Welcome to Tech Corner here on the NGC Marine website. I am Skipper Hank and my intention here is to try to give you some background into electric propulsion. I am glad you have been considering electric propulsion, because it is a very exciting thing.
First of all, the handsome guy on the left is not me, it is my great grandfather, whom has been an inspiration to me, even though I never knew him. He spent all his life sailing the high seas, up until he retired around 1930. On and off, when in port, he would be a model for artists and photographers. There are several paintings of him. My grandfather was the person that took me sailing for the first time, I owe him everything when it comes to sailing skills, at least. Then I went on to get an MS degree in electrical engineering and have been working on designing electronic motor controls for electric motors since. So maybe you can see where this is going....
Why electric?
Electric is the system of the future. Lots of sources can produce electricity directly, such as wind, solar, hydro and even atomic. No existing source can produce oil, other than the already slowly draining sites around the world. Storage problem, you might say. Well, we just have to take it to the next level. If you think that electric energy cannot be stored efficiently, please consider this. As an example, we can look at the output of a windmill. Assuming this one is placed in an area with lots of wind so that supply is pretty much constant. Electric energy is supplied to a grid, but still, there are peak periods that usually take peak capacity as well. A system is designed to operate at maximum efficiency at a high output, but what are we going to do with the lull periods? Simple enough, these days it has been proven that hydrogen is a very clean energy resource, lots of different vehicles can run on hydrogen as well as they do on gas, but where does hydrogen come from. It is generated be a chemical involving water and electricity. You see what I am getting at? Excess capacity of electricity production that exists outside peak usage periods can be used to produce hydrogen that can be liquefied and stored easily. That is your storage problem solved.
When hydrogen is burned as a fuel, the byproduct, the waste is, yes, WATER. That by the way can be recycled to make more hydrogen. How clean can you get!What does that have to do with electric propulsion, you may ask. As part of the energy storage equation we should not forget about batteries that can be charged from the same source as we earlier used for consumption and storage. Charging batteries would be adding to the consumption part of the picture. Now why wouldn’t we just use hydrogen engines? Simple, hydrogen engines will still be internal combustion engines that have horrible low speed performance. Here is where the electric motor comes in, superior low speed performance. For applications the require occasional high speeds, a hybrid solution is the most like and efficient. For lower speeds, such in sailboats, the electric motor would be the predominant propulsion component. With an optional requirement for hybrid for generation of electricity for a continually rising need for additional power onboard and for backup during heavy weather conditions.
How did I get into the electric propulsion business. Especially electric propulsion for sailboats and other smaller vessels. There are several reasons, the first one is I have been sailing since I was only a few years old and back then, no or very few sailboats had inboards or other propulsion than the sails. Another big part is that I, as an engineer, have been designing controllers for electric motors for over a quarter of a century now, so there is nothing mysterious about electric propulsion to me. As a mater of fact, with a strong background in sailing AND in electric motor drives, I found almost natural to combine the two. That is why you are reading this!
A few interesting numbers before we get started on the heavy stuff:One comment you hear a lot when electric propulsion is discussed: “ You need a ton of batteries for this, they are liable to sink your boat”.Well, another of many misconceptions. Here is a comparison from our demonstration boat:
DIESEL ELECTRIC
Engine + Fluids 302 lbs Thoosa System 66 lbs
Fuel Tank (25Gal) 176 lbs Batteries 508 lbs
2 Batteries 122 lbs
TOTAL 600 lbs 574 lbs
As it comes out, a slight advantage electric.
Think about this: Burning fuel in an internal combustion engine does not exactly give you 100% efficiency. 35% of the fuel’s energy is lost to heating the surroundings, another 25% is lost to heating the air or water (exhaust) and vibrations and a few percent is lost in the mechanical transmission. This leaves about 38% of the fuel’s original energy content to create movement.
This only accounts for the fuel losses, besides that there are additional energy needed to run oil and water pumps, not to mention the generator. These losses can amount to a few Hp at maximum speed.
It is not that I have anything against diesel engines, they have served marine vessels and other applications well for quite some time. But here is an alternative that must be considered. The electric motor. The reasons have been listed elsewhere on this website. Here we will be concentrating on the performance from an engineering perspective.There are several different types of electric motors, but for propulsion the two types that are most common are AC and DC motors. Each of these types can divided up into several categories based on design and principle of operation. They all, however, have one thing in common: They display a constant output shaft torque regardless of speed up to rated speed. That includes 0 RPM.
Torque is what drives the prop shaft and drives your boat forward, not horsepower or kW. We all know that HP or kW are the only measure or performance the is mentioned for internal combustion engines. As we shall see shortly, there is a very good reason for that!First we will look at the basic equation for motor output, typically what is available at the shaft. We will use metric units, which makes the equations simpler. If P is the output power, N is the shaft torque and the number of rotations per second multiplied by 2×pi is indicated as w, this is the relation.
