B737-300 FLT CREW HAS A TRACK HDG DEV DURING THE BOACH 2 RNAV DEP FROM RWY 25R LAS.

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WE WERE CLRED ON THE BOACH DEP BY CLRNC DELIVERY AT LAS. PROGRAMMING OF THE FMC WAS NORMAL AND RTE WAS VERIFIED FOR THE DEP. FMC UPDATE WAS ACCOMPLISHED TAKING THE ACTIVE (RWY 25R) FOR DEP AND THE NAV SWITCHES WERE PUT IN THE NAV POS. TKOF WAS NORMAL. AT 400 FT; LNAV WAS SELECTED. INITIAL STEERING GUIDANCE WAS A SLIGHT R TURN. THE FO STARTED TO FOLLOW THE GUIDANCE. REALIZING THE GUIDANCE WASN'T CORRECT; I SELECTED THE LEGS PAGE; SELECTED RBELL TO THE BOTTOM OF THE PAGE AND BACK UP. I PUSHED THE 'EXEC' BUTTON TO GIVE US STEERING GUIDANCE TO RBELL. THE STEERING GUIDANCE GAVE US A SLIGHT L TURN WHICH APPEARED TO BE CORRECT. ON CHK IN WITH DEP; THEY ASKED IF WE WERE CORRECTING BACK ON COURSE AND WE ADVISED THAT WE WERE. AT THIS POINT WE WERE PASSING 1000 FT. WE SELECTED AUTOPLT AND CLEANED UP THE ACFT. THE ACFT BEGAN A L TURN THAT SEEMED APPROPRIATE FOR THE DEP; HOWEVER; IT TURNED PAST THE HDG FOR ROPPR WHICH IS THE NEXT POINT ON THE DEP. I RESELECTED THE LEGS PAGE AND NOTED THAT THE ANP SHOWED AN ERROR OF APPROX 10 NM. REALIZING THAT WE COULDN'T FLY THE RNAV DEP WITH THAT ERROR; WE ADVISED DEP CTL THAT WE WERE UNABLE THE RNAV DEP AND REQUESTED VECTORS. THEY GAVE US A VECTOR TO JOIN THE MCCARRAN 3 DEP; WHICH WE DID. ONCE ESTABLISHED ON THE DEP; I RECHKED THE ANP ON THE LEGS PAGE AND IT WAS APPROX .3 NM. I RECHKED IT PERIODICALLY THROUGHOUT THE FLT AND IT STAYED WITHIN LIMITS. I DON'T KNOW WHY WE HAD THE ANP ERROR WE DID ON DEP. THE ROUTING FOR THE DEP WAS CHKED PRIOR TO DEP AND THE RWY UPDATE WAS ACCOMPLISHED; BUT THERE WAS A SIGNIFICANT POSITIONAL ERROR ON DEP. IN THE FUTURE; WHEN DEPARTING FROM ARPTS THAT REQUIRE AN RNAV DEP; I WILL QUICKLY CHK THE ANP ON THE LEGS PAGE PRIOR TO TKOF. SUPPLEMENTAL INFO FROM ACN 696577: THE DEP'S FIRST FIX WAS RBELL; WHICH SHOULD HAVE BEEN STRAIGHT AHEAD 6.2 NM. I INITIALLY HESITATED TO TURN AND INFORMED THE CAPT THAT SOMETHING WAS WRONG. HE RESELECTED RBELL ON THE LEGS PAGE AND EXECUTED. I FLEW THE FLT DIRECTOR COMMAND BARS; HOWEVER; IT OVERSHOT THE FIRST FIX. AT THIS POINT TWR ASKED IF WE WERE CORRECTING TO GET BACK ON COURSE. LOOKING DOWN; I NOTICED ON THE LEGS PAGE; THAT THE RNP .3/ANP 9.7.