Am I reading the KD cruise timetable correctly?

That is correct. You have to change boats at St Goarshausen to continue to Boppard. It is a 1 hour 40 minute change time.

That's what it means.

You can either wait at the St Goarshausen dock for the boat to Boppard at 7:40 (arriving Boppard at 18:30) or take the ferry across to St Goar and walk up the hill, 250m, to the station. From there trains leave at 16:38, 17:00, 17:45, and 18:00. Those trains will get you to Boppard in about 12 minutes, sooner than the 17:40 boat will get you there. I personally don't think the scenery between St Goar and Boppard is worth the 1H40 wait.

Or, there is a little ferry across the Rhein from the right bank (in the middle of nowhere) to Boppard. Since you are right there in St Goarshausen, you could take the train two stops, 11 minutes to Kamp-Bornhofen and walk 2.2km, 26 minutes, down the highway to the ferry, or you could go one stop farther, 14 minutes, to Filsen and walk back 950m, 12 minutes, to the ferry.

Yes, and be aware that they may make you pay separately for each boat. In June, we wanted to go from Bacharach to Boppard, changing boats in St. Goar. They would not sell us a ticket all the way to Boppard. We had to buy two separate tickets: Bacharach to St. Goar, then St. Goar to Boppard. Which basically doubled the price we expected to pay.

I don't know offhand what the ferry costs, but the train fare from St Goar to Boppard is 4,60€/P.

Just looked up the Loreley ferry, it's 1,70€ per adult.

The train fare from St Goarshausen to Kamp-Bornhofen or Filsen is also 4,60€/P.

Robert: The K-D boat that arrives in St. Goarshausen at 16:00 WILL TAKE YOU TO ST. GOAR IMMEDIATELY AFTERWARD.

The section you are cruising uses ONE boat that cruises downstream and upstream between St. Goar and Rüdesheim all day long. So that 14:15 boat from R'heim has to be in St. Goar by 16:05 in order to begin the 16:10 UPSTREAM journey back to Rüdesheim.

(It is unfortunate that the KD schedule is so unclear about this process. But you can see that 16:10 departure from St. Goar by looking at the upstream schedule for St. Goar > Rüdesheim.)

To reach St. Goar (and go to Boppard by train) you do not need to use the St. Goar ferry crossing. Just buy a cruise ticket from Rüdesheim to St. Goar and stay on the boat when it drops off passengers in St. Goarshausen.

To catch the later boat to Boppard, you would do the same thing. Step off in ST. GOAR rather than St. Goarshausen and spend your downtime there. The 17:30 boat will take you to Boppard.

https://www.k-d.com/fileadmin/KD_Fahrplaene/KD_Fahrplan_intern_2023_final.pdf

One thing the page above is very clear about... at the very bottom is the fare chart, Rüdesheim > Boppard is an "H"-level fare priced at €36. So I simply do not understand travel4fun's comment that a single ticket for R'heim > Boppard at this price might be unavailable at the ticket kiosk in Rüdesheim.

Not positive... but it is possible that a €25 ticket for R'heim > St. Goar was purchased and used... and while the R-heim > Boppard fare is €36, there is no €11 ticket available to supplement that first ticket for the later boat ride to Boppard. (In other words, the purchase of the first ticket to St. Goar required the purchase of the second ticket to Boppard.) In this case, KD will likely only sell the standard "C" level ticket for St. Goar > Boppard at €15, as the fare chart indicates.

A second possibility??? Maybe the first cruise was with Bingen-Rüdesheimer, which does NOT cruise to Boppard - and the second cruise was with K-D?

Thanks all!