P = N * w (1)
This equation indicates that output power is proportional to torque times RPM (to use a more commonly used indicator of shaft speed). As (1) indicates, it does not make much sense to talk about an engine’s (or motor’s) horsepower without knowing what the shaft speed is. It does not help much to know that HP or kW for an engine or motor is always indicated at rated speed. Back to torque and as previously stated, for electric motors, the torque is constant throughout the speed range, which is not the case for internal combustion engines.
Typical engine specifications show a torque curve that is fairly constant from around 2200 RPM and a few hundred RPM upwards to maximum speed, which in most cases is 3600 RPM , and it is at that speed where output Hp is given. The torque is dropping off as the maximum speed is approached, but not much more than 10-20 %. Going towards lower speeds from below about 2000 RPM, the torque for the engine drops steeply down to about 800 RPM which is the lowest speed that the engine can turn at, the idle speed. Any lower, and it would stall, mainly because all the torque that is produced is needed to turn the engine and all that is attached to it internally.
Link to PDF of graphs at bottom of the page
All this is outlined graphically in Fig 1. More specifically, the figure shows the significantly differences between internal combustion engines and electrical motors. These are the power characteristics, but they still indicate why an engine has to run at a very high rpm to produce any power. Looking at Fig 2, the torque characteristics, it becomes much ore evident why the electric motor is so superior to the engine at lower speeds. It is especially clear when the load characteristics are included in the same figure as shown.
The reason why there is a difference between the propeller curves is that the diesel curves not only include the propeller itself, but also all the other loads that the engine has to pull, such as gear box, generator, water pump, fuel pump etc, which are loads that do not exist with the Thoosa systems.
Link to PDF of graphs at bottom of the page
Notice the significant difference between Thoosa motor torque (Fig 2) and the torque needed to produce movement at a certain shaft speed (the Prop curve). All this difference indicates is how much torque is left for accelerating the boat. The difference is just about non existing alt low speeds and very small for the diesel, that is, up until the torque rises drastically. This is why it is so easy to stall an internal combustion engine (most of us know this from manual transmission cars) at lower RPM. This of course gives the electric motor with the Thoosa systems a huge advantage when maneuvering in and out of slips and docks.
Most performance curves for diesel engines include a fuel consumption curve, for electric propulsion, the equivalent in how the process consumes battery energy. This is relatively easy to figure out if the propeller torque load curve is known for a specific boat. The hard part is to find the load curve for the boat, but here is it where the various dimensions of the boat and propeller come in. The curve can be estimated pretty well from these parameters. Then we can add that the relationship between torque and current is
I = kI * N (2)
Where I is the motor current and N is again the shaft torque. kI is a constant for the motor used. Typically, the current drawn from the battery is a bit higher than the motor current itself, mainly because some is added to make up for losses in the controller.
Link to PDF of graphs at bottom of the page
Now we can figure out how the battery bank will be depleted depending on the speed of the boat. An example of this is shown in Fig 3. Remember this is not a general curve, but is calculated by NGC Marine for a specific boat / propeller combination. The battery curves are for the 3 types of MK batteries that we recommend. As can be seen, high speeds consume a lot of battery energy.
Now to round off the motor section, we will look at how the motor speed is controlled. At least in principle, we will get further into that when we go over the controller.
The speed formula is similar to the current – torque formula, but in this case the variable is armature voltage, VA:
S = kV * VA (3)
Where S is shaft speed and kV is a constant for the motor.
Link to PDF of graphs at bottom of the page
As indicated above, varying the voltage to its armature regulates the motor speed. This task is performed by the controller, which takes the input from the throttle and converts it to output voltage proportional to the commanded speed. Since the controller operates from a constant voltage it needs to employ a special technique called PWM (Pulse Width Modulation) to do so.
Fig. 4. DC Controller Principle
How does it work? Essentially what takes place is that one motor terminal is held at a constant voltage, either a 48V (Battery +) or at 0V (Battery -) and the other motor terminal is being switched between 0V and 48V. In order to accomplish this, each motor terminal is attached to two switches (see Fig. 4.), where only one of the switches can be on (if both were on, the battery would be shorted). Let us say that motor terminal (T2) is constantly connected to 0V through one of its switches and the other motor terminal (T1) is being switched between 48V and 0V at a certain frequency, then the voltage signal applied to the motor can be characterized by its pulse width, tp, and period, T. Examining the signal, we see that the motor terminal voltage is at 48V in the time tp and at 0V during the rest of the period, which has the duration of T – tp. One more interesting (and very important) relationship is called the duty cycle of the signal, D = tp/T, which indicates the fraction of the period the motor terminal voltage, is a 48V. In order to find the effective motor terminal voltage, we need to calculate the time average of the signal. Using the symbol from the previous section for armature voltage, VA, we see that:
VA = (48*tp + 0*(T-tp))/T = 48 * tp/T (4)
Or using the duty cycle from above:
VA = D* 48 Volts (5)
Fig. 5. PWM Voltage Waveform, voltage at motor terminals
This indicates that simply by varying the pulse width of the signal to the motor we can vary the speed of the motor. The word simply was used, but in real life it is a bit more complicated, but at least this describes the principle of the variable speed operation. Also, the period of the signal should be as short as practical, in other words the signal frequency should be as high as necessary to maintain a smooth
DC current. For typical motors, that means in the order of 15 – 20 kHz. This frequency range is chosen because there is a good balance between power losses in the switching elements and reasonable current ripple. Also the frequencies in that range is just above what most humans can hear giving a low audible noise for the equipment.
