ROUGH WATER TKOF DEMONSTRATION ENDS WITH A BLEACHED LA27 WHEN AIRSPD IS MISREAD AND ROTATION IS STARTED TOO SLOW.

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THE LA27 IS A LAKE AMPHIBIAN. DURING A WATER TKOF FROM LAKE BUFFUM; FL; I ABORTED THE TKOF AFTER XING THE SHORELINE; LNDG THE ACFT STRAIGHT AHEAD IN A GRASS PASTURE. THE AIRFRAME SUFFERED SHEET METAL DAMAGE TO THE UNDERBODY AND FLOAT SPONSONS. TKOF WAS INTO THE WIND; ESTIMATED AT 9 KTS. WAVES ESTIMATED AT 10 INCHES HT. I ESTIMATED 3000+ FT OF WATERWAY WAS AVAILABLE FOR TKOF. UPON ADVANCING PWR; THE ACFT ACCELERATION SEEMED NORMAL. ROTATION SPD IS 60 KTS. I GLANCED AT AIRSPD INDICATOR; SAW 60 KTS AND ROTATED WHILE APCHING SHORELINE. THERE WERE 2 OF US ON BOARD. I AM AN INSTRUCTOR AND WAS DEMONSTRATING ROUGH WATER TKOF TECHNIQUE FROM THE R SEAT. UPON ROTATION; MY L SEAT PLT SAID 'YOUR AIRSPD IS ONLY 45 KIAS.' AS THE ROTATION COMMENCED; THE STALL WARNING SOUNDED. THIS WAS MY LARGEST ERROR. I DEFINITELY MISREAD THE AIRSPD INDICATOR (PLACED ON FAR UPPER L-HAND SIDE OF INST PANEL). ROTATION OF A LAKE AMPHIBIAN INTO GND EFFECT FLT (WHICH I WAS AT) IS WELL KNOWN IN THE LAKE COMMUNITY TO PRODUCE NO ACCELERATION. YES; THE ACFT WAS NOT ACCELERATING; SO I REDUCED PWR AND ABORTED TO A ROUGH FIELD LNDG (GEAR UP) IN THE GRASS PASTURE AHEAD. I SECURED THE ACFT AND WE WALKED TO A NEARBY ROAD FOR ASSISTANCE. CONTRIBUTING FACTORS: OBVIOUSLY; NOT ALLOWING ENOUGH WATERWAY FOR TKOF. I COULD HAVE BACK-TAXIED FURTHER ON LAKE BUFFUM. YES; I HAD 3000 FT WATERWAY AVAILABLE; BUT IT IS A WEAK EXCUSE NOT TO HAVE USED MORE. THE WIND COULD HAVE DROPPED OFF DURING ACCELERATION. I NORMALLY TAKE ONE LAST LOOK AT WIND STRENGTH BEFORE ACCELERATION. I DO NOT REMEMBER DOING SO THIS TIME. I NORMALLY PICK A GO/NO-GO WATER ABORT POINT. THIS I DID NOT DO. THIS LACK OF DISCIPLINE WAS CLRLY A LARGE JUDGEMENT ERROR.