Thank you for the report. Checking the file versions history, this appears to date back to when these boats were first added to the pakset many years ago: the 30t weight seems to have been copied from the Thames Sailing Barge, which has the same 75t capacity. I have set the weight of all of the tug barges to 27.5t on account of them needing less weight than a sailing vessel. If you know the weight more accurately from an historical source, please let me know, and I will correct it.
As to the Blackwall Frigate, I think that that may have been deliberate owing to its particular characteristics, but I am afraid that I cannot now recall the details.