We had a 40E, we purchased it back when it was Pur brand, about 1998.
We used it in California and Mexico for six years, it made All our drinking water at about 1 gal an hour, on average. We rebuilt it once, changed the prefilter after ever use and cleaned them. A set of prefilters lasted about six months. We were very happy with it and it was reliable. After Katadyn bought them out I believe the quality and performance were the same. If I was to do it over I'd get the 160. The 40E was just to slow.
Sailing is the art of floating with occasional drifting.
5 knots isn't fast for a boat, but it's pretty quick for a house.