FAILED TO COMPLY WITH SID REQUIRING TURN AS SOON AS POSSIBLE.

Narrative

ASSIGNED SFO 3 DEP ON ORIGINAL CLRNC; CHANGED TO SHORELINE 8 DURING TAXIOUT. ATIS RPTED 1700' CEILING; 20 NM VIS; ACFT WEIGHED 345000#; 2000# UNDER MAX GROSS REQUIRING RWY 28R FOR TKOF. FLT WAS CLRED INTO POS ON RWY 28R TO HOLD. A COMMUTER WAS THEN CLRED FOR TKOF ON RWY 28L; FOLLOWED BY TKOF CLRNC ON RWY 28R. AT 300-400' THE TWR OPERATOR REQUESTED A TURN TO A 040 DEG HDG. WE COULD NOT ACCEPT THE TURN DUE TO ACFT WT AND PERFORMANCE W/O SEVERELY JEOPARDIZING SAFETY. THERE WAS A BROKEN LAYER OF CLOUDS AT 800' WHICH WE WOULD HAVE ENTERED IN THE TURN WITH 1000' TERRAIN; 3.5 NM NW. WE HAD VISUAL CONTACT WITH THE RWY 28L DEP; BUT WERE UNABLE TO INTERRUPT A CONSTANT STREAM OF TWR COM TO ADVISE. WE DID TURN TO 295 DEGS TO ASSURE ADEQUATE SEP FROM RWY 28L TFC. OUR FAILURE TO ACCEPT THE TURN TO 040 DEGS CREATED CONCERN OVER SEP FROM THE RWY 28L DEP. THE SHORELINE 8 DEP REQUIRES A CLB OF 425' PER NM WITH WHICH WE COULD COMPLY. IT ALSO REQUIRES A TURN 'AS SOON AS FEASIBLE' TO AN ASSIGNED HDG. IT IS NOT FEASIBLE TO COMMENCE A TURN AT 300-400'; OUR SINGLE ENG LEVEL OFF HEIGHT IS 720' ON A 295 DEG HDG. THIS DEP PROC IS ILL DEFINED AND NEEDS TO BE MORE SPECIFIC. IT SHOULD READ THAT IF UNABLE TO ACCEPT A TURN AT A SPECIFIC ALT; TO REQUEST A STRAIGHT OUT PROC.