Well, I either was either hugely more efficient than usual or, much more likely, I screwed up the malt quantities.
Target OG was 1049. The SG into the boiler was way higher than I expected so whilst the boil was going I stuffed some numbers into ProMash and could not quite believe the resulting numbers. It reckoned I was heading for an OG of 1061. My efficiency is normally about 70 to 75%. This OG would need an efficiency of 90%, not very likely given that I batch sparge and that I am not very precise. So.... my best bet is that I stupidly used 6kg of malt instead of 5kg. My scales will only weigh 1.5 kg at a time so I guess I may have lost count of the kilos. How hard can it be to count to 5 (or 6)?
Anyway, remedial action required. I boosted the boil volume by 4 litres and gave the boil an extra 20 minutes to boost the bittering, ending up with 5.5 gallons and an OG of 1054. Not perfect but my FV will not cope the extra few litres needed to get back to the expected OG. The IBU is a tad lower than I planned but I had planned on the high side anyway so I'm pretty sure it will all work out. Wort tastes great and the colour is perfect.
The little yeastie beasties are just starting to show signs of action.
