FO MAKING TKOF WITH CAPT IN IOE EXPERIENCE IN HVT ACFT STRUCK TAIL ON TKOF.

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I RECENTLY CHKED OUT AS CAPT ON THE HVT. I AM STILL IN MY FIRST 100 HRS. USUALLY; I WOULD DO ALL THE TKOFS IN THAT PERIOD; BUT ON THE NIGHT IN QUESTION; I DECIDED THAT IT WOULD BE BEST TO LET THE FO DO THE DEP. THE FACTORS: HVY AIRPLANE (310000 POUNDS); NIGHT; OBSTACLES ON TKOF END OF RWY; MINIMAL RWY FOR WT. I FELT IT WOULD BE BETTER FOR ME TO DELEGATE THE FLYING DUTIES TO THE FO (WHO HAS APPROX 3000 HRS IN TYPE) WHILE I SUPERVISED; OVERLOOKED PERFORMANCE; AND COORDINATED. TKOF ROLL WAS UNEVENTFUL AND A NORMAL 8 DEG ROTATION MADE. I CALLED 'V2;' AND ALL 3 CREW MEMBERS FELT THE PLANE LEAVE THE GND. THE FO DID THE SECONDARY ROTATION TO 12 DEG; AND WE FELT THE TAIL SKID STRIKE THE RWY. NO SERIOUS DAMAGE WAS CAUSED; BUT I STILL DO NOT UNDERSTAND WHY THE TAIL STRUCK. ONE FACTOR: THIS PARTICULAR ACFT DOESN'T HAVE AN 18 DEG FLAP DETENT - - OUR NORMAL TKOF SETTING. 23 DEGS MUST BE USED. THIS RESULTS IN A SLOWER THAN USUAL TKOF AND SLUGGISH ELEVATOR CTL; PLUS REDUCED INITIAL CLB DUE TO HIGHER FLAP SETTING (MORE DRAG). PERHAPS THIS LED TO SOME OVER CTL AND AN EXCESSIVE RATE OF ROTATION TO 12 DEGS; BEFORE THE PLANE HAD A CHANCE TO REALLY BEGIN ITS CLB.