It seems that little is ever truly discussed about regenerative capabilities (or regen) of an electric motor on a yacht. Those who don’t have electric motors often tout that regen is fictitious and doesn’t actually produce any usable power, while those with electric motors seem to keep quiet about it.
I wish to break that silence and go into the details of regen and what it can actually do.
Regen occurs when you are sailing and the force of the water over the propeller causes the propeller to spin. Since the propeller is connected to the motor, the motor will also spin. With a small electrical current supplied to the motor, the magnets in the motor will begin to act as a generator and produce power that will feed back into the battery bank. This process can add a lot of power to your batteries, giving you more range under power later while also supplying power to run your yachts electrical systems.
But how does it do this and how does regen work exactly? Regeneration is the transformation of kinetic energy into electrical energy. This takes work to accomplish, and leverage really helps here.
The magnet inside the electric motor is resisting movement thanks to the electromagnetic field created by supplying a small amount of electricity to the wires that are wound up around the motor. To make the magnet spin, you need enough torque on the shaft (work supplied by the propeller) to overpower the force of the electric field and cause the magnet to rotate. You also need the magnet to spin quickly to pull the electrons in the field around fast enough to create a flow of electricity. So, you need fast spinning and you need lots of torque.
As you can imagine, it all comes down to the propeller, as this is the structure that is going to create the spin and supply the needed torque to the motor. The more blades your propeller has, the more effective it will be at being turned by passing water. Also, the longer the blades are (larger diameter of the propeller), the more torque can be generated.
Imagine that you are trying to change a tire and the lug nuts just wont budge. You are faced with two options: get stronger or get a longer wrench.
Getting stronger will apply more force to the same wrench which will then supply enough torque to the suborn lug nut and remove it. This is the equivalent to sailing faster to cause more water to rush by the propeller and force it to spin.
Getting a longer wrench allows you to multiply your force thanks to leverage which will supply more torque to the stubborn lug nut while you are still applying the same amount of force on the wrench. This is the concept behind “foot pounds”. If you hang 1 pound from a 1 foot wrench, you would be exerting 1 foot pound on that stubborn lug nut. But if you hang that same 1 pound from a 2 foot wrench, you would now be exerting 2 foot pounds on that same lug nut!
Having a larger diameter propeller means that the force of the water at the tips of the propeller blades will have much more leverage and will generate much more torque. This will allow you to spin the magnet in the motor with more torque which will in turn generate much more power than if you had a small propeller which supplied much less torque.
Sadly, due to clearance issues, it is impossible to simply “make the propeller huge” on a yacht, as you are limited by the space and clearance of the hull structures around your propeller.
It seems that the minimum propeller size to generate any appreciable regen is 16 inches. Larger propellers produce more torque and therefore can generate much more regen power, while smaller propellers don’t seem to have enough torque to overcome the electromagnetic field of the motor and generate sufficient amounts of power.
This is where the bad reputation gets further muddied. Smaller yachts that are used as day sailors tend to also have small propellers, so they are not being used for long enough distances and have propellers that are too small for the task at hand. Larger yachts have larger propellers and are often cruised over long distances. There are many more day sailors than there are blue water cruisers, so the number of stories of “it doesn’t work” vs “it works great” gets skewed.
I feel the best way to look at regen is not to view it as instant power being added back into your batteries, but more in the view of power that can be harvested over a distance that is sailed. Regen needs a few key ingredients to work, you need a large propeller with many blades and you need speed. We are equipped with a 16 inch diameter three blade propeller, which apparently is the smallest size and number of blades to get any appreciable regen from, and are able to generate around 2 amps @48v while sailing at 5 knots, and 6 amps @48v when sailing at 6 to 7 knots. We never seem to sustain speeds of 8 or 9 knots for a long enough time to retrieve the really high amps that are being produced, so for all intents and purposes, I ignore them. If we are sailing at speeds less than 5 knots, regen is insignificant and we actually turn the system off so that the propeller can free wheel without slowing us down much.
So, imagine that we are sailing along at 5 knots and producing 2 amps @48v. This means that in 1 hour, we would have sailed 5 nautical miles and generated 2 amps @48v. If we sail at this speed for 10 hours, we would cover 50 nautical miles and generate 20 amps @48v (or 80 amps @12v). At the same time, if we managed to sail at 7 knot for that same distance, you would only sail for 7.1 hours but have produced 42.8 amps @48v (or 171.2 amps @12v) in that same distance! As you move faster through the water, you will spin the propeller more quickly and that will generate much more power; but you do have to sail the distance to extract the power for your batteries.
