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.
In the display above, you can see that it is producing 6.0 amps at 51.22 volts DC, and 0.30 kilowatts because the motor is spinning at 247 revolutions per minute.
What this all means is that our electric motor is producing 300 watts of power and that power is being fed back into our batteries. 6 amps might not sound like much, but this is in a 48 volt system. When the power is stepped down to 12 volts to power the yachts electrical systems, those 6 amps are the equivalent of 24 amps in 12 volt!
Suddenly, the tiny electric motor is producing the same amount of power as three 100 watt solar panels, but this is occurring all the time! It can be high noon on a cloudy day, or in the middle of the night, the motor will produce this kind of power when you are sailing through the water with enough speed to turn the propeller.
So, why is regen not typically talked about? Well, the numbers are not representative of what you are getting at every single moment, but more of what you could get in an hour. People like instant results and don’t want to wait around for it, but they don’t seem to realize that they have to wait for everything anyways. Solar panels take hours to charge batteries, and so do generators, but these are things that people usually set and forget. Regen requires the boat to constantly remain in motion and if your speed falters, so will your power production. If you are hand steering, then you are going to have to work for that power! If you have a balanced sail plan and wind steering, then it becomes just as passive as your solar panels, wind gen, or genset.
In this example, 300 watts added to the battery bank would only be there after a full hour at this speed. If you are puttering around for a few hours, 300 watts per hour might not add up to anything appreciable, which is why regen has gotten a bad reputation. When you are passage making and sailing around the clock for days, these numbers become very real and very important.
Hour: Watts / Amps @12v / Amps at 48v
1: 300W / 24 amps / 6 amps
2: 600W / 48 amps / 12 amps
3: 900W / 72 amps / 18 amps
4: 1200W / 96 amps / 24 amps
5: 1500W / 120 amps / 30 amps
6: 1800W / 144 amps / 36 amps
7: 2100W / 168 amps / 42 amps
8: 2400W / 192 amps / 48 amps
9: 2700W / 216 amps / 54 amps
10: 3000W / 240 amps / 60 amps
11: 3300W / 264 amps / 66 amps
12: 3600W / 288 amps / 72 amps
13: 3900W / 312 amps / 78 amps
14: 4200W / 336 amps / 84 amps
15: 4500w / 360 amps / 90 amps
16: 4800W / 384 amps / 96 amps
17: 5100W / 408 amps / 102 amps
18: 5400W / 432 amps / 108 amps
19: 5700W / 456 amps / 114 amps
20: 6000W / 480 amps / 120 amps
21: 6300W / 504 amps / 126 amps
22: 6600W / 528 amps / 132 amps
23: 6900W / 552 amps / 138 amps
24: 7200W / 576 amps / 144 amps
300 watts per hour tallies up in the table.
What this means is that if you only sail at 6 to 7 knots for a few hours, you will only gain a few hundred watts, just the same as you would produce from a 300W solar array in peak sunlight hours. This will give you a few watts to play with here and there but probably nothing you would really notice.
If you go out for a daysail and plan on having the regen charge you back up to bring you back into port, but you only sailed for 2 hours, you can only expect to see up to 12 additional amps in your motor battery bank if you managed to sail at speeds over 6 knots for the entire time. This is really insignificant and probably where the bad reputation comes from.
When you are sailing a long distance, and are under sail at speeds over 6 knots for around 12 hours, you could expect to see up to an extra 72 amps in your motor bank! This is a significant amount and will provide you with the power needed to bring your yacht back into port with ease.
When we were crossing the ocean, we were sailing for days (almost a month actually) and we had full batteries the entire way!
Our motor bank is 210 amp hours @ 48 volts and our house bank is 525 amp hours @ 12 volts.
If we left port with completely empty batteries (which was not the case) we would have been able to charge the house bank fully in 22 hours, and have the motor bank completely charged in the next 35 hours! This means that in 57 hours, we could fully charge all of our batteries without using a single drop of fossil fuels; just by sailing fast!
In our case, we actually left with full batteries, but all of our electronics needed lots of power on the crossing, and they were all supplied completely by the regen of the electric motor.
Now, not all electric motors have regen, so it is imperative that you make sure the one you are looking at does. Our electric motor is a QT20 (Quiet Torque 20kw) from Electric Yachts
This motor is a plug and play design, allowing you to have all the benefits of a well engineered system that simply drops into your boat. It gives you all the information you need to properly operate it while maximizing your range and motoring time. It also has a very simple user interface to set the regen level you desire. When we were looking for which type of electric motor to use, we noticed that many of them had complicated procedures to regen, and some of them could easily begin to draw power if you were not paying close attention! This one is pretty much fool proof, as regen occurs when the throttle lever is in the neutral position and power consumption occurs when you move the throttle lever forwards or backwards.
Regen is a truly remarkable feature of an electric motor which I feel makes it the best motor to have for blue water passage making. If you are a coastal cruiser who only does short trips, regen might not be very apparent to you, but the moment you head out on your first long leg, regen can keep you charged up.