A PA32 pilot declared an emergency during climb after takeoff because of a rough running engine and landed at a nearby Air Force base. Maintenance determined that water had accumulated in the recently fueled tanks and had not settled out before the tanks were sumped.

Narrative

After departure at approximately 900 MSL the engine began running rough at full climb power. At low altitude and low airspeed I reported the problem to Departure Control; they asked if we could make it back to the departure airport which would require 180 degree turn and flying over water 6 miles. I declared an emergency and asked for permission to land at an Air Force Base; the runway directly off the nose which would only require reducing power and landing straight ahead. Approach turned me over to the Tower which cleared us to land on the runway. The landing was an unauthorized landing and required to be reported. The aircraft was checked by an A and P Mechanic and cause of the rough running engine was determined to have been caused by water ingestion. Fuel system was sumped and cleared of all water. Pre-incident: The aircraft was fueled just prior to takeoff. During the pre-flight the tanks were sumped and no water was found and the run up all checked good. The only explanation we could determine was the water had not had time to settle to the sumps prior to take-off. In the future I will demand the tanks to be topped after landing and not wait until the next departure to insure water has adequate time to settle in the sumps.