We are considering a ferry trip on the Rhine &/or Rhone &/or Seine in the fall of 2020. The plan is for about a 3 or 4 week excursion. We have been on cruises before but we would like ability to stop and spend 2 or 3 days in a centre if we like it. Any assistance on ferries, accommodation, itinerary and attractions would really be appreciated.
Are you willing to mix in other types of transport? I would not expect to find ferries everywhere, since trains, buses, and cars are probably all faster, and bridges span most rivers at logical places. And while I'm aware of some good aggregate web sites for sea-going and international ferries, domestic river ferries may require you to search individually by location.
www.kdrhine.com is not the only ferry on the Rhine, but it's a good option for tourists between Cologne and Mainz, for instance.
For a general look, use the Search box top center for "Middle Rhine". That will bring up mostly one to three day trips, but it will give you an idea of the Rhine options.
Be careful about the use of the word ferry. Like the word cruise, it implies things that may not be precisely correct regarding these two rivers. While I'm not crazy about calling the KD boats a "Rhine cruse", it does distinguish their North-South service from the very short East-West "ferries" that connect two towns on opposite sides of the Rhine. I don't mean to berate you on a technicality! I just want you to know what the boats are.
I like the Middle Rhine, but it's not a 3 week visit. Even if you figure in Cologne and Koblenz (which are poor uses of the boats, when the train is cheaper and ten times faster, and the scenery outside The Middle Rhine is missable) you can't get three weeks on the German Rhine. I don't think creating a trip the entire length of the Rhine is worth the travel time (and it can't be done by boat ... ), but there are swell places to see near the river. I might add that our Business-hotel room on the river in Arnhem came with earplugs! It could have been due to Queens Day, but I think it was because the river is largely, industrial, noisy, and un-scenic. That description does not apply to the Middle Rhine.
I don't know how important budget is to you, but because there are discounts for train and KD boat use on the same day, a multi-day KD pass may or may not be the cheapest deal for your group. There also can be group ticket discounts that could work the other way. It depends on exactly where you want to stop. (There is a romantic notion that you hop on and off the KD boat based on whim and visual beauty. But the truth is that the boats are infrequent enough that it is naive to think of them like a midtown bus service in a big city.)
Are you maybe considering a barge or narrowboat charter, where you can stop or go as you please? I’ve never done it, but I’ve read at least one excellent trip report on this site about narrowboating in England, and a book about canal barging in France. And there are lots of sites for renting barges in France, here's just one: French Waterways. I have no personal experience, but it’s on my list to do soon.
I have not heard about any options for taking ferries longer distances so I thought I’d offer these ideas instead.
Ferries go back and forth across a river usually because of lack of bridges, ships go up and down.
Have you thought about a bike and barge trip perhaps? They have them on the Mosel and many other rivers. It might satisfy your sightseeing wishes and be slower than a cruise.
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Perhaps you could find an Itinerary with one of the river cruise companies that would work for you. Unfortunately, they're very expensive and the longest cruise is typically about 15 days. Combining a river cruise with some self guided travel would probably be a workable solution.
You might find it helpful to visit a knowledgeable travel agent in your area, as they may know of a product that fits your needs completely. Some of the high end travel planners can offer custom Itineraries but a service of that type will again be on the expensive side. If you'd like to look at something like that, have a look at THIS travel expert.
KD is the only river ferry I've been on that seems to fit your idea. If there are ferries on the Rhone or Seine, I've completely missed them.
I agree with the terminology points. A ferry is something that goes from one side of the river to the other. Any day boats going up and down a river will be short trips in particular well touristed sections, like the Rhine Gorge, Bingen - Koblenz - Cologne(Köln); and may be summer-only.
Your best option is to pick towns and link them by train.