46 CFR § 10.232 - Sea service.
(h) Day.
(2) On [vessels] authorized by [46 U.S.C. 8104]) and [46 CFR 15.705], to operate a two-watch system, a 12-hour working [day] may be creditable as 1 1/2 [days] of service.
So problably that tug is not authorized.