Yes, V is speed or velocity,
thrust is Power * efficiency / speed
and you see that thrust is reduced with speed if Power and efficiency is held constant.
efficiency is increased slightly with speed normaly untill cavitation come in.
efficiency is the result of boat speed not the water.
propellers have there openwater efficiency
and hull its hull efficincy, with the propeller doing its job at the back.
Imperial Power is a horse lifting 550 lb-ft/sec or 33000 lb-ft/min (76,04 kg-m/sec)
metric horse lift 75 kg one meter in one sec.
5 hp will then make 5*75*0,5/1,8= 104 kg of thrust at 3.5 kts (4 MPH) with an efficiency of 50%
thats for theory.
in that case of have 50 kg of thrust at zero speed, then let go. in practise the prop will let the Engine rev more at higher speed, if perfectly governend (sp) the rpm will be held constant but fuel and Power will be reduced with higher speed due to unloaded prop, and the prop will probably have a better bite at some speed then static.
to make that test correctly to get the mass of the boat, the thrust need to be monitored and held constant.
Ps, I do calculate propellers for airplanes.