AN SMA PENETRATED SEA CLASS 'B' AIRSPACE.

Narrative

RNT TWR USED NON STANDARD TERMS FOR DEP TO N FOR VFR XING OF SEATAC. WE WERE CLRED FOR L DOWNWIND DEP ON RWY 33. RNT TWR WAS ADVISED TWICE OF INTENTIONS TO CROSS SEATAC. NO INSTRUCTIONS WERE GIVEN TO US TO CONTACT SEATAC TWR AND NO MENTION OF S SOUND DEP PROC. WE PROCEEDED S ON 150 HDG EXPECTING RNT TO SAY SWITCH TO SEATAC. ACROSS FROM SEATAC; WE CALLED RNT AND ASKED TO SWITCH TO SEATAC. SEATAC WAS UNPREPARED FOR US AND TOO BUSY FOR IMMEDIATE XING. BY THIS TIME WE WERE WELL S OF SEATAC AND ELECTED TO CROSS UNDER THE B SPACE. WE TURNED W AT 6.1 DME TO CROSS UNDER AND WERE ENJOYING THE VIEW WHILE MONITORING SEATAC TWR. DIRECTLY S OF THE FIELD; WE HEARD SEATAC HOLD DEPARTING TFC FOR TFC IN THE DEP CORRIDOR. I LOOKED AT THE DME AND REALIZED IT WAS US; AT 5.7 DME. IN TURNING W FOR THE SHORELINE. I INADVERTENTLY CUT OFF THE ARC OF CLASS B SPACE. WE IMMEDIATELY TURNED S TO CLR THE SPACE. RNT TWR SHOULD HAVE TOLD US WHEN TO CONTACT SEATAC. WE DID NOT ASK FOR S SOUND DEP SINCE WE DID NOT HAVE A COPY OF IT IN OUR CHARTS.