Hybrid electric power for Blu Emu

This post will be controversial for some readers, but we like the idea of electric power for our Blu Emu. This post is an introduction to why we want to use it, what we would use it for, and how we intend to actually implement it. At the moment, this is all based on research and ideas, and not one electron has been harmed during the making of these ideas!

In the future, maybe we will become the Electric Blu Emu

The idea of using electricity to move Blu Emu, using electric motors, is tempting because:

  • it is much quieter inside the boat and outside the boat than diesel or petrol engines
  • it is possible to use no fossil fuel during use (it does for the construction of the parts, of course) as electricity can be generated from other means
  • it has much less ongoing maintenance compared to diesel motors as it has fewer moving parts and their operation is mechanically simpler


  • it is much more expensive to install in a boat that already has running diesel motors
  • it limits the range you can travel a lot (no ocean crossings and possibly no multi-day trips). We want to go long range cruising sometimes.
  • It limits the speed you can travel, which becomes a safety issue on longer trips where speed=weather safety.

There are two ways people have found around the problems if they want to go cruising:

  1. Have a really large boat (like Tûranor), AND go slow (5kn), AND spend lots of money – thus solving only two of the three issues: or buy a slightly smaller boat that is custom-built for lightness AND is expensive (US$2,400,000) AND is slow when using electric motors only, something like the Silentwave’s (which coincidentally also only solve the same two of three issues):
  2. Don’t have an electric-only boat if you want decent speed and decent range!

We have bravely decided to go with 2. as we already have a boat and consequently don’t have the money for option 1 (as the current boat takes up all available money, as is inherent to boats…and horses I’m told!). So our solution won’t be electric only.

However, we do want all of those nice benefits. The world is also changing, and there are more and more examples where non-polluting propulsion will be needed to use some areas of water (such as Amsterdam, UK, and many other parts of the EU).


The next best choice therefore is to have a boat which can run on electric power sometimes, preferably sourced from renewables, and diesel at other times when it is really needed. This is known as a “hybrid”. This is the same choice as is happening for cars – some are electric-only (like the Nissan Leaf or Tesla) and some are hybrids (Toyota Prius).

Hybrids can eliminate the issue of range, as you can run on diesel just like any other boat (until you run out of it).

The other two issues of speed and cost need to be dealt with, and this starts to depend on how you design and build a hybrid. There are at least two types of hybrids, and the two we are interested in are serial and parallel:

Serial Hybrid
  • Serial hybrids have only the electric motor connected to the propeller shaft and driving the boat. The diesel motor is separate and its purpose is be a source of electric power to the electric motor, usually buffered through a battery bank.
    • The advantages are
      • simplicity as there is only one motor on the prop shaft and the other connections of the system are easier
      • flexibility in that the electric motor is usually quite small, and the larger diesel motor can go anywhere on the boat rather than in front of the prop shaft
      • the size of the electric motor means the actual engine rooms can be smaller.
    • The disadvantages are
      • speed is only as fast as an electric motor can make you go; you can get very large electric motors like the 250kW on the Silentwave (335hp)
      • if you already have diesel engines, two of the advantages of size and flexibility aren’t useful
      • retrofitting an existing boat, which has engine rooms already and where the position of weight has already been factored in, means some advantages aren’t
  • Parallel hybrids have a diesel engine driving the propeller, and add an electric motor to also drive the propeller. Sometimes there is a clutch to disconnect the non-driving engine/motor.
    The advantages are
    • the diesel still drives the propeller when it needs to, so full diesel power is available,

    • the electric system is separate from the diesel, so if anything goes wrong with it such as a lightning strike, then the diesel may still be usable,
    • it can be retro-fitted to an existing boat.
  • The disadvantages are
    • there is extra complexity in making sure the two play nicely in propelling the boat
    • you still have all the maintenance, size, weight, position of your diesel engines – you are adding to the boat, not replacing
Parallel hybrid

Time for a little aside: there is a different between a motor and an engine, and I will endeavour to use the too properly in these blogs. For the difference, I can’t do better than this from MIT’s School of Engineering:

