There has been a lot written about ‘Hybrid’ yachts, often with some hype about being eco-friendly, having green credentials, etc., but with very little information of substance to support the claims.
Hybrid, in the context of motor yachts, just means there is more than one system capable of providing propulsion and/or electrical power.
Apart from a couple of innovative examples, all these systems rely on burning diesel fuel (MGO/MDO) as the primary energy source. This is sometimes glossed over where batteries are concerned.
Hybrid motor yachts and tenders have also been compared to Hybrid vehicles, but completely ignore the fact that they generate power from coasting or braking to increase their efficiency – something not available to yachts.
So, in this piece, I take a more in-depth look at Hybrid systems in an effort to provide some much-needed clarity on their benefits. I also question whether it is a transitional technology, or if it will be here for the longer term?
No Standard Operation
When it comes to superyachts and their operation, this is true. It’s what makes optimising the propulsion and electrical package challenging.
Main engines (ME) must be able to deal with variable speeds e.g. manoeuvring, cruising, maximum speed.
Generators (GEN) must handle variable loads and the ‘cycling’ of equipment that is dependent on the operational mode e.g. underway, guest-on, guest-off, in port or at anchor.
Given the variability of speed and load it can be difficult to size the main engines and generators for their best performance. This matters, because running engines at optimal load increases efficiency, reduces fuel consumption and emissions such as CO2; this is where ‘Hybrid’ can help…but, more on that later!
A Little History
Traditionally, superyachts used diesel main engines to provide propulsion power, and diesel generators to provide the electrical power for auxiliary and hotel loads – independent systems.
Fig 1. Traditional – ME to drive the propellors and GEN’s that power the hotel and auxiliary loads.
Although Diesel Electric has been used in shipping for many years it was not until the launch of M/Y Limitless in 1997 that it started to receive attention in the superyacht industry. And, in 2005 we saw the launch of M/Y ICE a Diesel Electric yacht that also used Azipods – interestingly both were built by Lurssen.
The story goes that many shipyards were reluctant to adopt Diesel Electric due to the complexity, engineering risks and the associated costs of developing a new platform – concerns that still apply to any new technology. However, their benefits, and the demands from yacht owners and their technical team saw a steady increase in use – especially on the larger yachts.
Fig 2. Diesel Electric – GEN’s provide electrical power for propulsion through Permanent Electric Motors (PEM’s) and the hotel and auxiliary loads.
Diesel Electric has many benefits. Having a number of generators (can be different sizes) that can be brought on-line for propulsion, and the hotel and auxiliary loads, meant the engines could run at optimal loads with better efficiency. It also provided a high degree of redundancy, greater flexibility in the technical layout – no longer limited by shaft lines – and, reduced noise and vibration which improved comfort.
You could say Hybrid is a mix of ‘Traditional’ and ‘Diesel Electric’ systems, combined with batteries (BAT) and sophisticated Power Management System (PMS) to optimise engine performance and efficiency.
The diagram below is an example of a Hybrid system.
Fig 3. ‘Hybrid’ Arrangement
Propulsion can be driven by ME’s, PEM’s or both (Boost), electrical energy can be generated from the ME and/or GEN’s and stored in batteries. The PMS takes care of managing the loads for propulsion, hotel and auxiliary systems.
The 83m Feadship, M/Y Savannah, delivered in 2015, is often cited as the first Hybrid motor yacht; though, just like Diesel Electric, it was not new to shipping. Since then we have seen am increase in its use across a variety of different sized motor yachts.
As mentioned, although diesel fuel is the primary energy source, there are some examples of innovative Hybrid motor yachts that use ‘Alternative Energy’. These include Artifact that uses solar, and the recently announced Lurssen project that will use Methanol in the energy mix.
No doubt the use of alternative energy will increase over time as the industry strives to reduce its carbon footprint.
It’s All About Efficiency
There are different types of marine diesel engines, two-stroke, four-stroke, slow speed, medium speed and high speed, each with their own performance characteristics. We use four-stroke high speed diesels on superyachts for both main engines and generators.
Whilst diesel engines are the most efficient internal combustion engines (ICE) at approx. 45% energy out vs. energy in, there are challenges to improve this – we may be close to the max with modern engine designs and materials.
What may not be so obvious, is that diesel engines have an optimal load; a point where the power generated requires the least amount of fuel. For many marine diesels this is at or near the Max Continuous Load and can be found from a table or graph of Load (kWh) vs. Specific Fuel Oil Consumption (g/kWh) in the engines test/technical file.
The example below is the SOFC for fixed speed 340 kW generator.
Fig 4. SOFC for 340kW GEN
The lowest fuel consumption 205 g/kWh is at 100% load. As the load decreases fuel consumption increases at a shallow and near linear slope until about 40% load, where it starts to rise exponentially.
The effect of running at or near optimal load on fuel consumption can be seen in the below.
Fig 5. Using one GEN at 80% load instead of two at 40% load can reduce the fuel consumption by approx. 9%.
Providing the generators are sized correctly it is unlikely they would run at less than 40% load – a point to bear in mind when looking at fuel efficiency claims for ‘Hybrid’ systems as they may be based on the more extreme ends of the SOFC curve.
