to modern viking: for your sailboat 18 hp are good. I sailed the river rhine with a 30 foot sailboat and about 5metric tons with a Volvo
MD 10 with about 10 HP. Slow on the river against current
but on the French canals the speed is limited to 6 km/h about 3,3 Knots.
From Northern Europe to the Med I would prefer to sail from the Netherlands
to gain the River Maas and then over the Canals to the Med, or if one is patient to sail up the River Rhine in very short distance from the bank (keep a vivid eye on the chart as there is sometimes construction under the water
level to brake current
and one should avoid to hit that) up to Koblenz and then up the River Mosel (really beautiful, only very slow current) to France
For the River Seine I would recommend only to pull the mast in Rouen, as there are many cranes in the commercial
harbour and building supply for construction for transporting the mast on board. If you start one hour after low water
from Honfleur you may reach Rouen in one Tide (my own experience so far).
I have crossed France
during my active Sailingtime several times and so have now a little catamaran
what I use as a River Cruiser actually on the River Saone.