The distance from Calais to Boulogne-sur-Mer is 21 miles according to the most reliable European travel planner, ViaMichelin: http://www.viamichelin.co.uk
The only ferry company on the Dover-Boulogne route, LD Lines, withdrew their service 4 days ago.
From Dover you can currently travel to Dunkirk (with Norfolkline) and Calais (with P&O or Sea France). Norfolkline doesn't accept foot passengers (but, if you borrow a pushbike you can cross for a tenner each way). All services accept cars.
Craft:
Are you sure that there's still a service from Folkestone to Boulogne? I thought that it had closed. (Hoverspeed certainly no longer exists). The only service to Boulogne that I know of is with the Euroferries daily sailing from Ramsgate.
The distance from Calais to Boulogne-sur-Mer is 21 miles according to the most reliable European travel planner, ViaMichelin:
http://www.viamichelin.co.uk
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