What size hybrid drive should I buy?

1 KW of electric power can replace 3 or 4  horsepower of diesel power.   Size and pitch or propellor, type of packing gland or dripless gland, gearbox ratio all affect the efficiency of an electric drive.   Diesels are rated at peak horsepower at peak rpm which is rarely achieved on a boat.   Diesels also contend with two very inefficient water pumps, inefficient alterators and belt losses.    Considering 1 KW = .746 h.p., it becomes obvious just how well suited electric propulsion is for boats.

Do I need an all electric boat or a diesel electric hybrid?

As a boat approaches boat speed, the power required increases exponentially.    If you want to cruise at boat speed for extended periods of time you will need a hybrid system with a diesel generator.    If you are a daysailer, and you are content to travel at 75% of boat speed, then an all electric boat is right for you.  

How long will my batteries last?

How long batteries last depends on two major factors, boat speed and battery size.   Since power increases exponentially as boat speed approaches hull speed, it is adviseable to travel at less than full boat speed.   A typical 30 foot boat draws 3 KW at 4 knots but over 6 KW at 6 knots.    Not only does it require exponentially more power to travel at full boat speed, batteries become less efficient at higher current consumption.    Battery powered electric boats are not designed to travel at full boat speed for long periods of time.    It is easier to think of it this way, if your

Not all batteries discharge the same amount.   AGM and Gel Batteries can be discharged by 50%.   We recommend Odyssey Batteries which can be discharged up to 80%.   Odyssey batteries use Pure Lead, Thin Plate design and are rated at 400 cycles at 80 % discharge.    

To calculate range, first convert amp hour ratings to kilowatt hour ratings.   But remember, amp hour ratings are dependent on discharge time.   Odyssey Batteries 2 hour rate is 71 percent of their 20 hour rate.   The PC 1800 Odyssey Battery  has a 20 hour amp hour rating of 214 amp hours, and a 2 hour rating of 153 amp hours.   If we use the 2 hour rating, only 80 percent is useable or 122 amp hours.   If we multiply 122 amp hours x the average voltage of 12.2 volts we get the 2 hour kilowatt rating of 1.5 KWH.    Multiply by 4 and you have 6 KWH's in 4 PC 1800's.    The two hour range at 3 KW / 4 knots would be 2 hours or 8 nautical miles.

The PC 1800 Odyssey Battery  has a 1 hour rating of 136 amp hours.   Only 80 percent is useable or 108 amp hours.   If we multiply 108 amp hours x the average voltage of 12.2 volts we get the 1 hour kilowatt rating of 1.3 KWH.    Multiply by 4 and you have a one hour rating of 5.3 KWH's in 4 PC 1800's.   The one hour range at 6 KW at 6 Knots would be 53 minutes or 5.3 nautical miles.

How do I make my batteries last even longer?

The ultimate point of sail for an electric sailboat is motor sailing.   When the wind is very slight your sails luff.   It takes a very small amount of energy, 1  KW, to move the boat along at 2 knots.    There is almost always 1 or 2 knots of wind out there, so now you have 3 or 4 knots of wind and you are moving along at 2 or 3 knots.  

The 8 hour rate of the PC 1800 is 186 amp hours of which 150 amp hours is available multiply by 12.2 volts to get 1.8 KWH multiply by 4 (for 48 volts)   = 7.3 KWH    At 1 KW at 3 knots your light air range is 7.3 hours or 22 nautical miles.

What does efficiency have to do with designing a powerful hybrid drive?

The most important factor regarding an electric drive installation is the efficiency of the components.   Overall efficiency of the system is calculated by multiplying the individual efficiency of each of the components.    Our system utilizes a Mars brushless, permanent magnet motor (93% efficient), a Sevcon controller (93% efficient) and a Edison gearbox (97% efficient).   System efficiency is .93 x .93 x .97 or 83% efficient.   What this means is that 17% of the power that is consumed generates heat.   So when the motor is drawing 4.7 KW, 17% of that or 800 watts is being converted into heat.   

Why is heat a problem with electric drives?

Electricity does not like heat, all conductors become less conductive when they are hotter.   If the engine room is unable to dissipate 800 watts of heat in this example, the system will overheat.   When the system overheats, the controller either reduces power output or shuts it off completely, something you do not want to happen.   When you select efficient components you create less heat.   All systems create heat because nothing is 100 percent efficient.   Dissipating the heat ensures that the system stays cooler.   Heat sinks, fans, adequate ventilation and water cooling are several ways the heat can be dissipated.

What's the difference between different types of electric motors?

A DC motor accepts direct current, while an AC motor accepts alternating current.    Here's the interesting part though - all electric motors are AC motors.    DC motors convert DC to AC mechanically through the armature and brushes.   The Mars brushless motor is a 3 phase AC motor.   The controller is where the DC is changed into AC.   AC motors are desireable because they do not require brushes.   Brushes require maintenance and also spark so are not ignition proof.

A motor shaft turns because the magnet that is attached to the shaft ( the rotor),  is inside a moving magnetic field created by the stator.     At least one of these two magnets must be an electromagnet.   Most motors use two electromagnets.   An electromagnet is a created by wrapping a wire around a metallic object and then applying electrical power to that wire.   Effectively this is a short circuit which creates heat.    Permanent magnet motor employ rare earth permanent magnets in their rotors, thus eliminating the need for the second electromagnet, thus increasing efficiency.

Brushless, Permanent Magnet motors are the most efficient motors available today.

What does the controller do?

The Sevcon Permanent Magnet AC controller which we use for the Mars motor is a variable frequency inverter which converts DC power into 3 phase AC power.   Controllers are programmable for maximum current, how quickly they can accelerate and regenerative characteristics.

What voltage system is the best for my boat?

Higher voltage is desirable because it requires less current, however higher voltage requires more safety equipment.    You have DC and AC voltages on a boat.    DC voltage is considered dangerous over 50 volts while AC voltage is commonly employed and considered safe up to 120 volts.   Our electric drives are DC between the batteries and the controller and then AC between the controller and the motor.      With respect to the design, anything in excess of 50 volts must be handled carefully and fully insulated with a secondary sheath enclosing the high voltage DC cables.   My recommendation is to pick a voltage that allows you to not exceed 100 amps in current.   If you need 4.8KW, use a 48 volt system.   If you need 7.2 KW, use a 72 volt system.

How do I effectively manage the amperage within the system?

Amperage creates heat.    Lower voltage systems require higher amperage to create the same power.   Remember that Power is Voltage multiplied by Amperage.   Reducing amperage is always desireable.  However lower voltage is safer.   You are going to have to deal with amperage and minimize its effects on the system.    Use high quality, large cables for your main power cables.  2/0 cables have an ampacity of 330 amps so are well within their designed use.   Larger cable has less voltage drop so the engine uses a higher voltage.   This is especially important when the electric engine is functioning at full power.  

Why is Gear Ratio so important?