This is also relevant to main engines though because they are able to run at variable speed, the difference is less pronounced – one of the advantages of variable speed generators as fitted to Artifact.
In the example below of a 3600 kW main engine, the SOFC slope is shallow and near linear from 100% load down to 25% load, where it starts to rise exponentially. There is about a 5% difference in fuel consumption between 25% and 100% load – a much wider load range and lower fuel saving compared to the fixed speed generator.
Fig 6. SOFC for 3600 kW ME
Unless ME’s are significantly over-sized to eke out that extra 0.5 knot of top speed, the only times when running at less than 25% load would be whilst maneuvering and/or motoring at sub-optimal speeds – you can determine this by comparing power/speed graph with SOFC.
What About Batteries
Batteries are just one form of Energy Storage System (ESS) others include devices such as capacitors and flywheels but, they all need an external energy source to charge them e.g. generators, solar, shore power.
Batteries perform a number of functions in a Hybrid system.
Each of the above helps to improve performance and efficiency of the yachts systems by keeping the engines operating at their optimal load, reducing fuel consumption and CO2 emissions.
Batteries with sufficient capacity can also power propulsion and/or the hotel and auxiliary loads for short distances or time.
Engine Free Operation
Whilst it is clear that batteries are an integral part of a Hybrid system – though not essential as I have sailed on a Hybrid yacht without batteries – whether they offer significant benefit when used for ‘Engine Free’ operation is another matter.
Engine Free, often described as ‘silent’ or ‘zero emissions’ mode, is perhaps a little more nuanced than might be suggested.
One reality is that emissions are not really reduced or prevented, they are merely displaced by time and location and do nothing for the overall CO2 footprint of the yacht – you still need to burn diesel fuel to generate the electrical energy stored in the batteries.
When it comes to noise, I also found that on larger yachts that with proper isolation, sound insulation and dry stack exhausts, it was difficult to tell if a generator was running in the guest areas. It was often the air-conditioning that emitted the most obvious noise and shutting down the a/c often resulted in an eerie silence, even with generators running.
With the above and, given the known issues with Li-ion batteries, from their production and imbedded emissions, to safety, life-cycle and recycling, I think the use for Engine Free operation needs to be carefully assessed. The current battery technology does not provide sufficient capacity for any reasonable level of autonomy without significantly impacting the interior space and weight.
In addition, if installing large battery banks for Engine Free running, you may need to upsize the generators as the total amount of energy required is not diminished, it is just generated over a shorter period of time. You may also lose some efficiency gains due to the power required to keep the battery banks at optimum temperature.
A Hybrid Solution
The following example provides flexible modes of operation.
Fig 7. Fully Integrated Solution
A. Stationary – Hybrid GEN/BAT for electrical loads
B. Low Speed – Propulsion and electrical loads from Hybrid GEN/BAT
C. Cruise – Mechanical propulsion, Hybrid GEN/BAT electrical loads
D. Performance (Boost) – Combined mechanical and electrical propulsion/loads from Hybrid GEN/BAT
The main purpose of these modes provides operational flexibility to ensure the optimal engine load, performance and efficiency.
Fuel Saving and CO2 Reductions
Whilst we now know fuel can be saved by running diesel engines at optimal loads, the more difficult question to answer is; by how much?
One DNV study ‘Electrical Energy Storage For Ships, published in 2020’ suggested that the ‘fuel saving potential’ for yachts was around 5 – 10%. Engine manufacturers and yacht builders tend to be a little more “optimistic” with the potential savings and benefits often using emotive words to describe their benefits.
Where you measure the ‘delta’ from matters, as using the extremes will result in significant differences in SOFC, but may not reflect the real-World use – the time the engines spend at least efficient load may have very limited effect on overall consumption.
It is important not to focus on the headline figure, but to evaluate the potential savings across a range of speed and load scenarios that are expected with the yachts use. The engines technical files will help identify the real savings.
Is It Worth It
The evidence confirms that a well-designed ‘Hybrid’ system can have a positive impact today in terms of more efficient yachts and reduced fuel consumption. According to the IMO fuel coefficient, every ton of MGO/MDO emits 3.206 tCO2. So, on that metric alone, the answer would have to be yes.
Unfortunately a ‘Hybrid’ yacht will be more expensive which no doubt affects more widespread use. When viewed purely on a cost/benefit basis, it may be hard to justify the extra cost against the potential fuel savings, and reduced operational costs due lower engine hours and extended maintenance intervals. Depending on use, the break even point may be a many years away and lay outside the envisaged ownership timescale. So as long as it’s an ‘option’ from builders cost will probably trump environment.
In the evaluation process, it worth considering the cost of diesel fuel may increase in the future due to carbon tax, and renewable fuels such as HVO (drop-in diesel) will be significantly more expensive. So, the additional cost today, may be sound investment for the future?
Finally, whatever energy is used in the future, it will likely be less energy dense than diesel fuel, and certainly more expensive. This means power management and efficiency will become even more critical and Hybrid technology will remain a fundamental component, especially in mixed energy solutions.