More or less what I concluded, though I had to go back a step and find the distance between the two ports (http://worlddistance.com/) (in fact, the departure point is Maldon, famous for salt) and the average speed (7 knots, give or take, assuming not especially fabulous wind) of said boat -- which in the end is being written as a Thames barge.

Then double the result to take into account tide, weather, sandbanks, wrecks, hostile vessels ...

Am grateful it didn't turn out to be days, as there is only so much plot you can fit onto a small cargo ship in the 18th century (and I'm not writing a pirate story this time round!)

Many thanks for confirming my tentative guesses!