A321
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 1
Situational Awareness; Communication Breakdown
ATC Issue All Types; Conflict Airborne Conflict; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Separated Traffic; Air Traffic Control Issued New Clearance
Company Policy; Aircraft
It was an overcast day with ceilings too low to do pilot to pilot Visual Separation; but I could still see aircraft some ways out to achieve Tower Visual Separation. Departed Aircraft X (A321) and then next in line departed Aircraft Y with what would normally be plenty of space to achieve 3 miles of separation. Aircraft X's speed on departure was not uncommonly slow immediately off the departure end; but then did not accelerate at all which is not typical. They stayed constant at about 170 knots.When Aircraft Y tagged up I saw that there was not going to be 3 miles of separation immediately; but still had some room to achieve Tower Visual Separation. So I was still thinking Aircraft X should be speeding up at any moment and that 3 miles of separation would develop before I lost Visual. I held on to Aircraft Y and was going to wait for required separation to develop. I then pretty quickly realized that Aircraft X was showing no signs of speeding up and decided to cap Aircraft Y's altitude. At the time Aircraft Y was showing either 2400 feet or 2500 feet and I instructed them to maintain 3000 feet. In response Aircraft Y said that he would descend to 3000 feet as he was actually at 3300 feet at the time. I observed its Mode C was indicating him at 2500 feet. At that point my Local Assist asked if I was going to leave them at 3000 feet and ship them to departure. I said yes and he called over to coordinate it. During the attempt to coordinate the departure controller was combative; unprofessional; and completely unhelpful and made no attempt to maybe help resolve the situation. After coordination had been made I shipped Aircraft Y to departure.
SEA Local Controller reported an A321 climbed at a slower than normal and expected airspeed resulting in the subsequent departure overtaking the A321.