The drag is very low compared to the boat drag. The hydro will not impact your speed at all.
Its depends on the size of the propeller and the speed of navigation. At 6 knots, we estimate that the average drag is 10 kg. To calculate the loss of speed for a particular boat, you need to compare the total drag of the hull at 6 knots with 10 kg of drag of the hydrogenerator at the same speed. Our immersion and lifting trials at a constant speed have not shown any significant difference on the speedometer.