New research reaches some surprising conclusions about which propulsion systems might actually have the lowest carbon footprint. Theo Stocker digs into the numbers

A white sail on a blue sea. What could be cleaner than that?’ Sailing nerds might recognise this quote from the all-time greatest sailing film, Wind. Or maybe that’s just me. Anyway, it’s easy to assume that drifting around with the breeze has little impact on the environment, let alone the global climate. But that assumption may be further from the truth than you think.

Certainly, the marine industry is tiny compared to other emitters of greenhouse gases, but it still registers on the scale. Recreational boats account for less than 0.1% of global greenhouse gas (GHG) emissions, specifically 0.7% of transportation carbon dioxide (CO₂) emissions in the United States and 0.4% of transportation CO₂ emissions in Europe.

Those numbers sound small, but then consider that there are estimated to be 50 million recreational craft globally, with as many as one million new boats being added to that number every year. New research has now been published by the International Council of Marine Industry Associations (ICOMIA) as they seek to plot a way forwards for the leisure marine industry.

The rather bleak headline is that if you want to move away from fossil fuel propulsion, you could easily generate a larger rather than smaller carbon footprint, whether you’re looking at biofuels, hydrogen, electric or hybrid propulsion systems.

The good news, however, is that the devil is in the detail. Make a few compromises to things like range and performance, conduct an honest appraisal of how you actually use your boat, and utilise new and emerging technologies, and you can reduce your boat’s carbon footprint, whether you’re buying a new boat or not.

John Kerry, with his granddaughter, signing the 2016 Paris Agreement on behalf of the US. Photo: Xinhua / Alamy Stock Photo

Paris climate agreement

The Paris Climate Agreement, a legally binding international treaty that seeks to limit global temperature increase to 1.5°C above pre-industrial levels, requires signatories to reduce global carbon emissions by 43% by 2030.

While the marine leisure industry might be a tiny proportion of global emissions, it is still a contributor. Given recent protests against superyachts and the perception of sailing and boating as an elite, luxury pastime, ICOMIA wanted to be able to work with regulators to reach policy decisions based on data rather than emotions, assumptions or ideologies.

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There was, however, a ‘vacuum of data’ and while regulators, companies and customers were all trying to take positive steps environmentally, there was very little cold, hard evidence guiding them, leaving potential for flawed choices and the potential dismissal of the best options for reducing the marine industry’s carbon footprint. To fill this void, they commissioned the most detailed research project ever undertaken on the subject in the marine industry.

The result of the project is the 550-page report Pathways to Propulsion Decarbonisation for the Recreational Marine Industry. So far, the report and its data has been shared with the European Commission Department of Growth, the US Dept of Energy and Governments in the United Kingdom, Spain and Sweden.

Infrastructure for new technologies to be adopted is an essential part of assessing their carbon footprint. Photo: Scharfsinn / Alamy Stock Photo

The study

ICOMIA set out to create a robust and holistic analysis of the full environmental impact of all currently available propulsion technologies for recreational vessels under 24m (80ft).

It was to consider the full-life ‘cradle-to-grave’ impact of building, using and disposing of leisure craft. The report covers first-of-its-kind primary research study from Ricardo, a leading global engineering consulting firm. The resulting data took two years to compile and was subject to thorough peer reviews in an attempt to make this the most robust data possible.

ICOMIA’s aim was not to make a pronouncement on what technologies are green or not, but to provide benchmark data against which governments, boat builders, technology developers and you, the sailing public, can make informed choices.

The study covered nine types of vessel up to 24m, including inflatable tenders with small outboards, runabout day motor cruisers, fishing boats, pontoon boats (a large sector in the US),
personal watercraft, sailing yachts, inland waterways craft, large displacement motorboats and performance motor yachts.

Steve Bruce, UK MD for ePropulsion, is sceptical about ICOMIA’s report. Photo: Scharfsinn / Alamy Stock

The study was broken down into six steps: exploring the decarbonisation options; conducting a Greenhouse Gas Life Cycle Assessment (LCA) to ISO 14044 and 14067 standards including the manufacture, use phase and end of life for energy converters and energy carriers; assessing the total cost of ownership of each system, including purchase, operation and maintenance; analysing the implications of life expectancy, maintenance, performance, safety and availability of each system; analysing the infrastructure implications, before ranking the suitability for each craft type and usage case.

