Emergency Electric Propulsion

Designing an electric system requires more than just spinning a propeller. Much of an electric boat focuses around safety and emergency planning. This article covers several key decisions to design an electric system that delivers far more than propulsion. Instead, our electric system ensures reliable safety.

Electric Systems that Keeps You Safe

1.0 Introduction

Electric sounds fun, but can it keep you safe?  Especially in emergency scenarios.  When you’re fighting against the storm and struggling to stay off the rocks, will the electric propulsion hold up to the job?  We need those features in our electric propulsion system.  How to design an electric system fit for emergencies and ready to keep you safe?

2.0 Keeping it Safe

THIS STUFF CAN KILL YOU.  Electric propulsion means large currents and voltages. This is not something to experiment with and learn as you go.  The electric system needs to protect you, even when things get damaged or in an emergency. 

Before starting on an electric propulsion project, make an honest assessment of your own expertise.  Do you have the skills and background knowledge to install your own electric system that will keep you safe?  Do you understand all the standards involved?  If you answered with anything less than a completely confident “yes”, hire an expert. 

But what level of expertise?  The more impressive, the more expensive.  I recommend you start with a marine electrician.  These people do the physical installation; they know the nuts and bolts for the wiring.  More importantly, they will know the limits of their own knowledge. 

Matching all the components can be tricky and may reach past the capabilities of an electrician.  This is when you call in the engineer.  Engineers go into the reasoning behind the rules and know when they can step outside the normal standards.  This can be important since electric propulsion is still fairly new, with standards organizations still refining their rules. 

3.0 Emergency Power

Imagine you run a pure electric boat, with all the power in the batteries.  Now suppose that a storm caught you near a lee shore, and you spend all night running full power to pull away from that lee shore.  That propulsion used all your power and the batteries run dead.  Time to call for help.  With what?  Dead batteries mean no electronics.  No radio.  No options.

When incorporating electric power, you need to plan for emergency power.  After you go through every last drop of propulsion power, you still need a reserve to turn on electronics and basic lighting.  This reflects the larger complexity of the power system.  You also need to consider that your propulsion may not run at the common 12V power.  It can require a dual voltage system, one for propulsion and another voltage for the hotel loads.

There are two major options for emergency power.  The first option is to isolate a smaller 12V battery on an automatic charge relay (ACR), sized for minor electric needs. (Figure 3‑1)  The ACR keeps the battery charged, but disconnects it during discharge, to prevent the emergency battery draining.  Then if the worst happens, you can force the ACR to “ON”, which now connects this battery as the emergency reserve.  Just enough power to run a radio and basic navigation. 

But how to connect the main propulsion battery?  Use DC to DC voltage converters to link your hotel loads, with your propulsion batteries, which run at some higher voltage.  This setup provides two major benefits:  relatively simple, and all your charging sources coordinate with one common point:  the propulsion battery. 

But this solution doesn’t work well for less common voltages.  For example, one large motor required 144V from the propulsion batteries.  You may struggle to find a DC converter for these less common voltages.  This brings the second option:  wire two separate battery banks. (Figure 3‑2)  One for propulsion and one for hotel loads.  Don’t try to connect them at all.  If the propulsion batteries run dead, you still have plenty of power to run electronics on the hotel battery. 

I don’t prefer this option.  It adds a lot of complexity.  You need to essentially wire two completely independent electric systems.  Two sets of charging sources.  Two AC connections.  And you need to be very careful to avoid accidental connections between the two systems.  That can make grounding and fault tolerances a challenge. 

Remember, the electric system still needs to protect you when things go wrong.  Running two independent systems creates more combinations and more ways for things to go wrong.  I would absolutely consult with an electrical engineer for a system of this complexity.  My recommendation:  only go with dual systems if you have no other choice.  But the preferred option is to use a single system with an emergency battery.

4.0 Ship Performance

Slow and steady are not always the safest option.  We still need a minimum power level to fight against a storm and control our heading.  But how much?  Obviously, we don’t expect a small 15 ft. yacht to fight against a hurricane.  Table 4‑1 provides some basic estimates for minimum power on yachts of different sizes.  But the answer varies wildly, depending on ship type, expected storm conditions, weight, etc.  In fact, many yachts had conventional diesel engines which were smaller than these minimum power requirements.  The only truly accurate estimate is custom analysis for your specific vessel.

Minimum power requirements become a highly personal decision.  When considering this, focus on storm survival.  About half of the minimum power came from needs for storm survival.  And it clearly depends on the size of the storm and the waves.  Just don’t undersize the electrical equipment to save money.  Remember, an underpowered ship can cost your life, which is far more expensive than any electrical equipment.

5.0 How to Store Energy

With electric, the real challenge is finding a way to store all that energy.  We need a massive reserve for extended propulsion.  Batteries require more volume and weight than the conventional option of a generator + fuel.  Figure 5‑1 shows that for very small energy storage, the battery is the better option.  But for anything large scale, the generator and fuel quickly outcompete the battery.  A large yacht running for two days could easily require 2000 kWh of energy.  On that scale, the generator and fuel are over 1100% less weight than a battery.

This doesn’t make generators perfect.  They come with downsides:  they lack the ability to recapture energy from renewable charging sources (wind + solar).  Batteries and generators both offer different capabilities.  This means any electric yacht will likely be a hybrid setup, storing energy in multiple energy mediums.  You have three big options in that arena:

1. Methanol fuel cell
2. Diesel generator
3. Pure battery

Methanol fuel cells seem like an environmentally friendly option.  In terms of energy density, they lie somewhere between a battery and a diesel generator.  These fuel cells require some careful planning.  First, the fuel cell doesn’t operate like a generator; it supplies power at a much slower rate.  This helps for slowly charging a battery, but don’t use it for a burst of power.  Second, methanol is not widely available everywhere in the world.  Plan on staying in range of confirmed methanol suppliers.  And finally, methanol is not as clean as you want.  True, it releases far less emissions than a diesel engine.  But most methanol in the world still distils from fossil fuels.  In terms of CO₂ emissions, it is not much better than a diesel generator.

Diesel generators are the common option for electric propulsion.  They store huge levels of energy in the form of diesel fuel.  And they emphasize the advantages of a hybrid system.  You can size the batteries to only work for low energy propulsion.  But assume the generator is required when using full power for any length of time.  This gives the best of both worlds in terms of responsiveness and endurance. 

Pure batteries are also possible, but only for a few hours.  If you go with pure battery and want extended cruising, I guarantee that sails will be the main propulsion method.  For this scenario, the battery mainly supplies your hotel loads, works for getting in and out of port, and provides emergency power if you need to move without the wind.  Even in this case, most sailors include a small generator as emergency power. 

The final hybrid selection depends on your usage and the requirements for emergency maneuvering.  Again, this comes back to storms:  how easily can you avoid one.  For those emergencies, generators work great to supply large power in a hurry.  Outside of those emergencies, the generator rarely turns on.  A hybrid system can provide all the safety of the current designs, with a fraction of the fuel consumption.

6.0 Conclusion

Designing an electric system requires more than just spinning a propeller.  Much of an electric boat focuses around safety and emergency planning.  We need to consider minimum power for storm survival.  How to store sufficient energy.  Most boats go with a hybrid option.  Finally, don’t forget the emergency reserve.  In a worst case scenario, with no juice in the batteries, you still need a way to call for help.  These decisions show that the main effort of electric propulsion is not spinning the propeller.  Instead, our electric system delivers something far more important:  reliable safety.

7.0 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

8.0 References