Roughly 6 of my last 10 beers have fermented very quickly, but with no real krausen forming. These are all beers that I have brewed before and have seen more sedate fermentations with thick krausen. I have looked at my brew logs and can't figure out what is different. The new ones are fermenting out in a day or two instead of 4 or 5. I have reduced my pitch rate so I don't think I am over-pitching. I thought I might be over-oxygenating but a couple have used SNS starters and it is hard to imagine them being over-oxygenated. The latest brew was bubbling within 8 hours of pitching, with bubbles and some foam appearing around the edges. The next morning the foam was gone and the bubbling was furious and the liquid was swirling and circulating. According to my floating hydrometer it has gone from 1.050 to 1.020 in 28 hours and is still going strong. I have seen this behavior with OGs from 1.050 to 1.093 with a variety of yeasts. I had one Scottish ale that did this act of foaming a bit, then going to swirling with no foam, then after it was fermented most of the way it rapidly formed a 1" thick layer of krausen for a few hours, then the krausen dropped and fermentation was done.
If I had a stainless fermenter I wouldn't know anything except that the beer was fermenting very quickly, but since I can see it I fret. The final result is good, but perhaps not as good as the previous ones with more stately fermentations. I am planning my next brew right now, an amber ale, and am rolling around lots of ideas on what to change but none of them seem convincing. Any suggestions?