I've done a couple of dark beers recently that have finished lower than expected (Dogma 1.072 -> 1.012, Mild 1.036-1.008 despite mashing high), so if anything I was concerned about finishing too low, hence the lactose. I guess I could always leave the lactose out and if I finish too low just boil it up and add it after fermentation.
I see your recipe calls for DME, I would get some extra as backup, just in case you don't hit your numbers. Big grain bills can be a bit of struggle to extract all the sugars.
It may be an idea to add the brown sugar late in fermentation also, so that the yeast tackles the more complex sugars first.
Thanks. I'll definitely have some additional DME just in case. The last couple of big-ish beers I've done have has inefficient mashes. In fact, this is the reason I'm adding DME at all! I've lowered my brewhouse efficiency accordingly so hopefully I'll be on target for this one. I hadn't though about staggering the brown sugar, but it makes sense and I'll do it.
