Boat props need to withstand groundings, flotsam etc so for longevity they cannot be long and slender. A short blade cannot absorb much power, so now you have to add more blades and make them wider. All the above is bad for efficiency.
A prop does not care what fluid it operates in. Fluid Density and viscosity will determine the blade drag and thrust at a given rpm, so rpm needs to match the available shaft torque.
Guys who have swapped out their stock trolling motor props for rc aircraft props report higher top speed and less current draw.
These props actually have higher efficiency underwater than in air. It is due to a thing called Reynolds number. A higher number results in a lower drag coefficient for a given blade foil section.
The motor's efficiency is another variable that has to be juggled to get best performance on limited power.
It consumes a given power most efficiently at a very specific rpm and voltage. So to establish these values I built a basic dyno. The brushed motor acts as a generator. When its wires are connected to a string of bulbs, it provides a torque load on the brushless motor. The scale is used to measure this torque and a photo transistor gives a speed reading.
I measured efficiencies of >90% at both 13V and 26V.
For anyone interested in the technical details of the design process, I have started a series of video tutorials. Already covered are solar panel and dc motor basics. Next up wil be prop selection, hopefully with some first-hand test results.
Brushless motor dyno - YouTube
My version of a water-cooled heatsink. Ali flatbar straight into the drink.
Oh, and be careful where you put down a brushless motor. The magnets can easily pull in a small screw through one of those vent holes, leading to unpleasant noise and heartache when you start it up. I almost learned that lesson the hard way.
I am also building larger outriggers, the old ones do not have enough buoyancy to cope with the increased weight of the new panels. To make things even more top-heavy, I plan on raising the panel mounting points to give me space for tilting the panel fore and aft. Even just 10deg tilt to either side will allow the panel to catch significantly more sunlight later in the afternoon.
The outriggers will themselves be mounted high enough to give me 10deg heel for when the sun is from the side.
Well as races go, mine didn't.
Some electronics went pop 10 sec after launch, just as I was starting to accelerate nicely.
The toasted circuit was not a critical component but we never managed to get the motor speed control to power up again. This morning on the bench it was purring away again as if nothing had happened...
The panel now has freedom to tilt 10deg either way to better align with the sun.