1 day for Berg Eltz and Koblenz to Bacharach River Tour?

You will save time if you keep Koblenz OUT of your cruise and if you cruise from south to north (faster.) Also, keep the cruise short - Arrive in Bingen early from Frankfurt or wherever you are and catch the 9:30 boat out of Bingen to St. Goar (10:55) - that is the most scenic section - then proceed to Moselkern by train (via Koblenz) and do the walk to - note spelling - BURG Eltz - (Burg = castle, Berg = mountain) and back. On your return trip by train, stop off and see one or more of the west bank towns - Boppard, Oberwesel and Bacharach are each a little different but worth seeing. Trains run all hours of the day and make it much easier to stop off than if you were to try this by boat (since there are only a few boats per day.)

The other option is to see Marksburg Castle instead of Burg Eltz. Marksburg is terrific as well - you can STAY on that 9:30 boat all the way to Braubach (also a beautiful old town) where you'd arrive at 12:20 and walk to the castle on foot in 20 minutes. This way you'd have more time to explore those west bank towns I mentioned above. Braubach's old town is a good place for a meal.

By the way, while we pronounce Burg and Berg the same, in German they are not just spelled differently but are also pronounced VERY differently.

Burg - Say Boo-uck (quickly to make it sound like one syllable.)
Berg - Say Beh-uck (in the same way.)

Russ, great phonetics lesson.

" I'm trying to find out if I can cram in Berg Eltz and a Rhine River trip from Koblenz to Bacharach in the same day."

Here is the K-D Rhein river schedule.

" Does that mean there is no time to check out the towns along the way?"

Yeah, pretty much so.

I would never do Koblenz to Bacharach. That's not the best part. Just do St. Goar to Bacharach (actually, it's better to do Bacharach to St. Goar, with the current. Faster.).

This is great advice. So I do the river cruise FIRST on the way to Eltz. The hike to Eltz is also part of the appeal for me. Then stop at any of the towns on the train ride back if I still have time.

I'm still a bit worried if this is too much to tackle on my first full day in Germany. And I have some work to do the next day. How long would that river trip take?

The river trip from Bacharach to St. Goar takes 40 minutes. According to the Bahn, the walk from the train station in Bacharach to the KD dock takes about 3 minutes; according to Google Maps it takes 7 minutes. Having walked it, I'm inclined to agree with Google. In St. Goar, the train station is just up the hill from the dock.

If you only ride the ship between Bacharach and St. Goar you will have missed a major part of the beautiful scenery and lots of castles.
Train from Frankfurt to Rüdesheim, then ride the ship from here to Braubach or St. Goar or Boppard. Use the trains for the remainder of your sightseeing. For me, Burg Rheinfels is the most stunning fortress on the Rhine and the most interesting.

It's just that way with advice... you now have 3 different plans from 3 different posters.

We all agree on this for a trip to Eltz:
"...do the river cruise FIRST on the way to Eltz. Then stop at any of the towns on the train ride back if I still have time. "

I am on the same page with Jo on this:
"If you only ride the ship between Bacharach and St. Goar you will have missed a major part of the beautiful scenery and lots of castles." It takes just 40 boat minutes to enjoy the Bingen-Bacharach segment as well (otherwise you can try to catch a few blurred images from the train window - the train trip takes about 10 minutes.)

But I would not board in Rüdesheim for 2 reasons. 1.) The extra 15 minutes on the boat do not offer additional sightseeing since that time is spent crossing the river to Bingen only. 2.) The hourly trains that get you to R'heim arrive inconveniently in Rüdesheim at :04 minutes after the hour (if they are on time.) The KD boats depart at :15 after - this isn't an adequate window to get off the train, get yourself to the KD dock, buy a ticket and board. This would mean leaving Frankfurt at 6:53 a.m., arriving in R'heim at 8:04, then waiting an hour for your 9:15 boat departure. IF YOU BOARD IN BINGEN you get to have breakfast... you leave Frankfurt at 7:47 and arrive (after one change of train in Mainz) at the Bingen Rhein Stadt train station at 8:59 - which leaves you 31 minutes until your 9:30 boat leaves the dock.

Whether you take the train from Frankfurt to Rüdesheim at 6:53 or to Bingen at 7:53, the cruise boat will not be any faster, and you won't get to Eltz any earlier.

Boppard, St. Goar, and Braubach are all great places to visit and you could have a great day without going to Eltz. But with your tight day and your goal of visiting Eltz, I side with Lee on getting off in St. Goar. You've cruised the best part - now get yourself to Eltz ASAP. You will have about 30 minutes in St. Goar for a quick look around and maybe a snack. Board the next train and you should be in Moselkern just before 13:00. Do you have this walking guide to Eltz?