Black Triangle over Lake Kezar in Lovell Maine in the Late 1980's

It was in the summer time in the late 1980’s at night. I was with my family looking at the stars on a boat in the middle of lake Kezar in Lovell, Maine when a very large black triangle ship silently flew lowly over us.
I didn’t see any lights, but I could be wrong. I did see what looked like windows on the side. Not 100% sure about the windows.
I don’t remember it taking on water, but it could of before we saw it. It flew directly in the middle of the lake, and from the projected flight path it might of took prior it could of spent a few miles over the lake.
After it flew over us it went into the mountains. It last about 5-10 minutes.
Unfortunately I don’t remember the exact date, but know it was in the summertime in the late 80’s. My family can verify the sighting, but they don’t remember the date either. As far as I know an early date for a black triangle sighting.
Witness indicates that date of incident is approximate.  PD