B733 FLC INCORRECTLY INPUT FMS SEA DEP SID AND TURNED EARLY FROM ASSIGNED RTE.

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PDC CLRED VIA BLUIT 3 DEP J90 MWH. RTE WAS NOT XCHKED AFTER RECEIVING PDC AND WE NEGLECTED TO LOAD BLUIT 3 DEP IN THE FMC (BUT WE HAD BLUIT AS THE FIRST FIX AFTER SEA). BRIEFED USING LNAV AS ROLL MODE AT 400 FT. CAPT'S LEG TO FLY. DURING 10 MIN TAXI OUT; WE NEEDED TO GET PAX RECOUNT FROM FLT ATTENDANTS DUE TO DISCREPANCY BTWN FLT ATTENDANT COUNT AND MANIFEST FINAL NUMBERS. WE DID NOT NOTICE LACK OF SID IN FMC DURING BEFORE TKOF CHKLIST. AFTER TKOF WE ENDED UP TURNING DIRECT TO BLUIT PRIOR TO 5.5 NM FIX ON SID. DEP CTLR ON FREQ 119.2 ADVISED US THAT WE TURNED EARLY AND GAVE US A 030 DEG VECTOR FOR THE CLB. DEP CTL MADE NO OTHER COMS TO US REGARDING THE EARLY TURN. SUPPLEMENTAL INFO FROM ACN 509000: HAD FMC COURSE ON PNF (FO) SET 158 YET STAYED IN AUTO BECAUSE THOUGHT WAS BETTER FOR AUTO-UPDATING FOR FMC DEP. AFTER TKOF AT 400 FT; LNAV SELECTED AND FOLLOWED FMC WITHOUT ADEQUATE XCHK OF SID. OUR BUST. CLASSIC CASE OF CREW BEING SLIGHTLY MORE RELAXED DUE TO EXPERIENCE LEVEL IN COCKPIT. ASSUMPTIONS MADE THAT SHOULD NOT HAVE BEEN ASSUMING TOO MUCH. THE OTHER GUY HAS GOT IT. GOOD REALITY CHK FOR ALL INVOLVED.