Medium Transport; High Wing; 2 Turboprop Eng
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 9.5
Confusion; Distraction; Situational Awareness; Training / Qualification
ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Air Traffic Control Separated Traffic
Procedure; Human Factors
Aircraft X departed straight out on the Seattle 4 departure and Aircraft Y was put into position by LCE (Local Control East). As Aircraft Y was going into position S46 called and stopped our automatic turns to 020 heading and I told LCE expecting them to cross aircraft in front of the departure with both aircraft now being straight out instead of turning the second aircraft. LCE departed Aircraft Y and issued visual separation instructions to the pilot which he accepted.  After Aircraft Y became airborne I realized that they were going faster and out climbing Aircraft X; I told the LCE trainee that they needed to do something with Aircraft Y; LCE then stopped Aircraft Y at 2000 ft. I then called departure Sector A and told them that Aircraft Y was stopped at 2000 ft and was told 'I would expect so' by the departure controller. As I was on with the departure controller I incorrectly assumed LCE had climbed Aircraft Y to 2100 ft because the altitude readout on the STARS showed the aircraft at 2100 ft and they were approaching a 2100 ft MVA area North of Boeing Field.
Aircraft X departed SEA climbing straight out climbing to 120. Aircraft Y departed SEA climbing straight out climbing to 030; with +VV in the secondary scratchpad. Almost immediately Aircraft Y began speed overtake of Aircraft X. LA (Local Airport) called me on Dep A to coordinate and advise Aircraft Y was stopped at 020 due to the overtake with Aircraft X ahead with the same departure procedure. At this point Aircraft Y was nearly directly beneath Aircraft X. I immediately issued a turn for Aircraft Y to establish divergence so I could climb the aircraft.It appears the tower controllers at SEA are unfamiliar with aircraft characteristics. If this continually reoccurring problem cannot be trained and fixed I would recommend discontinuing to allow SEA to use visual separation between departures; and instead make them use standard IFR separation. This problem could be remedied by just taking aircraft characteristics into account prior to launching IFR aircraft.
Seattle Tower and Seattle TRACON controllers reported of an operation where the second departure overtook the first departure. Controllers used altitude to separate traffic.