Electric Yacht System Design

Going electric requires you to design a whole custom electric system for your ship. The challenge with electric systems is the interaction. The requirements for one minor component may determine key settings for the whole system. Today I cover some key settings for the electric system. After deciding these points, the...

Picking the Right Components

1.0 Introduction

So you want an electric yacht?  You did your diligence to sort out myth from fact.  And now to make the big purchase.  Actually, purchases.  This isn’t like buying a used car where it only comes in the color you see.  Going electric requires you to design a whole custom electric system for your ship.  It’s more akin to building a custom home, where the decisions multiply and pile up until they overwhelm you. 

Don’t worry, we can organize this.  The challenge with electric systems is the interaction.  The requirements for one minor component may determine key settings for the whole system.  The only option is to start with one strategy, pick your components, and keep adjusting until everything matches.  Today I cover some key settings for the electric system.  After deciding these points, the complexity of shopping should simplify into a few simple paths.

2.0 System Voltage

The first choice:  picking the voltage for the DC system.  Don’t be afraid to go over 48V.  Initially, all electric vendors wanted to stay under 50V DC, for two reasons.  In the electrical world, low voltage is generally considered to lack the strength for a fatal electric shock. [1]  The human body alone offers enough resistance to avoid instant death.  Due to this premise, most electric codes get lax for requirements under 50V DC.  That was the second reason for low voltage systems.  But low voltage does not mean low risk.

Even if we avoid instant death, low voltage offers plenty of injury.  Especially in the wet conditions of a ship, where electric resistance can change depending on the contact surface.  That low voltage may be riskier than you think.  50V DC will still do a lot of damage.  Think about this practically.  You don’t see people grabbing the posts of car batteries, and those are only at 12V DC.  Electricity at any voltage still requires protection.  From a system design perspective, higher voltages present the same risk as low voltage and require the same careful protection.  So go ahead and consider higher voltages.  Think about 96V DC, or even higher.  And expect that protecting the system comes as part of the design.

The risks don’t really change with higher voltage; what about the advantages?  Generally, higher voltage means more torque for the same size motors.  (But this can be limited by the heat dissipation from the motor.)  And higher voltage means smaller wires.  Less copper required to conduct the same amount of power.  Copper is heavy and expensive, costing $8-$10  per foot for large wires.  All things we want to minimize.  My advice:  go with the highest voltage you can tolerate for propulsion power. 

But remember that you are selecting a system voltage.  This only works if you can find components at every point in the system to work for that voltage.  That creates a problem for the hotel loads (lighting, electronics, refrigerator, etc.).  Most hotel loads run at 12V DC.  This requires a dual voltage system:  one set of circuits for propulsion, and a second set for all the hotel loads.  A DC voltage converter switches between the two system voltages.  That makes the DC voltage convert a critical component.  Voltage converters DO NOT come in every shape and size.  Make sure you can source the equipment to work with both system voltages.

3.0 Parallel or Serial Electric

Even with massive batteries, you probably can’t muster sufficient energy for extended use.  Most large electric setups require a hybrid system, which combines electric batteries with a diesel generator.  When planning a hybrid system, your first choice is a parallel or serial hybrid setup.  (Figure 3‑1, courtesy of Hybrid Marine UK).   In a serial hybrid system, the generator supplies power directly to the batteries, which then power the electric motor.  All power gets transmitted through the electric system.

In the parallel hybrid system, the engine is directly connected to the propeller shaft, with a gearbox that also includes an electric motor.  The engine and motor work together to deliver the full power output.  Personally, I think parallel systems are the superior choice for larger yachts.

1. Larger yachts require a high power output, and it becomes a challenge to source electrical components which meet that massive power demand.  With a parallel hybrid, the engine supplies the majority of the power directly to the shaft.  This allows a much smaller electrical system.  Less weight and less cost.
2. Either the engine or the motor can power the propeller independently.  This creates redundant propulsion in case one component runs into mechanical problems.
3. The electric motor takes over for low speed propulsion, and the engine shuts down.  This works great since these low RPMs kill engine efficiency. 
4. You can couple the engine and motor together; use the motor as a generator for hotel loads.  Given the larger size of the motor, this creates a massive electric supply for hotel loads. (The propeller can be completely decoupled for this.)
5. You can also couple the propeller to the electric motor only and use the motor as a generator.  On sailboats, this allows you to recharge the battery from the boat’s forward motion.

Parallel hybrid systems make sense for large yachts.  And major boat manufacturers have started using them. (Figure 5‑2)  On a smaller scale, the serial electric option may be preferable.

4.0 Available Space

Where do you plan to put all the equipment?  It seems like a small detail, but space becomes a challenge on yachts.  A typical electric propulsion requires several components:

1. Massive battery packs
2. Battery chargers
3. Large inverters
4. Electric motor
5. Motor controller
6. Solar chargers
7. Giant wires
8. Busbar connections
9. Digital controllers and interface

Unlike a diesel engine, these components are not tightly integrated into one unit.  They get mounted on your boat and spread across the bulkhead.  It should be accessible for maintenance.  You need 2-3 closets of space, or more depending on your battery capacity. (Figure 4‑1)  Before jumping onto electric propulsion, make sure your ship fits all the equipment.

5.0 Conclusion

Going electric grants you a lot of options . . . and dozens of decisions.  You create a custom design, tailored to your individual needs.  And this comes down to a few key decisions:

1. Pick your system voltage.  Can you find all the components at that voltage?
2. Pick your battery.  This depends on your lifestyle.
3. Parallel or serial electric.  I prefer parallel.
4. Where do you plan to put everything.  Leave room for a tidy layout.

These key questions very quickly filter the choices and ease the workload of designing your electric system.  Just take it one stage at a time.  And don’t be afraid to change your mind.  You got this.

6.0 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

7.0 References