The first thing I wanted to tackle was the motor and propeller, which ones to choose. Naturally I wanted to do this the scientific way, calculate what force is required to move the boat, then calculate the force produced by propellers and then determine the motor. However, this proved a lot trickier than expected. First of all, to calculate the force required to move the boat you need to understand a little about hydrodynamics. There are many factors which affect the force, just imagine pulling a box through the water compared with a streamlined shape, the latter obviously being more efficient. Some of these factors are the length of the boat, the amount of water displaced, size of wetted area and so forth. After searching for a formula to calculate the force I soon realised that without a physics or marine engineering degree this was going to be extremely tricky. So I was going to have to measures the forces myself. Time for an experiment!
Luckily my friend Phil and I put together a small prototype boat. Even though this boat would be nothing like the final one, as it’s made of wood rather than fiberglass, and it’s square rather than streamlined, I figured it would still give us a rough idea of power requirements. So with a boat in hand we headed down to the local pond. I’d already done some work on the electronics so there was a GPS in the boat that would send it’s speed to a dashboard on my laptop.
To measure force we used our scientific instruments, the suitcase weighing scales! This was then attached to the boat and we would pull it along at different speeds and record the force shown. What ensued was lots of shouting, “speed up”, “slow down” as we’d initially intended to pull the boat at specific speeds (so we could capture specific data points), but it was just easier to record the speed at the time of measurement. We also added different weights, so we could see how this affected the force. We expected the data to be fairly noisy and not very accurate seeing as the string we were pulling the boat along with, wasn’t always perfectly horizontal, and there was also a small delay in reading off the speed (so that would be slightly inaccurate). However we were impressed with the pattern that appeared in the data.
You can see that as the speed increases, the force increases almost exponentially, which is as expected, and as more weight is added, more force is required. To pull the boat at 2kn with 7kg of weight, about 1500g of force is required. If we were to extrapolate and work out how much force would be required to move the boat at 20kn, this would not be possible as increasing the force won’t always make the boat go faster, as there’s a limit to how fast boats can move.
A quick bit about boats…When a boat moves a wave is created as it cuts through the water. The speed this wave moves is relative to the length and speed of the boat, and a crest forms at the bow of the boat. When the boat moves slowly the wave dissipates past it, but as the speed increases this will eventually keep the crest of the wave at the bow. The creates a lot of friction and the boat is not able to overcome this wave and travel faster. However, with tremendous power and a planning design, the bow of the boat is able to lift out of the water, therefore overcoming this resistance and surfing on top of the wave.
So there is a theoretical limit to how fast the boat can go, but this is also backed up in the data. If we extrapolate the 7kg dataset and see how much force is required to move the boat at 10kn it would take almost 60kgs of force, and for a boat, which weights less than 10kgs it’s not feasible to generate this level of force!
Now the we know the forces required at different speeds and weights, this can help us select the correct propeller and motor.
Read part 2 for more experiments about choosing the correct propeller and motor.