Used the fast ferry a few times this year with the Chim, no problems other than they pack you in really tight and there's a few corners going up and down the ramps that nearly need 3 point turns! This is on the Portsmouth/Cherbourg route.
Took one to Ostend, on the way back it was very rough force 6-7. It was so rough we huged the coast to Calais then done a quick dash to Dover. They do bob around alot when going acrossed the current and I did fear the worst, but the car was OK.
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Perhaps it would be better on a normal boat? 