Fast moving silent craft over water

On a summer afternoon in 1966, three of us went sailboating on Mountain Creek Lake near Grand Prairie, Texas.  It was a small sailboat, and we were occasionally rocked by big waves as speedboats navigated close to us.  We had sailed from the east side to the west side of the lake near Dallas Naval Air Station.  We then were working our way back across the lake, but by sunset, the wind died down to almost nothing, and we were somewhat stranded in the middle of the lake.  As it got dark, we worried that some of the speedboats might not be able to see us as they continued zooming around the lake.
Around 9:30pm, I spotted what appeared to be a rather tall yacht coming towards us at a fairly fast speed (30 mph est.).  As it passed us within 40 feet of us, all we could see were several round and lit-up porthole windows facing us.  There was no headlight in front of it.   Although it was eerily silent with no sound of a motor, we were just happy that it had missed us and were more concerned with the waves that would soon hit us, and possibly tip us over.  We all gripped the sides of our boat waiting for the shock waves, but no waves ever came to hit our boat.  
To this day we have no explanation of the incident, except that this craft that passed us defies any description we can come up with.
Witness indicates that the date of the sighting is approximate.  PD