The report is still pretty broad brush strokes, and there are plenty of caveats to the findings. How and where boats are used, what technology is installed, and the supply chains in each of those products have an impact, and ICOMIA says that it is keen that the data acts as a baseline for people to innovate around and find ways of cutting emissions in every part of a boat’s lifecycle.

Even so, some are sceptical about the report. Steve Bruce, Managing Director (UK) of electric engine manufacturer ePropulsion said, ‘I have to say I am extremely disappointed.

It would appear to me that the report has been written with a bias against electrification and has failed to consider several relevant factors, such as the ability to re-purpose and ultimately recycle batteries.

‘It also seems to be based on facts that do not appear to be in line with what we are being told by boat users in our daily conversations, so I would like to better understand precisely where the report writers have decided to select their data from and why.

‘The small amount of hours they suggest boats are used for does not correlate with the actual use profiles we are being asked to support.’

From a user point of view, electric propulsion has many benefits over combustion engines. Photo:

What the research doesn’t do

The report’s authors were keen to emphasise that they were not aiming to stymie certain solutions, but rather to identify the real-world use cases in which these systems are the best to choose, and to highlight where more development and investment is required.

They also noted that the research focuses on propulsion systems, while there are a whole host of other areas in which a pleasure vessel could reduce its carbon footprint, from building materials and manufacturing processes to supply chains, shipping, and disposal. Similarly, how a vessel is used will have a big impact.

ICOMIA CEO Joe Lynch explained, ‘This isn’t to tell people what to do but to help them make the best decision for their use case and allow industry to find solutions, by providing a set of data. It is a point in time study for most popular types of vessel currently in use. We hope to expand this out to more use cases and more emerging technologies, such as foiling, in due course, and aim to turn the work into a more usable life cycle analysis tool as a basis for solid decision making to help decarbonise the marine industry.’

The research did not interrogate the use of recycled goods, for example repurposed pre-used car batteries, or the re-engining of existing yachts, and nor did it pit one type of vessel against another.

It also didn’t give weight to local considerations, such as the need to reduce pollution or noise in ecologically sensitive environments, or increased user enjoyment or a reduction of maintenance.

‘This is a decarbonisation report rather than a usability report,’ said Lynch. ‘There are many other reasons to choose electric-based criteria such as noise, smell, cost, maintenance and so on,’ acknowledging the focus is very much on the global carbon emissions of a vessel’s lifecycle.

Electric cars have a very different set of parameters, meaning they are more able to offset the additional carbon emitted in their manufacture. Photo: Kenny Williamson / Alamy Stock Photo

Cars vs Boats

One of the big assumptions the report seeks to interrogate is that because electric cars, or ‘battery electric vehicles’ (BEV), are thought to be the most environmentally friendly solution for land-based transport, the same will apply on the water. The problem is that there are vast differences between cars and boats, according to the data.

For a start, propelling a boat over a set distance takes roughly 10 times more energy than it takes to move a car. Then you’ve got the fact that most cars are used much more regularly and for
longer periods than cruising boats.

This means that of a car’s total lifetime carbon footprint, less than 20% of it is in its manufacture, with almost 80% in its usage – energy supply, exhaust emissions and maintenance – and a bit in its end-of-life disposal. For a boat, the research shows that while some yachts are heavily used, many lie idle for great chunks of time, and they arrived at an average annual usage of a sailing boat’s engine of just 24 hours a year, based on data from engine manufacturer service records and data from the US Environmental Protection Agency.

For the various kinds of motorboats, this increases to between 35 to 48 hours a year. Only a commercially operated rental jet ski had an average use case of over 100 hours a year.

Proportionately, as much as 50% of a boat’s lifetime emissions come from its manufacture, 10% from scrappage and 40% from its usage (and even less when it comes to sailing boats). This gives you a much shorter lever with which to balance out carbon already emitted with a reduction of carbon in usage.

Small dinghies are now frequently powered by electric motors. Photo: Benjamin Sellier

Supply chain carbon

The research analysed the carbon contribution, or global warming potential (GWP), of each stage in the supply chain and lifecycle of each propulsion system, for each of the vessels being considered.

