With a traditional diesel engine, it’s kind of all or nothing. Typically, you are either sailing or motoring. Motors sailing is an option but generally not an enjoyable one.
With electric you will find yourself almost always motor sailing. Why? Because it have’s fun and no noise.
Let’s face it, the wind is usually not quite as strong we would like. If you want to make your propeller "invisible" it doesn't take much power at all. If you want to add a little speed, help point higher and get you through lulls, it takes just a little more power.
For those reasons, we recommend our customers going with the most efficient propeller, regardless of drag. This way you will have the more range when needed and still have the option of no drag by adding a few watts into the system.
But after a long or small time period of motoring, power is going to be needed. Is it possible the prop generates electricity and charges the battery?
A significant point to make in this article concerns the effectiveness of a marine propeller when acting, as a turbine. Marine propellers, generally fail as turbines due to their design and blade orientation. Generally it makes more sense that they would be far less effective at being driven by water passing over them, than they are at driving through the water.
Certainly, propellers are drive us better forward than they do in reverse. BellMarine technisians observed that, to make your propeller 100% effective as a turbine, you'd have to be able to reverse pitch by 180 degrees. Indeed, the Dutch Electric propulsion company, Bellmarine powered by Transfluid SpA, has developed and patented a regenerator system for reversing propellers for their electric drive units that they claim is twice as effective at generating power while free-wheeling under sail.
Nevertheless, after some practises and examples BellMarine was able to generate some amount of power via free-wheeling of a traditional sailboat prop. And yes it's another one way, to make your prop "invisible" with net zero output and the potential for a few extra watts incoming - with no extra investment or equipment for a new propeller - so the system works also without a new propeller, but with the old one .
As long as the "water force on the prop" is greater than the "throttle input", it should regen?
It’s not that simple.
Let’s say you throttle up and are cruising at 5 knots and that it takes 1kw to do so. Then the wind comes up and you transition to sailing and you are able to throttle back. As that happens, at some point the propeller will be driven backwards and regen will occur.
Without knowing the specs of other controller, other motors and other propellers, we can’t say much more than this. We DO know that there’s roughly a square relationship between power required and speed for a displacement craft. And that’s a square loss both ways. If it takes 1kw to drive the boat 5knots, you should not expect more power in regen than sqrt(sqrt(1kw)), or less than 6watts. That’s a trivial amount of savings. Far greater is the savings you get by simply being able to throttle down, saving power significantly.
What is going to be the amount of regenerated power?
The amount of regenerated power, would be approximately the NET of the amount generated less the amount input. If you've tried turning the motor shaft by hand, you've observed that it seems to want to "step" through its rotation. This is the effect of the alternating resistance and attraction felt as the permanent magnets on the rotor pass by the stator windings. To free-wheel a sailboat propeller, power is sent to the motor in order to energize the stator and overcome the latent resistance.
Once the resistance if equalled, the additional force of the water passing by the prop will turn the motor faster than it would otherwise be turning given the low power input, and -- voila -- the motor becomes a generator, transforming the surplus of energy into electricity flowing back into the battery.
Although I haven't seen any scientifically oriented research to support it, my thought is that in an electric boat scenario, it makes more sense to have a three or four-bladed prop, as large as will comfortably fit the boat and with moderate pitch to allow some amount of regeneration with the ability to make the prop "transparent" when sailing at slower speeds. Two-bladed, variable pitch, and folding props simply won't work as well “because” they're lower drag!
May I take this one step further? What happens if at the same time you are solar generating at 10 amps and regen at 5 amps? Would this simply add up to 15 amps charge?
Sure, that’s a start. But it depends.
The engineers reading this will be able to affirm or correct my thought on this, but assuming the currents you're referring to, are at the same voltage, then, yet, I believe they would simply add together.
If the regen current alone causes your pack voltage to get near its rated peak, you’d expect regen current to “back off” if another source also pushes charging current into the pack.
And assuming that BOTH of these respect voltage limits, then it’s a race condition, but suffice it to say that no, it will not simply add to 15amps of charge the arithmetic is different. Now, IF that happens, you could consider pulling out of regen by pushing the throttle forward until you maximize power from the solar panels while getting greater speed due to less drag. Overall, a win-win. A similar thing should be considered if running a genset with battery charger while underway.
So how much current are going to have at regen mode?
Would the expected regen current at 500 rpm be roughly the same as the current required to push to 500 rpm - minus friction and "throttle input"?
As noted above, no. Again, let’s say you need 1kw to spin the prop 500RPM. We know there is a square relationship between power and speed. Sqrt(1000) = 31.6
Now, you are driven by the wind and the prop is now spinning 500RPM backwards. We expect, due to this square law, sqrt(31.6) or 5.6watts of power might be able to be captured. Now, the devil is in the details.
We know that a prop’s loss is greater with speed and load.
So, if the water is spinning the prop at 500RPM, while the speed is the same, the load depends on the propeller, the motor/controller and would ideally be set to deliver max power to the pack (e.g. via MPPT).
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