What Master 100 unlocks#
Master 100 GRT lets you operate USCG-inspected passenger vessels (Subchapter T or K) within the route and 100 GRT tonnage limit on the MMC. Whale-watch boats, dinner cruises, larger fishing party boats, and ferries can fall under this credential. Without a Master endorsement, the OUPV lane is capped at six passengers for hire on an uninspected vessel. If the unresolved question is whether the trip is private, bareboat, or passenger-for-hire in the first place, run that through the captain's-license trigger guide before comparing Master tonnage. Master authority is federal and identical in every state; for the nearest USCG Regional Examination Center (REC) and local charter hubs, see the state captain's license guides.
Sea-service requirements by route#
Master less than 100 GRT is route-specific. The Near-Coastal route requires 720 days of deck-department service on ocean, near-coastal, or Great Lakes waters; inland-water service may substitute for up to 360 of those days. The Great Lakes/Inland route starts from 360 total days, and Great Lakes authority requires 90 days on Great Lakes waters. Current NMC checklists also apply a recency screen of 90 days in the past 7 years. For exactly how the USCG counts a service day, credits Great Lakes and inland time, and records it on CG-719S, see the full USCG sea time requirements guide — or put your vessels straight into the sea time calculator to see which tonnage band your service supports.
How the 100 GRT limit is calculated#
The Coast Guard does not award 100 GRT just because the course or page says Master 100. Under 46 CFR 11.422, the tonnage limit is based on the size of vessels in your qualifying service. The current NMC Near-Coastal checklist shows the 100 GRT limit at either 180 days on vessels of 51 GRT or above, or 360 days on vessels of 34 GRT or above. The Great Lakes/Inland checklist uses 90 days on 51+ GRT or 180 days on 34+ GRT because that route has a 360-day base requirement. If your boat-size history is smaller, the endorsement may be issued at 50 or 25 GRT instead.
How big is a 100-ton vessel? Gross tonnage is volume, not weight#
A 100-ton captain's license is measured by volume, not weight. Under 46 CFR 69.9 a "register ton" is a unit of volume equal to 100 cubic feet, and tonnage is defined as the volume of a vessel's enclosed spaces — so a 100 gross register ton (GRT) vessel encloses roughly 10,000 cubic feet of measured space, not 100 tons of displacement. That is why boats that look modest by length can still carry a 100 GRT number, and why two vessels of the same length can measure very differently once superstructure and enclosed volume are counted.
Because gross register tonnage is volumetric, there is no fixed length that equals 100 tons. The Coast Guard prints the tonnage limit from the vessel's measured gross register tonnage on its documentation (46 CFR 69.209), not from feet or weight. As a rough working range, a 100 GRT inspected passenger vessel is often in the mid-60-foot to roughly 100-foot class depending on hull form and how much enclosed volume the design has — but always confirm the number on the vessel's tonnage documentation rather than estimating from length. The same volume rule sets the limit on your credential: the 100 / 50 / 25 GRT band comes from the measured tonnage of the vessels in your qualifying service, as covered in the calculation section above and 46 CFR 11.422.
Exam differences vs OUPV#
The Master less-than-100 GRT exam (e.g. ONC06 Near Coastal) covers the same topic ground as OUPV but splits Deck General/Safety into two separate modules (Q160 Deck General and Q161 Deck Safety) and tightens the regulations content. Like the OUPV exam, it includes the 90%-pass Chart Plot module (Q163) — running fixes, set & drift, danger bearings, magnetic variation/deviation — so plotting is shared ground with OUPV, not new: an OUPV holder has already passed a Chart Plot module (Q172 Near Coastal / Q358 Inland). What raises the bar is the broader question bank and the higher sea-time requirement.
For Rules prep, treat COLREGs Rule 34 maneuvering and warning signals as a checkpoint before scheduling: it is where one-short, two-short, astern-propulsion, overtaking, and danger-signal questions cluster. Then compare the Master modules against the USCG captain's license exam format guide and decide whether a USCG-approved captain's license course or the REC self-study path fits your plotting needs. The USCG exam prep guides hub keeps the ATT letter, pass marks, retest window, and module sequence in one place.
Endorsements#
Optional add-ons are route- and service-specific. For sail or auxiliary sail authority, 46 CFR 11.428 requires 12 months of sail or auxiliary-sail service on the Near-Coastal route, while 46 CFR 11.455 requires 6 months for Great Lakes/Inland. Commercial Assistance Towing is different from a towing-vessel officer endorsement: under 46 CFR 11.482, an OUPV, Mate, or Master needs an Assistance Towing endorsement to perform assistance towing unless an exception applies, and the add-on is earned by passing a written exam or completing a Coast Guard-approved course — not by a blanket six-month tow-service requirement. The authority still stays inside the scope of the underlying MMC.