It assumed a like-for-like comparison of energy storage and range, rather than the ‘optimised’ systems referred to later. While these vessels would not be usable in the real world, it made it possible to compare the carbon footprint of equivalent processes within the supply chain.

In the figures given, an inflatable dinghy using an electric outboard can eliminate around 40% of its total GWP from its usage emissions (energy, tank to wake) alone, compared to the petrol-driven baseline. However, this has to offset an almost three-fold increase in its raw materials (propulsion) carbon footprint and a manufacturing footprint that is around two-and-a-half times higher.

Using sustainable marine fuel caused a far bigger increase in the craft’s total footprint, in which the energy well-to-tank actually doubled the craft’s footprint.

For sailing yachts, there is a proportion of the vessels’ total GWP that is attributed to the hull and structure’s raw materials that is several times higher than that of any of the propulsion systems’ raw materials, including electric, though manufacturing impacts are significant.

Mineral extraction and the processes involved are a large part of batteries’ carbon footprint

By far the worst option for a sailing boat was hydrogen, where the well-to-tank impact of the fuel accounted for 40% of the vessel’s total GWP, giving a hydrogen-propelled vessel a carbon footprint twice the size of a fossil fuel-propelled boat.

Both electric and hybrid systems’ GWP were 50% higher than the baseline, with much of this coming from raw materials, manufacture, and surprisingly, maintenance, given electric systems’ almost maintenance-free usage, due to the necessary replacement of the batteries (estimated life span of 12.5 to 15 years).

Even sustainable marine fuel had a higher total impact by about 12% before use-case optimisation, due to a higher energy well-to-tank impact.

Darren Vaux, President of ICOMIA, says, ‘There is a lot of carbon in the supply chain of the batteries, and because there is such low utilisation of hours, it’s very hard to offset. Sailing craft life is long, so the batteries have to be replaced during the course of its life because they don’t have the same longevity.

‘The fascinating thing is that electric motors’ torque profile, lack of noise and all of that are absolutely ideally suited for marine. The challenge is the energy storage, both in terms of the energy density, and also the life of the batteries and the carbon embodied in them from most battery manufacturers.

‘Where manufacturers who operate in a country where they have a high green-energy mix, and a supply chain for manufacture in a factory with a very low carbon footprint, then there’s a competitive advantage to say, “I’ve got a battery that has a very low carbon footprint,” and that will address some of the carbon issue. The energy density of batteries is still significantly lower, but this may be satisfactory in some cases.’

Fitting any kind of alternative propulsion to runabout day boats like this RIB requires a significant compromise.

Realistic comparisons

The five power systems weighed up were all systems that use existing technologies currently available commercially. These were: conventional petrol or diesel internal combustion engines (ICE) as a baseline; sustainable fuels used as drop-in alternatives for fossil fuels in ICEs; hybrid fuel (fossil/sustainable) and electric systems; battery electric drives, and hydrogen ICE or fuel cells.

In order to calculate the life cycle assessment of vessels equipped with the various alternative propulsion systems, it was unrealistic to substitute in systems that provided like-for-like power and range.

As Patrick Hemp, ICOMIA technical consultant explained, ‘We get a lot of questions around the bottom-up approach and why H2 ICE and Battery Electric needed to be optimised. The main reason Ricardo had to do this was because the current baseline speed/range is simply not possible with Hydrogen ICE or Battery Electric (too much weight and volume) and hence, a downsizing was required.’

Fitting any kind of alternative propulsion to runabout day boats like this RIB requires a significant compromise

Runabout motorboat

As an example, a small runaround motorboat may have a typical range of 14 hours and 166 miles. To give the same range in a single duty cycle (one tankful or battery charge) with hydrogen propulsion, fuel storage would need to be around 430% larger than the ICE system and would be 350% heavier, giving the boat a displacement 56% greater.

With electric propulsion, the volume of the batteries for this range would be 360% larger and 820% heavier, with a vessel displacement 133% heavier.

In the calculations, the researchers decided to optimise the power system so that it could still fulfill the way most people use the boat, but with a propulsion system as close to the mass, volume and performance of the existing system as possible.

