Day Rhine cruise- To or from Bacharach

I don't know if I would even want to spend a ½ day cruise. That's an awful long time to spend on a boat. Even down the river (NW), with the current, it's an awful slow trip. The scenery doesn't move by very fast. Suggestion: start in Bacharach and go down the river (40 min) to St. Goar. On the way, you'll get to see the mid-river toll castle, Pfalzgrafenstein, and the Lorelei. Get off in St. Goar and see Rheinfels castle. If you still want to spend more time on a boat, take next boat on to Braubach or the train to Boppard and the K-D boat from there to Braubach and see the Marksburg castle. If you've had enough boat travel, take the ferry across the river to St. Goarshausen, then the train down to Braubach.