So far, we have only used two of the four switches that were mentioned above. What if we now tie T2 to 48V instead of to 0V? If we still switch T1 between 48V and 0V, we see that if T1 is at 48V, the voltage across the motor terminals T1 – T2 = 0, but if T1 is at 0V, T1 – T2 = -48V. As we can see, there is a minus ( - ) in front of the 48V, where we above had T1 – T2 = 48V. It means that a negative voltage is now across the motor terminals and if inserted into (3) we get a negative speed! Or the motor is turning in opposite direction!
Selection of the right batteries is obviously very important for perfect system performance. As the third major component in the system and the energy storage and supply of the system it is essential that it is kept in the best shape at all times. There are a few things that are necessary to know to make this happen. Selection of the correct battery type is the on the top of the list. The only type to use is a sealed lead acid battery, such as a GEL or AGM, They are designed to handle deep cycling, which is the only type battery type to use for electric propulsion. These batteries are more expensive than flooded cell batteries and they require special chargers to last for the maximum number of cycles, but they will last for almost 10 years if the recommendations are followed. And they are sealed, so there is no out gassing and acid spillage as is the case with cheap batteries. And a charger that can handle the precision charging is more expensive that the regular automobile type chargers, but as mentioned earlier it all adds up to considerable long term savings.
The electric propulsion systems use a number of batteries in series, depending on output power. 4 to 6 to be exact. The higher voltage helps keeping a high efficiency for the systems. For a given amount of power (which is voltage x current), the higher the voltage, the lower the current. The lower the current the lower the losses in cables and other parts of the system. Lower current also means lower risk for electromagnetic interference. All these issues are very important to safe and reliable operation of your boat.
The capacity of the battery, usually measured in Ampere-Hours, is a measure for how much energy is stored and how much can be drawn, of course. There is one important thing about Amp-hrs ratings of batteries, however, the rating indicated is determined at a constant current draw of 20A. Increase the current draw and the Amp-hour rating diminishes, the equation that describes this behavior is known as Peukert’s equation. The effect of this can be seen in Fig. 3. above. The amount of operation time is not inversely proportional with the current draw, it decreases somewhat quicker than that.
First of all, the handsome guy on the left is not me, it is my great grandfather, whom has been an inspiration to me, even though I never knew him. He spent all his life sailing the high seas, up until he retired around 1930. On and off, when in port, he would be a model for artists and photographers. There are several paintings of him. My grandfather was the person that took me sailing for the first time, I owe him everything when it comes to sailing skills, at least. Then I went on to get an MS degree in electrical engineering and have been working on designing electronic motor controls for electric motors since. So maybe you can see where this is going.... 
All this is outlined graphically in Fig 1. More specifically, the figure shows the significantly differences between internal combustion engines and electrical motors. These are the power characteristics, but they still indicate why an engine has to run at a very high rpm to produce any power. Looking at Fig 2, the torque characteristics, it becomes much ore evident why the electric motor is so superior to the engine at lower speeds. It is especially clear when the load characteristics are included in the same figure as shown.
Notice the significant difference between Thoosa motor torque (Fig 2) and the torque needed to produce movement at a certain shaft speed (the Prop curve). All this difference indicates is how much torque is left for accelerating the boat. The difference is just about non existing alt low speeds and very small for the diesel, that is, up until the torque rises drastically. This is why it is so easy to stall an internal combustion engine (most of us know this from manual transmission cars) at lower RPM. This of course gives the electric motor with the Thoosa systems a huge advantage when maneuvering in and out of slips and docks.
Now we can figure out how the battery bank will be depleted depending on the speed of the boat. An example of this is shown in Fig 3. Remember this is not a general curve, but is calculated by NGC Marine for a specific boat / propeller combination. The battery curves are for the 3 types of MK batteries that we recommend. As can be seen, high speeds consume a lot of battery energy.
As indicated above, varying the voltage to its armature regulates the motor speed. This task is performed by the controller, which takes the input from the throttle and converts it to output voltage proportional to the commanded speed. Since the controller operates from a constant voltage it needs to employ a special technique called PWM (Pulse Width Modulation) to do so. 
