Hi Larry, I believe that you are very miss informed as regards what Ship Finder does and does not do. First of all, while I agree it is not meant for real navigation, it still is an additional, emphasis on additional navigation tool, that happens to work only on the iPhone and iPad. Ship Finder acts like an AIS receiver. Nothing less and nothing more. As regards receiving AIS transmissions, Ship Finder does not care whether it is Class A or Class B.
This point is proven very easily by looking at Ship Finder at Portsmouth, NH. This is my own broadcasting station and it picks-up anything that is within receiving range. Commercial, recreational and USCG vessels.
Next go to Tampa Bay, where I was this past week to set-up another station. Again, you will see a mix of A and B transmissions.
HOWEVER, the point that needs to be made clear is, that Ship Finder and any other online display of AIS traffic must rely on AIS data feeds! These feeds are privately owned, most of them anyways. If the owner does not share his data, by sending it to Ship Finder / Marine Traffic etc., then you see a lot of nothing.
AIS broadcasts have a limited range. To cover a range of, let's say Newburyport to Portsmouth, something you can relate to, being where you are, would require multiple AIS broadcasting stations.
Presently, mine is the only one online, in Portsmouth. So, there are many holes along all coasts.
But, again, one important fact to point out here for everyone is, that an AIS broadcasting station does transmit A or B traffic. There is no filtering going on and if there is a signal, then you will see it. A, B or both.
Cheers,
Ronald