Hull Speed

The "Jolie Brise" has a 48 feet waterline lenght which is 2.4 feet at 1/20 scale.

hull speed

If you scaled down from the actual hull speed, the model's speed should be 0.5 knots. The formulae above shows the hull speed to be 2 knots. The model appears to be sailing 4 times faster. For a fuller explination, go to South Manchester Model Boat Club


There is a problem with the rudder glitching when the tiller is moved fast; it moves rapidily across from one side to the other before stopping in the correct position. The existing servo may not be upto the job. A trial was carried out in a local fast moving stream, to establish what Torque rating is necessary.

Rudder details

The boat was tied to a pole and readings taken for the force needed to hold the tiller at maximum angle. The force was so small it was difficult to get a reading on the spring balance.
The stream was flowing at 0.5m/s = 1 knot. (1km/hr = 0.54 knots).

    Force N     Distance cm     Torque N/cm  
Tiller 0.20132.6
Arm 0.8732.6
Drum 0.8776.1
Blade 0.8732.6

The rudder trial was carried out at about half max sailing speed. The torque of the rudder servo will have to be increased and no allowance was made for friction. The position/direction of the load on the drum is increasing the bending force on the servo shaft.