What Master 100 unlocks
Master 100 GT lets you operate USCG-inspected passenger vessels (Subchapter T or K) up to 100 gross tons. Whale-watch boats, dinner cruises, larger fishing party boats, and ferries fall under this credential. Without it, you are capped at six passengers under OUPV.
Sea-service requirements
720 days of sea service for Master 100, of which 360 must be on vessels over 50 GT. (For Master 50: 720 days with 360 on vessels of similar size; for Master 25: 360 days. For lower tiers, time on smaller vessels can substitute.) Near-Coastal Master 100 also requires 360 days of ocean / near-coastal time.
Exam differences vs OUPV
All four OUPV modules plus a Plotting (chart-work) module and a tightened General module on regulations. The Plotting module is the new ground for OUPV upgraders — running fixes, set & drift, danger bearings, magnetic variation/deviation. Pass mark on Plotting is also 70%.
Endorsements
Master 100 can carry Towing endorsement (separate exam, six months tow service required) and Sailing endorsement (additional sail service required). These are often added later as the candidate's career develops.