The Calypso washing machine has no center agitator, and uses very little water. The clothing is washed by the bottom plate of the washing tub moving in a bouncing rolling type motion. In this situation, the slippers felted, but didn't shrink! (Hmmm this might be great for felting purses, to get very little shrinkage!)
Some quick research on the internet on how to felt the slippers by hand found me with boiling water in one side of the split kitchen sink, and ice water in the other. Even shocking the yarn and hand agitating it didn't do much.
So then I tried saturating the slippers in water, then running them through the dryer. It took three iterations of that to get them to shrink to the right size. Third try was a charm, and now my Dad has toasty toes!
These slippers were made with Lion Brand Felting Wool in Midnight Blue and Ocean Blues, with one strand of each held together as one. I like the resulting subtle color pattern.