You can't sail from London to Cologne.
3 hours about
2 days
6 and half days
several days
100 days
how fast are you going?
approx 72 days
6 weeks
it takes about 2 hours by flight but im not sure how long it takes you to sail :/
If you are a sailor you will know the answer to this question. If you're not a sailor, don't try it.
There is no scheduled service between these two points and if you are a yachtsman you will know the answer.