K-D Rhine River Cruise

Beyond Boppard, the cruise is reportedly much less scenic (I never went beyond Boppard for that reason).
Conversely, the stretch upstream of Bacharach, from Bingen to Bacharach, is also very scenic.
I'd take the train from Bacharach to Bingen, start the cruise from there to Boppard, then head back to Bacharach by train. A roundtrip holds little interest for me, and the upstream direction is much slower on the boat.
Of course, you could stay all the way to Koblenz if you want to visit Koblenz, but then just take the train back upstream.

You could ask your hotel if they can pack you a lunch. But there are shops in Bacharach to get foodstuffs. There's also food on the ship itself, as I recall.

That's the way I read the schedule, but it might even be the same ship, just turning around. Dont worry, you can always make it back by train/ferry if your schedule gets off.

"I think it says we can leave Bacharach at 10am and arrive in Koblenz around 11:45"

Nope. you are confused because it's a confusing schedule. Board in Bacharach at 10, and the cruise ends in St. Goarshausen at 10:45. And you will have seen only HALF of the best part of the river. You should do as the previous poster indicated - take the train to Bingen, start there (9:15) and then cruise to St. Goarshausen (10:45, just opposite St. Goar on the river.)

Whatever section of the Rhine you think might be good for cruising, the KD cruise schedule is sending you a message. (Well, it's a poor communicator, as you know.... here's the deal...) The reason KD has only one cruise per day that continues as far as Boppard is NOT that they don't like to make money on that part - it's just that the demand for that part of the river between St. Goar and Boppard is lower; the part from St. Goar south is better. The cost/benefit relationship deteriorates from Boppard to Koblenz as well.

Another big cruise provider is Bingen-Rüdesheimer. They don't have any boats at all to Boppard or to Koblenz.

So I suggest you do the best part (Bingen > St. Goarshausen for example) and then hop on a train for Braubach (22 minutes away) and a tour of Marksburg Castle. It's by far the most interesting Rhine castle experience IMHO, and Braubach is picture perfect.

Braubach, half-timbered houses
Marksburg
Marksburg tour

Braubach is a great spot for lunch:
https://farm5.staticflickr.com/4143/4751924826_98980aa49f_o.jpg

Return to Bacharach by train via Koblenz (10 minutes from Braubach)

Next year's KD schedule might or might not get back to the normal 5 boats per day from Bingen. It's only 4 this year during high season for Covid reasons.

Thank you all so very much. This is extremely helpful!!

The boats have full menus, including alcohol, if that helps.

Go Bingen to Boppard on the boat. Visit other towns by train.. Don’t go upstream ( to the south ), it takes too long and the scenery is the same no matter which way you go !

You will miss 2/3rds of the castles if you only cruise from Bacharach to Koblenz. Go all the way down the river to Bingen and ride the ship up to St. Goar. Take the train back to Bacharach.
There is food and wine on the ship.

If you plan on ordering food or drinks on board the KD boat, be sure to bring cash. It was one of the few places I went to in Germany that did not accept credit cards.