While we see more and more electric cars on the streets these days, and an increasing number of electric boats on inland waters, fully electric cruising and offshore boats are still a rare sight. There are good reasons for this.
The Fundamental Differences Between Boats and Cars
There are two fundamental differences between boats and cars when it comes to electric propulsion. In cars, once you get up to cruising speed, the loads are relatively low; when you slow down and brake, you can recover some of the energy used to get up to speed. It is practicable for cars to carry sufficient batteries to have hours of driving and a 300 mile range.
With boats, the loads when accelerating are less than those for cars, but once you start to approach cruising speeds the hull resistance goes up drastically, resulting in continuously high loads, and then there is no slow down and braking energy recovery. You put these two things together and discover it is difficult to carry enough batteries to have more than an hour of operation at cruising speeds, resulting in an extremely limited range. And the faster you go the less the range, with the reduction in range being closer to exponential than linear. Almost always when you see the published range for an electric boat it is based on a slow boat speed in calm conditions with no adverse winds and tides.
Serial and Parallel Hybrids
Other than purpose-built boats with a large surface area covered in solar panels and a massive bank of lithium-ion batteries, there is no way to generate and store significant electrical energy on a boat without burning fossil fuels. With today’s technology, pure electric boats are doomed to short-range cruising.
Some high speed sailing catamarans can generate sufficient energy off a freewheeling propeller for modest subsequent electric propulsion but will still run up against the limitations of battery storage. And typically the regeneration capabilities do not live up to predictions and expectations.
For longer distance propulsion, a fossil-fueled generator is required, resulting in what is known as a serial hybrid. Unfortunately, as soon as it becomes necessary to fire up the generator, the overall efficiency of the system is often less than that of a traditional propulsion system! The more the generator run time, the less the serial hybrid system makes sense, especially given the additional expense and potential failure modes as compared to a conventional propulsion installation.
There is another approach to a hybrid propulsion system which is to add an electric motor to a conventional fossil-fueled engine. The electric motor takes care of all low-speed maneuvering and provides an energy boost in certain situations; the fossil-fueled engine takes over at cruising speeds and for sustained propulsion with the electric motor now being driven as a generator. This is a parallel hybrid (similar to a Toyota Prius). There are typically modest reductions in fossil fuel consumption with significant lifestyle benefits, but it is a long way from pure electric boating. To date, because of significant technical challenges, we have seen few well-developed parallel hybrid systems for boats.
High Energy Density Batteries and Shoreside Infrastructure
Fundamentally, to make electric boating viable for offshore cruising, or for any boat that requires more than a modest range under power, much higher energy density batteries are needed. In recent years announcements from major automotive companies about anticipated advancements in battery technology, with double or even triple the energy density of current batteries, have offered hope for the future of electric boating, but invariably these batteries have not come to fruition in the predicted timescales! In the meantime, every incremental increase in energy density expands the range of boats for which electric propulsion is appropriate.
But then we run into another problem. The more electric boats we get, the more likely we are to find a lack of shoreside infrastructure to support the charging needs. Unlike cars, where charging stations are becoming more widespread, many marinas and docks are not equipped with the necessary power supply to support a large number of electric boats. Currently, the Norwegian government is one of the few entities in the world investing in the infrastructure needed for widespread electric boating.
Conclusion
It is common for electric propulsion enthusiasts to grossly overestimate the capabilities of batteries and electric motors. Many who have applied electric propulsion to offshore boats have been disappointed and have ended up retrofitting their boats with conventional fossil-fueled propulsion engines. It's important to be clear-eyed, informed and objective about current technology.
At this time, electric propulsion is suitable for an ever-expanding range of boats with limited propulsion needs but is still not suitable for longer range propulsion or for extended runs at higher speeds.
Despite the potential, many have been disappointed with electric boating. It's important to be realistic and informed about current technology. Explore our Advanced Marine Electrics Course for more information!
If your interested in the past, present and future of boat propulsion, listen to Nigel's interview in the ShipShape podcast.
