5 Myths with Electric Propulsion

Electric power and electric propulsion are still growing industries for yachts and small ships. In this field, a few marketers may stretch the truth a little to make sale. This led to many myths and misunderstand about electric propulsion. Today I debunk five common myths.

Don’t Believe the Lies

1.0 Introduction

Don’t believe the lies!  Electric power and electric propulsion are still growing industries for yachts and small ships.  It’s the wild frontier of the marine industry.  And on the frontier, you find many geniuses . . . and a few thieves who want to bend the truth to make a sale.  Today we cover some common myths about electric yachts.

2.0 Electric Is Always Better

Electric power is not a miracle cure that magically saves the world.  The technology has limits, like any industrial technology.  When applying electric power to a small boat, you need to start by considering three facts:

1. Weight matters
2. Electric batteries fall far short of combustion engines
3. Electric propulsion comes with its own environmental impact

With ships, we always need to consider weight.  Extra weight means the ship sits lower in the water, which increases power requirements.  And electric propulsion often weighs more than combustion engines.  Often, our ship requires more power for the same speeds, just from the burden of extra weight.  Table 2‑1 shows an apples for apples comparison between electric propulsion and a combustion engine on a small 30 ft. sailing yacht, reproducing the exact same runtime.  The electric option was nearly 32x heavier.  This more than doubled the total ship weight!

Clearly, electric batteries are just too heavy.  Which brings up the second point.  Batteries require far more weight to store the equivalent energy of diesel fuel.  For any practical installation, this requires some type of compromise:

1. Supplement with generator
2. Recharge the batteries underway (sails or solar propulsion)
3. Accept much shorter runtimes

Batteries alone don’t cut it.  Electric propulsion requires a more complicated plan for energy storage and utilization. 

More complexity means more equipment.  More manufacturing.  More CO₂ produced.  Batteries come with their own environmental impact.  Just looking at CO₂ production, our batteries release about 28 t of CO₂ for their manufacture. [1]  Not too bad when you consider that the batteries are reusable.  This was only 10% of the CO₂ released from the equivalent energy in diesel fuel.  Assuming the batteries get recharged from renewable sources, plus several other assumptions.  Clearly, electric propulsion offers some benefits, but this incurs its own environmental cost.  You don’t get something for nothing.  Even in terms of environmental impact.

The real story behind electric propulsion always involves more complexity and nuance.  It’s not one solution that solves every problem.  Electric propulsion means more complexity, and new costs.  The value of that complexity depends on your individual goals.  We no longer ask if electric propulsion is a good idea.  We now need to ask if this technology works for your situation.

3.0 E-Power vs Horsepower

In the early days of electric propulsion, some sales agents tried to hide inferior motor performance by advertising its power as “E-power”, “kW”, or “Kilowatt power”.  They fostered this myth that electric power somehow operates different from conventional engines.  They want you believing that the two are not comparable.  This is absolutely wrong.

Power is power.  It doesn’t matter if that power comes from an electric motor or a combustion engine or a horse pulling a plow.  The only difference is the units of measurement.  Electric motors usually measure their power in units of kilowatts (kW).  And in the USA, engines measure their power in units of horsepower (hp).  But power is power, and you can easily convert between these two units.

You have every right to compare power in electric motors vs. combustion engines.  I do it all the time.  Spoiler alert: electric motors are generally less powerful than the same weight of combustion engine.  But doesn’t mean we need to hide the performance of electric motors.  Just remember that an electric motor brings other benefits in terms of reduced fuel consumption and less greenhouse   gas emissions.  Depending on your goals, an electric motor may have lower power but still offer higher value than combustion engines.

4.0 Peak Power

Marketing has another trick to overstate the performance of electric motors:  peak power.  When reviewing motor performance, they often advertise two power settings:  a continuous power and then a peak power which is around 25% higher.  Don’t depend on this peak power.  That’s a good way to burn out the motor. 

Heat plays a major factor in limiting the power of both electric motors and combustion engines.  Both of these produce large amounts of heat, which slowly raises the temperature in the engine/motor.  Left unchecked, this raises the temperature so much that the engine/motor melts, fuses together, or any number of other bad things to turn your expensive machine into a paperweight.  So we build in a cooling system to remove that heat from engine/motor.  But the cooling system only pulls out heat so fast.  It has an upper limit.  The continuous power setting of an engine/motor correlates to that limit on heat extraction. 

