Jeremy Harris wrote:
Power absorbed by the prop is proportional to the cube of prop rpm, so doubling the voltage doubles the rpm but the motor uses eight times the power.
This means that motor current will increase by a factor of 4 if you double the voltage.
I'm not so sure this is true for all situations or electric outboard designs.
Myself I have built a submerged electric outboard from the Magmotor S28-400 with a 2-blade plastic Motor Guide prop.
The electronic controller was designed to handle both 12V and 24V input, but usually I just go for 12V as it provides enough power.
The Magmotor S28-400 is a brushed motor with a peak efficiency at 84%.
By curiosity I wanted to see how much current it consumed during full blast at 12V vs 24V with freshly charged batteries.
At 12V WOT the outboard consumed 20Amps = 240W from the battery.
At 24V WOT the outboard consumed 50Amps = 1200W from the battery.
My findings is that doubling the voltage only increase motor current by 2.5
meaning 5 times higher power consumption.
This is just a litte bit more than standard resistive power increase by a factor of 4. Far from the 8 times power increase as mentioned.
Perhaps there is some other rules if using a 3-blade prop, possibly made from heavier metal instead of lightweight plastic?
I'm soon about to start building a new outboard with a Slotless Brushless motor at high efficiency (>96%) using a 3-blade Metal Machete prop.
The sinusoidal controller will handle inputs from 9V to 50V so the current consumption at various voltages will be easy to test.
But don't hold your breath, the new build will take a few years to finish due to nice family concerns as small children taking a lot of time
The physics I quoted are correct, doubling rpm cubes power. What I suspect happens in practice is that motor resistance comes into play and stops the motor from doubling in rpm when the voltage doubles; if the motor runs slower the prop absorbs less power. In that case, all that's happening is that efficiency is significantly falling off as current increases, as it does with all motors anyway.
All props behave the same way basically, whether made of plastic, wood, metal, carbon fibre or whatever. Even a bit of carved wood stuck on the shaft will follow the cube law power vs rpm characteristic. If the prop has overly flexible blades it can de-pitch with increasing blade loading, but this is the opposite of what's needed for good propulsive efficiency, so although it would reduce the motor torque (and hence current) with increasing rpm, it would very significantly reduce thrust gain with increasing rpm. I don't believe that the typical trolling motor props are flexible enough to do this, as they tend to be pretty stiff. The Kipawa after market trolling motor props might de-pitch slightly, as they use a fairly long swept blade that is cantilevered out a fair way from the hub, and that seems to be a bit more flexible than the standard Minnkota type prop.
The more blades a prop has the less efficient it will be. The downside is that often you need to add blades in order to keep the diameter down and still absorb the power, so lots of motors make a trade between efficiency and practicality. For example, my river boat was running a 13" two blade stainless prop, and at 380 rpm and about 85 watts that would push the 17ft 6", 500 lbs boat along at about 4 mph, our inland waterway speed limit. I've cruised with a friend with a smaller lighter boat, and his Minnkota was using around 200 to 250 watts to do the same speed. The main difference between the two of us was that my prop is around 85% efficient, the standard Minnkota prop is at best about 45% efficient (I suspect it's worse than that, TBH). Most off-the-shelf boat props are pretty inefficient, few are better than around 65%. Few people bother to match the prop to the boat hull resistance curve, either, which adds yet more inefficiency.
The very best thing you can do is pay attention to getting a good match between the prop characteristics and the boat hull resistance characteristic, then ensure you have the right reduction ratio so that you drive the prop at the optimum speed for its diameter and pitch. There is a very good free bit of software for optimising propeller design, I've used it a lot and it gives results that are always within a few percent of being spot on. Its called Javaprop, and needs fiddling with to work with water props, but it is worth getting to grips with, as the pay off is much less power drawn from a battery for a given performance.
Please ask questions on the forum, rather than by PM, as it helps others and you'll get a better range of answers.