B747-200 DEPARTED WITH INCORRECT WEIGHTS CALCULATED. THIS RESULTED IN REDUCED PERFORMANCE AND A TAIL STRIKE ON TKOF.

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AS INSTRUCTED BY THE CAPT; I WAS TO DO ALL THE PAPERWORK ON THE FLTS FOR TRAINING PURPOSES. INPUT THE DATA TO THE COMPUTER AND DID THE INS LOADINGS IN ZZZ. EVERYTHING WAS GOING AS SCHEDULED. OUR CLRNC WAS TO BACK TAXI TO RWY 31 AND WAS CLRED FOR TKOF. I WAS IN THE FIRST OBS SEAT WITH HEADSETS. TKOF ROLL WENT OK UNTIL LIFT-OFF; THE ACFT FELT A LITTLE SLOW EVEN THOUGH I WAS LOOKING AT THE ASI AND ALL SPDS WERE MET. AFTER TKOF I HEARD THE STICK SHAKER FOR MAYBE HALF A SECOND AND THAT TIME I LOOKED AT THE ASI AGAIN AND THE SPD WAS AT V2+10 KTS AS IT WAS SUPPOSED TO BE; WE KEPT CLBING; AND FOR A SECOND TIME I HEARD THE STICK SHAKER AGAIN FOR LESS THAN 1 SECOND AND I LOOKED AT THE ASI AND WAS READING V2+40 KTS; AFTER THAT WE CONTINUED CLBING. AFTER REACHING A SAFE ALT I WENT DOWN TO THE MAIN CARGO AREA TO CHK FOR CARGO SHIFT BUT THERE WAS NO INDICATION OF THIS; THEN I RETURNED BACK TO THE COCKPIT WHICH AT THIS TIME THE REST OF THE CREW WAS TRYING TO FIND OUT WHAT HAS JUST HAPPENED BECAUSE WE GOT A CALL FROM ZZZ TWR STATING THAT THERE WAS A POSSIBILITY OF A TAIL STRIKE AND THAT THEY WERE GOING TO CHK THE RWY AND LET US KNOW. ABOUT 10 MINS LATER; CTR TOLD US THAT ZZZ ADVISED THAT THERE WAS A POSSIBLE TAIL STRIKE; AT THIS TIME THE CREW CONSULTED THE APPROPRIATE CHKLIST AND CONTACT WITH THE COMPANY WAS ATTEMPTED FOR FURTHER GUIDANCE. AT THIS TIME THE OPERATING CREW WAS DISCUSSING WHETHER OR NOT TO DEPRESSURIZE THE AIRPLANE; BY THEN I NOTICED THE ACFT WAS PRESSURIZING NORMALLY. CREW WAS STILL NOT SURE IF TAIL STRIKE HAD IN FACT HAPPENED AND MAYBE MORE DAMAGE WOULD BE DONE BY DEPRESSURIZING THE ACFT. AFTER A BRIEF DISCUSSION WITH COMPANY DISPATCH AND MAINT THEY DECIDED TO CONTINUE AND THAT COMPANY WOULD ADVISE MAINT. AFTER TKOF; ONCE IN CRUISE; WE TRIED TO FIGURE OUT WHAT THE PROB WAS AND WE TOOK A LOOK AT THE PERFORMANCE COMPUTER AGAIN AND AT THIS TIME WE FOUND OUT THAT THE TOGW ON THE OPS WAS READING 277.1 AND NOT 366.5 LIKE I HAD INPUT FROM THE WT AND BAL AND THAT CAPT AND FE HAD CHKED BEFORE. MYSELF AND THE FE THEN PROCEEDED TO MAKE THE CORRECTIONS AGAIN AND AFTER CALCULATING THE TOGW CHANGED FROM 366 TO 277; AND THAT WAS SOMETHING THAT WE DID NOT CATCH THE FIRST TIME; THEN INPUT THE NUMBERS AGAIN AND THIS TIME THE COMPUTER HELD THE RIGHT NUMBERS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER USED THE ONBOARD PERFORMANCE COMPUTER FOR TKOF CALCULATIONS. THE COMPUTER APPARENTLY STORES WEIGHTS FROM THE PREVIOUS FLTS UNTIL CLEARED FOR SUBSEQUENT CALCULATIONS. IN THE PROCESS OF COMPUTING THE TKOF DATA; THE REPORTER FAILED TO CLEAR THE PREVIOUS INFORMATION. THIS RESULTED IN THE TKOF CALCULATIONS DROPPING BACK FROM THE ACTUAL 366500 LBS. TO THE RELATIVELY LIGHT WEIGHT OF 277100 LBS. FROM THE PREVIOUS FLT. THIS RESULTED IN THE USE OF INCORRECT V-SPEEDS FOR TKOF; AND SUBSEQUENT TAIL STRIKE ON ROTATION.