When aiming to achieve a range of 3 hours and 35 miles for the same small motorboat – a reduction of around 80% – a hydrogen system that was 61% larger and 13% heavier with a displacement increase of just 6% was achievable. For electric power for the same range reduction, the system was 23% larger, while being 81% heavier, with a displacement increase of 16%.

A sailing yacht has a huge fuel-free range if sailed, but most cruisers rely on auxiliary propulsion to keep making progress when conditions don’t suit. Photo: Richard Langdon

Sailing vessel

The calculation for a sailing boat used a baseline of a boat running on diesel, though the HVO fuel figures are identical. With a 21kW / 28hp engine and 70L fuel tank, the boat has a range of 24.5 hours and 147 miles, and there is no increase to mass, volume or displacement for either diesel or HVO.

Change to a hybrid electric drive system running a 21kW / 28hp ICE engine, with a 21kW electric drive. The fuel tank can be reduced slightly to 59L for the same range, but the system volume increases by 69% and would be 137% heavier, increasing the boat’s displacement by 6%.

For an electric or hydrogen system, the systems were specified to a range of 4 hours / 24 miles at an equivalent 6 knots, a reduction in range of around 84%.

The electric system, with a 21kW motor needed a battery capacity of 49kW, which resulted in a system that was actually 18% smaller than the baseline, though it was 61% heavier, and resulted in an additional 3% displacement.

Hybrid Marine specialises in diesel-electric parallel hybrid systems built around new Beta and Yanmar engines

The hydrogen-powered boat, with 21kW engine, had high-pressure fuel tanks to hold 3.7kg of hydrogen. This system was 49% larger than a standard engine and tank, and was 12% heavier, adding just 1% to the boat’s displacement.

Alternative propulsion systems were optimised in this way to enable realistic life cycle assessment comparisons to be made. Looking at the data then, sailors are free to make choices about whether they would be more happy to accept compromises to range, performance, the amount of space on board, and displacement, as well as cost.

Unsurprisingly, the report found alternatives are more expensive than the status quo, albeit within an enormous range. Electric systems as specified in the optimised use cases were 40% to 250% more expensive, 85% to 200% more for hydrogen, 25% to 115% more for hybrid, and 5% to 45% more for using sustainable drop-in marine fuel alternatives.

Large numbers of yachts remained unused or very lightly used, leading to just 24 engine hours a year on average

How much do boats get used?

While ‘battery electric on-road automotive vehicles’ (BEV) reduce CO₂ emissions by between 50% and 70% relative to conventional fossil fuel engines over their lifetime, the initial CO₂ created during the production of an electrical vehicle (EV) can be at least 50% more than a conventional ICE vehicle due to raw materials and the energy intensive process of creating the battery.

This means it takes an electric car 100,000 to 150,000km (62,000 to 93,000 miles) to get the break-even point where it is starting to have a positive impact on carbon emissions, which might take 3-5 years of fairly heavy use.

This also takes into consideration the fact that drivers’ range expectations have, on average, been reduced, so that one battery charge on your car will take you about 60% of the distance your old diesel or petrol car would have done. The cost, size and weight of the batteries would be prohibitive for a like-for-like range, and the environmental cost – mostly attributed to the batteries – would also be even higher.

For boats, those with very high use cases, and where small batteries with limited range can be specified, such as ferries and other commercial vessels, some of the new technologies really do add up in a similar way to cars. But the average leisure cruising yacht (though not, for example, a liveaboard cruiser), has just 24 hours of engine use a year, over its 45-year life span, according to the data.

Diesel engines remain the lowest impact propulsion to manufacture and install, but the emissions from the supply and use of them needs to be reduced

If this seems crazily low, marine surveyor Ben Sutcliffe Davies explained, ‘When I go to survey a vessel, I’m often amazed at how few hours have been put on the engines. It’s rare to find a yacht that’s done much over 50 hours in a year.’

Certainly, many boats will be used far more than the assumptions in this report, but there are also many boats that sit sadly idle, used for a handful of days or hours over the summer. The ‘use case’ of any boat is one of the key deciding factors in what propulsion technology will be the greenest option, and it’s worth remember that this report aims to look at the majority average rather than outliers.

