how to take a boat trip on the Rhine

You can find schedules and fares at www.k-d.de. You can buy your ticket right at the kiosk at the boat dock. Because of the strong current, St. Goar to Bacharach takes 30 min longer (70 min) up the river than down (40 min), so if you like boat trips, go from St. Goar to Bacharach. If you just want to see that stretch of the river, go from Bacharach to St. Goar. I went up the river and was so bored I couldn't wait to get to Bacharach. The fare is the same either direction. There are trains back to your start. The part between St. Goar and Bacharch, IMO, is the most interesting. You go under the Loreley, and right next to the mid-river toll castle, the Pfalzgrafenstein.

Do travel south to north, as Lee suggests. The Bacharach-St. Goar route is a mere 40 minutes. If you're interested in something a bit longer, I'd suggest starting in Bingen - where the impressive cliffsides house two castles and the Niederwald monument - and then continuing to St. Goar, a stretch that is also very picturesque and will add 7 or 8 castles/castle ruins and one hour to the experience. A second alternative would be Bacharach to Braubach (about 2 hours) which would deposit you at the foot of Marksburg Castle, the very best castle to tour along the Middle Rhine. The morning boats arrive at 11:20 and 12:20, giving you just enough time for a quick look around town or a meal before the 12 or 1:00 tour. The A 12:00 tour in English is routine in summer: www.marksburg.de While it's possible to do the whole area above - from Bingen to Braubach - that involves nearly 3 hours, and there is an element of tedium that sets in after a while. For me, two hours on the cruise is tops. Remember that the whole river has passenger trains on both sides which offer up nearly the same scenery much more quickly and let you hop on and off and explore the villages much more easily - the boat schedule makes that kind of exploration impossible. If you can, make a stop in Boppard, a larger village with lots of dining options and a fabulous chairlift ride: http://www.sesselbahn-boppard.de