In responding to a gCaptain private message I realized how the reg on ATB/ITB tyime interacts with another, the result is that it is possible to get an unlimited license, possibly without a tonnage limit, entirely with ATB time.
As noted in other dicussions, for the purpose of crediting sea time, an ATB is considered a “dual-mode” ITB. Per 46 CFR 11.211(d), time on dual-mode ITBs can be used at the aggregate tonnage of the tug and barge, credited at 1;2 (1 day credit for 2 days service) foir up to half of the required time on vessels over 1600 GRT.
46 CFR 11.402 requires all of the time for an unlimited license to be over 200 GRT, and half over 1600 GRT. If all the time is over 200 GRT, but less than half is over 1600 GRT, a tonnage limit is applied that is either the maximum tonnage that 25% of the time was acquired on, or 150% of the maximum that 50% was acquired on.
Assume you are a 3rd Mate and have service on an ATB where the push unit/tug is 1,200 GRT, and the barge is 8,000 GRT. 6 months of your time on the ATB is counted as 3 months service at the aggregate tonnage of 9,200 GRT. The remaining 9 months needed for upgrading to 2nd Mate can be from the ATB, counted day for day at the tonnage of only the tug/push unit, inthis case 1,200 GRT. You have only 3 months on vessels over 1600 GRT, so a tonnage limit applies. 25% of the time is the 3 months credited at the aggregate tonnage, so the tonnage limit on your 2nd Mate license works out to 9,200 GRT. Tonnage limits are rounded up to the next highest increment of 1,000 GRT, in this case 10,000 GRT. There are no tonnage limits at or above 10,000 GRT, so you get an unlimited 2nd Mate without a tonnage limit. But it will take 15 months of time.
You may now be thinking that if you use 12-hour days at 1.5 to 1, you can get the 2nd Mate in LESS time than the conventional vessels. Probably not. To get the 1.5 to 1 credit, the 12-hour day must be “authorized and practiced.” Most ATBs have 3 watches, so the 12-hour day is not practiced, and if the manning requirements correspond to 3 watches, it’s not “authorized.”