"Plan to use the KD to travel the rhine from sankgoarshausen to bacharach and back on one of the half day cruises."
Agree w/ Jo - the most scenic part is Bingen/R'heim to St. Goar/St. Goarshausen (1.5 hours.) If you cruise St. Goarshausen-Bacharach-St. Goarshausen, you see only half of the best part... and to do that, you spend 1.3 hours one way to Bacharach (very slow going in that direction because of the slow current) and 40 minutes back to St. Goarshausen through the exact same part you just saw.
So what makes sense from St. Goarshausen is to take the TRAIN first to Ruedesheim (around 25 minutes) and cruise back to St. Goarshausen. If you want to revisit Bacharach on the way, you can do a 1-2-hour cruise stopover there (check the limited KD schedule to make sure that's feasible) OR you could just start your day by crossing over to St. Goar on the ferry boat (runs all day long) and then break up your train trip south to Bingen (where you'll board the cruise boat) in Bacharach for 1-2 hours. Alternatively, visit a town you perhaps missed last time - Oberwesel is on the same St. Goar - Bingen train line and has a nice old town wall walk that you could easily do with 2 hours there:
http://www.oberwesel.de/en/wandern0/stadtmauerrundweg/
Using the train to get to Bingen or R'heim means of course that you don't pay any extra fee for the round trip cruise. And the train fare (E6.20) won't really cost you E6.20 additionally; KD offers a 20% cruise discount if you use the train to reach the dock. Just show your train ticket and you'll get about E4 off the one way cruise fare. So it's about E2.00 on top of the one way cruise fare - about the same as the additional cost of return supplement for the round trip cruise.
I haven't stayed in St. Goarshausen but I don't see a big problem with that... you are only a few hundred meters from St. Goar by foot + ferry boat whenever you like, and if it's OK to stay in St. Goar, it should be OK to stay just across the river, which is a LOT closer to St. Goar's town center than many inns with a St. Goar address. And St. Goarshausen is only 22 train minutes from Braubach, home of Marksburg Castle, the only intact medieval Rhine Castle you can tour.