DEPARTING BE20 CREW STARTED TURN ON COURSE INSTEAD OF MAINTAINING RWY HDG.

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INCIDENT OCCURRED ON THE INITIAL CLB PORTION OF THE KENT 3 SID FROM BFI; SEATTLE; WA. WE HAD BEEN CLRED TO CCR ARPT VIA THE KENT 3; J-1 TO BATTLEGROUND THEN DIRECT CCR. AFTER TKOF; THERE WAS SOME CONFUSION IN THE COCKPIT ABOUT THE DEP HDG TO INTERCEPT J-1 AND THE ACFT HDG INADVERTENTLY DRIFTED APPROX 30 DEGS R FROM RWY HDG (130 DEGS). SHORTLY AFTER ESTABLISHING CONTACT WITH SEATTLE DEP; THE CTLR QUERIED US ABOUT OUR HDG AND DIRECTED A TURN TO THE E TO 070 DEGS. DEP INSTRUCTIONS WERE VERIFIED WITH THE CTLR AND THE REMAINDER OF THE SID WAS FLOWN WITHOUT INCIDENT. NO KNOWN TFC CONFLICTS WERE ENCOUNTERED. CAUSES: INCIDENT WAS CAUSED BY THE PIC MISREADING THE DEP PLATE; RESULTING IN THE HDG CONFUSION AND SUBSEQUENT HDG DEV. FACTORS: THE FOLLOWING FACTORS CONTRIBUTED TO THE INCIDENT: 1) LACK OF FAMILIARITY WITH DEP AIRFIELD AND LCL ENVIRONMENT. 2) PRE-SUNRISE DEP. 3) INADEQUATE REVIEW AND BRIEFING OF SID BY BOTH CREW MEMBERS. CORRECTIVE ACTION: COMPANY SOP'S REVIEWED AND CHANGED TO REQUIRE SID CHALLENGE AND RESPONSE ACTIONS FROM BOTH CREW MEMBERS DURING PREDEP BRIEFING.