UNEXPECTED HIGH SPEED BOAT TRAFFIC CAUSES INSTRUCTOR PLT AND STUDENT ABOARD FLOAT EQUIPPED PA18 TO BECOME AIRBORNE PREMATURELY. UNABLE TO CLIMB OR MANEUVER DUE TO INSUFFICIENT AIRSPEED; THE ONLY SAFE PATH WAS TO FLY UNDERNEATH THE BRIDGE DIRECTLY AHEAD OF THEM.

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WE WERE FLYING THE WILLAMETTE RIVER JUST S OF DOWNTOWN PORTLAND; OR; IN PREPARATION FOR HIS SEL AND SES CHK RIDE. WE HAD JUST DEPARTED THE SEA PLANE BASE AND BACK 'WATER' TAXIED APPROX 500-600 YARDS DOWN RIVER TOWARDS DOWNTOWN IN ORDER TO HAVE PLENTY OF ROOM TO EXECUTE A NORMAL WATER TKOF UP RIVER AND INTO THE WIND. IT WAS A BEAUTIFUL DAY AND THERE WAS MODERATE BOAT TFC ON THE RIVER. WE CLRED THE AREA AROUND AND BEHIND US; COMPLETED OUR TKOF CHKLIST; AND EXECUTED A TURN INTO THE WIND AND UP RIVER. IT SEEMED THAT AS WE MANEUVERED INTO POS AND AFTER WE TURNED ALL OF OUR LIGHTS ON; THAT ALL THE BOATS IN THE GENERAL AREA PRETTY MUCH MOTORED TOWARDS THE SIDES OF THE RIVER. WE BEGAN OUR TKOF RUN AND FOR A SPLIT SECOND OUR NOSE WAS HIGH AS WE WERE COMING ONTO OUR 'STEP.' ONCE THE NOSE OF THE ACFT LEVELED OUT; WE ACCELERATED RELATIVELY QUICKLY. AS WE WERE NEARING FLYING SPD A CIGARETTE BOAT; TRAVELING AT VERY HIGH RATE OF SPD; CAME AROUND THE CORNER UNDER THE BRIDGE AND WAS UPON US. I BELIEVE THE BOAT OPERATOR NOTICED US JUST IN TIME BUT BECAUSE OF THE SPD HE WAS CARRYING HE COULD ONLY ALTER HIS COURSE SLIGHTLY. HOWEVER; IN DOING SO HE CUT RIGHT IN FRONT OF US. WE WERE TRAVELING TOO FAST TO CHOP THE THROTTLE AND MANEUVER AS NOT TO CREATE A HAZARD TO THE OTHER BOAT OPERATORS OR OURSELVES; YET TOO SLOW TO FLY. WHEN WE HIT HIS WAKE WE BECAME AIRBORNE. WE GRABBED A HAND FULL OF FLAPS SO NOT TO SLAM BACK INTO THE WATER. ONCE IN GND EFFECT WE ACCELERATED TO A SPD TOO FAST TO AVOID ANY OBSTACLES WHICH MAY HAVE COME ACROSS OUR PATH AND YET TOO SLOW TO CLB ABOVE THE BRIDGE BEGINNING TO FILL OUR WINDSCREEN. WE WERE LEFT WITH ONLY ONE SAFE OPTION...FLY UNDER THE ROSS ISLAND BRIDGE. WE WERE APPROX 5-10 FT ABOVE THE WATER 40-50 FT UNDER THE BRIDGE. WHEN WE CLRED THE BRIDGE IT WAS VX THEN VY TO AN UNEVENTFUL CLB TO 1500 FT HEADING S UP RIVER. THAT WAS WITHOUT A DOUBT TOO CLOSE FOR COMFORT. AS THE PIC; THE ULTIMATE RESPONSIBILITY FOR THE SAFETY OF FLT LIES IN MY HANDS AND I BELIEVE WE MADE THE SAFEST DECISION FOR THE SITUATION WHICH DICTATED SPLIT SECOND THINKING.