There are also other factors in determining the continuous limit.  But for this simplified explanation, continuous power means the cooling system exactly matched the heat production from the engine/motor.  It holds the engine/motor at a steady temperature and can continue that until the end of time.

With peak power, we go past that limit.  Technically, engines can do this too, but it’s a little easier to implement with electric motors.  In either case, you now run on borrowed time.  The cooling system can’t keep up with heat generation.  Heat builds up and temperatures slowly rise in the motor.  It takes several minutes, but temperatures eventually climb to the danger point.  Thankfully, these motors normally have a built-in safety switch.  It triggers before you do any damage and completely shuts down the motor.  But once it’s off, it stays off.  The motor needs to cool down before you can turn it on again, which takes several minutes.  So why even have a setting for peak power?  Because peak power only works for short bursts. 

Sometimes a burst of power can save your life or your ship.  Peak power provides a wonderful bonus feature in electric propulsion.  But don’t rely on it for constant operation.  When selecting your motors, always go with the continuous power setting.

5.0 Direct Drive is More Efficient

Some vendors claim they offer a direct drive option for maximum efficiency, where the motor shaft is directly connected to the propeller shaft with no gearbox.  Why?  Who ever said that direct drive was more efficient?  Yes, a gearbox or belt drive may lose 2-4% of the power due to friction losses.  But forcing a bad match between the propeller and motor may lose 10-15% of the power by forcing the propeller into a bad speed.

The propeller and motor both prefer different rotation speeds for maximum efficiency.  In general, propellers want slower speeds around 150 – 500 RPM. (This varies wildly with propeller size.)  Somewhere in that range is a perfect rotation speed where the propeller works best at converting that rotational power into forward thrust.  But the electric motor probably doesn’t like this speed.  Electric motors often compensate for lower torque by operating at higher rotation speeds.  One electric drive rotates at 1140 RPM, well above the preferred range for a propeller.  On a direct drive system, these two speeds must match.  So which do you pick?  Do you want an efficient electric motor attached to a bad propeller, or a bad electric motor attached to an efficient propeller?

Thankfully, with indirect drives, you don’t need to pick.  The gearbox shifts that high RPM from the electric motor down to the lower RPM of the propeller.  This gearing also steps up the torque going into the propeller, a major help when creating an electric motor that runs on limited DC voltage.  This also adds flexibility in matching the propeller and the motor.  Gearboxes can be ordered from a range of gear ratios.  That makes it much easier to find the right combination between motor and propeller.  Yes, the gearbox sacrifices a little efficiency in the transmission.  But in the process, we gain more back by ensuring both the motor and the propeller operate in ideal conditions.

6.0 Slow is Safe

Run longer by just going slower:  this is the common marketing explanation when discussing battery life.  For cruising boats, this extends to the alluring possibility of infinite runtime.  Travel slow enough, and solar panels can recharge as you go.  Motor each day and recharge that power with renewable energy.  But the math doesn’t add up.  This may be possible with sail propulsion, but not with solar alone.  Slow propulsion has its limits.

Solar power for propulsion depends on running at extremely slow speeds.  To balance the energy input from solar power, most ships are normally limited to around 2-4 knots speed for 8 hours per day.  Sounds great for a lazy vacation.  But this ignores the safety aspect.  Propulsion is not just a means of going from point A to B.  We also need that electric motor to control the ship in a storm.  To fight waves and prevent the ship from crashing into a cliff.  The propulsion needs to supply a minimum power level for more than a day to maintain that control through storm conditions.

This is why reliable electric propulsion still includes an electric generator.  It provides a backup to ensure reliable power supply for long duration in a storm.  Reliability is key.  We can’t give that up when switching to electric propulsion.

7.0 Conclusion

Electric propulsion is not a drop in replacement for combustion engines.  And this may be the biggest myth of all.  We tend to compare electric propulsion as a replacement for the current way of doing business.  But electric only makes sense if we change our thinking about ship powering.  Different values of what matters in our ship and our society.  Do we only measure success on minimizing cost?  Or do we also measure it based on cost to the environment?  Do we still need to maintain the same level of reliability and safety? (I think so.)

So before weighing your options, eliminate the myths of why you are doing this.  Start by asking what makes electric propulsion worth it for you?  Clarify that goal first, and the rest gets easier.

8.0 Acknowledgement

Many thanks to Jeff Cote from Pacific Yacht Systems for helping with much of the background and practical knowledge of these articles.  For more information on yacht electrical installations, check out their YouTube channel at:  https://www.youtube.com/c/PacificYachtSystems

9.0 References