As technologies and devices evolve, language must stay on its toes if we expect to understand each other when we talk about them. English-speakers are particularly flexible at adapting to progress. They’re willing to coin new terms, modify old meanings, and allow words that are no longer useful to pass from common usage. “The etymologies of ‘motor’ and ‘engine’ reflect the way language evolves to represent what’s happening in the world,” says MIT literature professor Mary Fuller.
The Oxford English Dictionary defines “motor” as a machine that supplies motive power for a vehicle or other device with moving parts. Similarly, it tells us that an engine is a machine with moving parts that converts power into motion. “We use the words interchangeably now,” says Fuller. “But originally, they meant very different things.”
“Motor” is rooted in the Classical Latin movere, “to move.” It first referred to propulsive force, and later, to the person or device that moved something or caused movement. “As the word came through French into English, it was used in the sense of ‘initiator,’” says Fuller. “A person could be the motor of a plot or a political organization.” By the end of the 19th century, the Second Industrial Revolution had dotted the landscape with steel mills and factories, steamships and railways, and a new word was needed for the mechanisms that powered them. Rooted in the concept of motion, “motor” was the logical choice, and by 1899, it had entered the vernacular as the word for Duryea and Olds’ newfangled horseless carriages.
“Engine” is from the Latin ingenium: character, mental powers, talent, intellect, or cleverness. In its journey through French and into English, the word came to mean ingenuity, contrivance, and trick or malice. “In the 15th century, it also referred to a physical device: an instrument of torture, an apparatus for catching game, a net, trap, or decoy,” says Fuller.
In the early 19th century, the meanings of motor and engine had already begun to converge, both referring to a mechanism providing propulsive force. “The first recorded use of ‘engine’ to mean an electrical machine driven by a petroleum motor occurs in 1853,” says Fuller.
Today, the words are virtually synonymous. “Language evolves to take on new tasks,” she explains. “Without thinking about it, we adapt to new meanings and leave the old behind.” We talk about our computer’s dashboard, unaware that in the 1840s, the word referred to the board at the front of a carriage that stopped mud from being splashed on the coachman. Similarly, the term “search engine” harks back to the older meaning of “engine” as a contrivance, suggests Fuller. First used in 1984 to mean “a piece of hardware or software,” the phrase may have been informed by Charles Babbage’s 1822 use of “engine” to mean a calculating machine.
The related word “engineer” was first used in 1380 to describe the constructor of military engines like siege works and catapults, and by the early 18th century, referred specifically to the maker of engines and machines. The OED lists a second definition of “engineer” as well. “It is synonymous with the older usage meaning ‘artifice,’” says Fuller. “An engineer is an author or designer of something, a person who contrives a plot, a schemer.” A definition one can only hope will soon pass from common usage.

So I will talk about diesel engines and electric motors.

Back to the plot… so there are the two types of hybrid. Both types can have the power augmented through renewables – solar and/or wind power augmenting the electric drive.

Also clear from the above for us is that since we have an existing boat, AND that our boat can go much faster than electric motors by themselves would let us, AND that our diesel engines that work perfectly well and don’t have to be removed at great expense, then the parallel hybrid is our best option.

Solar augmented, parallel hybrid

Our choice is therefore solar-augmented parallel electric hybrid. For many boats, this is not a very viable option as they simply do not have the solar area to have a decent collection of solar energy, AND they are also not designed for efficiently going through the water (ie. for less fuel consumption). We though have a power catamaran which is wider than many monohulls (though not as wide as most catamarans), we have a motor yacht so suffer less shading on the solar panels than sail boats, so decent amounts of solar-augmentation is possible and since the boat is a catamaran, it should take less power to go through the water than many other vessels.

Notice that one of the benefits is not “saves money on fuel”! Fossil fuel saving can be a by-product of the choice and solution, but it also may not be – it depends on how the boat will be used.

Generally, the following is true for us and most hybrid electric yachts (not all electric boats, some of which are designed…differently! I’m talking about larger displacement, semi-displacement or planing yachts and not hydrofoiling fast taxis):

  • if you want to speed more than 6-7kn, choose diesel unless you have a huge battery AND don’t have to go far
  • if you want to go far on electric,
    • and can use diesel assist, then you need to go slowly (say 4-5kn) AND have a good size battery AND have diesel to add to the battery
    • and only electric, you need to go very slowly (3-4kn) AND have a good battery AND have large solar augmentation

Everything else depends on those two. There are many electric yachts that only travel short distances (<20nm) and use the electric power only to enter or leave a port. These are typically sailing yachts that spend the rest of their time under sail with no propeller propulsion.

The next post in this series will discuss our expected usage patterns for hybrid electric with solar augmentation, and then I will provide the revised electric plan for Blu Emu.

3 thoughts on “Hybrid electric power for Blu Emu”

    1. Hi Carsten,
      Thanks for the query. We had the boat ready for Europe and in the UK, and then covid lock down happened. Australia, where we are, was one of the most stringent. After 6 months of became clear we wouldn’t be in Europe soon, so we decided to relocate the boat back to Australia and start discovering more around the pacific.
      So Blu Emu is in Australia. Work and life has meant little opportunity, but we are planning a trip along our coast for April this year. Europe will have to wait!
      We still want to travel more in Europe, but initially may have to be on a hire boat in the next while.


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