AFTER PURCHASING THE 2 ENG ULTRALIGHT, THE PLT MODIFIED IT BY INSTALLING 2 ADDITIONAL ENGS. THESE WERE 6 HP PIONEER ENGSINSTALLED IN THE #1 & #4 POSITIONS. JUST PRIOR TO THE ACCIDENT, WITNESSES OBSERVED THE VEHICLE FLYING OVER THE CENTER OFA LAKE AT APRX 250 FT & AT HI SPEED. REPORTEDLY, IT ENTERED A RIGHT BANK, WHICH CONTINUED, AND SUBSEQUENTLY, THE VEHICLEDESCENDED & STRUCK THE WATER IN A NOSE 1ST ATTITUDE. AN INVESTIGATION REVEALED THAT ON AT LEAST 2 OCCASIONS, AN OUTBOARDENG HAD INADVERTENTLY SHUT DOWN, INFLT. A FRIEND OF THE PLT HAD FLOWN THIS SAME VEHICLE & HAD EXPERIENCED AN OUTBOARD ENG SHUTDOWN. HE CONFIRMED THAT FULL OPPOSITE RUDDER WAS REQUIRED TO MAINTAIN CONTROL. ALL PROPELLERS WERE FOUND BROKEN,EXCEPT #4 WHICH WAS NOT DAMAGED. FLEXIBLE FUEL LINES WERE FOUND INSERTED IN THE FUEL TANK TO PICKUP FUEL FOR THE #1 & #4ENGS; HOWEVER, THEY WERE NOT SECURED TO ASSURE PROPER DEPTH. THE VEHICLE WAS NOT EQUIPPED WITH A SHOULDER HARNESS. THE PLT HAD NOT RECEIVED ANY KNOWN PLT GROUND TRAINING. HE HAD FLOWN THE ACFT APRX 5 HRS AFTER IT WAS MODIFIED WITH 4 ENGS.