Hi Laurent
I don’t know which harbour you use, but according to this research data, the current at Zebrugge is around 4 Knots.
20 Kmh is equivalent to 10.7 knots but there are so many variables you can’t say exactly what speed you’d get. Not forgetting that you can use the sails to supplement the output of the outboard. If there is more than (say) 8 knots of breeze you’d probably go as fast with the sails and no outboard and of course a small boat like the Weta can hug the shore to get out of the current.
According to this test, the Torqueedo would give you a speed of around 6 knots as shown here.
A 2.5hp petrol engine would be faster and could allow the boat to plane – the little Super Chibe 1HP motor would give you around 3-4 knots – depending on wind/waves. A 2.5 engine should be good for about 5-6 knots. There’s a test on a small boat here which gives speeds for different outboards.
Use a GPS or a GPS App on a smartphone like iRegatta to check your speed.
