The Merit U18 Boys Youth Cup
Whitstable made the most of their early dominance, Euan Sallis setting up Underhill for a close-range opener on 18 minutes.
Within a minute it was 2-0 as Dexter Lee rounded keeper Bobby Crabb and slotted home into an empty net.
Whitstable had goalkeeper Henry Duley sent-off for a professional foul with 20 minutes left when he halted Blijaris Tambe’s progress outside the penalty area.
Chatham pulled a goal back with five minutes left when Lewis Bain scored against stand-in keeper Sallis but they were unable to force extra-time despite their numerical advantage.
Read the full report and reaction on the kentonline.co.uk website here.