There is a time in the life of every father and son when you have to sit down and say, 'We need to talk about propellers'.
Accordingly the basic facts are up there for you to print off and discuss with friends behind the bicycle sheds, but here are a few pointers for when it comes to fitting out our (experi-) mental maritime drone.
Longer wings are more efficient. Swifts for instance have relatively long and thin wings and they never need to come to ground, sleeping and having sex in flight, which I had thought only airline pilots could do.
As propeller blades are rotary wings it follows that the larger the blade and the slower it's rotation the less energy they require to produce the same amount of lift (or thrust if we're talking forward motion as with the boat). A helicopter is up to twenty times as energy efficient as a VTOL jet in the hover, which is why jet-packs for people will only ever be rich-boys toys.
Turning to motors, these have a KV rating that specifies the RPM per volt and ours are the slowest of the three available for that model. Looking at the recommendation on the handout, you can see that the slowest motor is paired with the largest propeller at 20". Turning a smaller propeller faster produces more thrust, but there is a sweet-spot at which the motor is best matched to voltage and propeller, and that is using the 18" instead of the 16" propeller.
We have as drag-racers say, however, 'race what few brung' and in our case it is a 22" carbon-fibre propeller, a 280KV motor and a 22.2V battery-pack. What we have also brung, however, is a boat with only 10.5" clearance from the motor axis to the monoski and therefore we have to swap the 22" carbon-fibre propeller on hand for 18" or 20".
Given the 20" provides the greater thrust (four kilos instead of three flat out, albeit off the back of 24V) then we may as well run with that?
Given too that the prop will penetrate the waterline in part at some stage, that rules out carbon-fibre whose properties ~ like the price ~ are rarely forgiving. Anyone who knows me, too, will know that I don't want to steer further revenue toward the same people who supply ESCs and motors that are not easily paired.
The happiest takeaway from this is that our motor comes with an adapter to suit regular RC props that simply screw on with a single bolt, the way wheels on sports cars used to back in the day. This in turn opens up a plethora of options in terms of propellers on the market, all of which (like that pictured) are four or five times cheaper than carbon fibre too.
Nicks in alloy props can be sanded out, whereas carbon-fibre propellers have to be thrown away... we call it progress, which is why the planet will be here long after we've gone.
