As Lee says, the 9:15 from Bingen (which is actually the 9:00 from Rüdesheim) and the 14:30 from Bingen (the 14:15 from Rüdesheim) do not stop in St. Goar enroute to St. Goarshausen.
However, recent cruisers on these NORTHBOUND boats have reported that after stopping in St. Goarshausen at 10:45, the boat then crosses the river and stops in St. Goar. This in fact is what's happening. KD Schedule:
Take a look at the SOUTHBOUND boat schedule from St. Goar. In the column on the far left, do you see the 10:55 departure time? And do you see the 10:45 departure from St. Goarshausen just above it in the same column? The same northbound boat that arrives in St. Goarshausen at 10:45 turns around there and becomes a SOUTHBOUND boat - with its first stop in St. Goar.
The 14:15 boat from Rüdesheim does the same thing... ride to St. Goarshausen (16:00) then stay on to make a St. Goar exit (16:10.) Then the boat continues south all the way to Rüdesheim (19:00.)
Can we be sure it's the same boat? Yes. The page below shows which boats and docks are active this week and which not. The 5th row of boxes down addresses the boat in question - the "MS Boppard", which is abbreviated as "BO." And the corresponding routes/schedules are attached to this boat:
On Saturday the 10th, the 16:00 boat from St. Goarshausen is canceled (which means the 16:10 from St. Goar is as well.) "Fällt aus!" means canceled.
On Sunday the 11th, the 9:00 "BO" boat from Rüdesheim is canceled.