In order to get to the ‘break even’ point on an average GRP cruising yacht, where the new propulsion system’s reduced usage emissions have compensated for its increased manufacturing emissions, you will need to do 60 hours of electric motoring a year (compared to 24 hours average), every year for its 45-year life span.

To get to a point where you have cut your emissions by 50% – the benchmark in the automotive industry – you will need to increase your usage by 600% above the average, or 168 hours of motoring a year on your electric engine. The figures are almost identical for a hybrid system.

Liveaboard and long-term cruisers clearly have a much higher use case, giving them longer to offset carbon with alternative technologies. Photo: Crew Eastern Stream

For a hydrogen system, which has a manufacturing footprint slightly lower than that of battery systems, the break even point comes sooner, with a modest 52% usage increase (36.5 hours), but with a slightly higher carbon footprint in use, a 50% reduction in emissions compared to a diesel engine will take a whopping 192 hours of motoring a year.

Surprising findings

Once the use cases and optimised propulsion systems have been taken into account, for many recreational sailing yachts that match the assumed use case, opting for electric propulsion in a new yacht or converting a used boat to hydrogen, hybrid or electric propulsion will be worse for the environment than sticking with a fossil fuel internal combustion engine (ICE).

For most vessels and average use cases, the best way of reducing environmental impact for the time being is to retain conventional ICE engines, but switch to sustainable synthetic or biofuels such as hydrotreated vegetable oil (HVO) or e-petrol, the report claimed.

Doing this in a sailing yacht will, over the vessel’s 45-year lifespan with an average annual usage of 24 hours, reduce the vessel’s global warming potential by around 35%, compared to using fossil fuels in an ICE engine. The same benefit applies to both new boats and the numerically more significant existing fleet of boats already in use. Switching to electric or hybrid propulsion may in fact increase the vessel’s global warming potential by over 35%.

If you can find somewhere to buy HVO biofuel, this will be simplest way to immediately cut your carbon

Of the nine vessel types analysed, electrification was only the greenest option for a commercially operated personal water craft used for 156 hours per year over a 12.5 year lifespan. The highest impact of going electric was for a displacement motorboat, used for 48 hours a year over 45 years, where the global warming potential went up by over 80%.

If your vessel, its systems, their manufacture, and your usage don’t match the averages used in this report, then the life-cycle assessment might reach different conclusions, particularly if your usage is higher than average, but it’s worth considering the lifespan emissions when making these decisions.

How we choose to build, propel, and dispose of boats could be limited if their climate impact isn’t addressed


The research concluded that renewable diesel fuel, specifically hydrotreated vegetable oil (HVO) ICEs can provide the largest global warming potential (GWP) reductions compared to existing ICE propulsion, but only if the fuel is produced using waste feedstocks so that it’s not taking resource away from global food production, and gives the marine industry the greatest chance of decarbonising by as much as 90% by 2035 without compromising a vessel’s range or performance.

That’s all very well, and marinas with existing fuel supply infrastructure should be able to adapt easily, but it would require a huge increase in the supply of HVO in order to facilitate that change, and that capacity just isn’t there at the moment. As the report notes, ‘There is considerable uncertainty over the availability of these fuels through to 2035, and caution must be taken to guarantee that e-fuels are produced using low-carbon electricity sources and biofuels are produced with low GWP feedstocks.’

It also concludes that because electric-only systems ‘may have a higher GHG contribution from raw materials and manufacturing than conventional propulsion systems’, vessels that have low-usage cases are unlikely to ‘yield a reduction in greenhouse gases’ over their lifespans. Boats that are in frequent and prolonged use may be more likely to reach the break-even point, while hydrogen and hybrid systems may, in some cases, be the greenest option.

ICOMIA is now working on making a full life-cycle assessment tool available to the industry so that boats and use cases can be examined on a case by case basis. The report is a snapshot of the LCA for each type of boat, and a series of assumptions made about its usage.

ICOMIA President Darren Vaux, explains, ‘These use cases should cover the majority of the market, but there will be outliers that don’t fit these use cases. We are trying to move the dial on meeting the Paris Agreement for decarbonisation of the marine industry. At the moment, the only thing that will get us there is drop-in HVO fuels.

‘We need to focus policy-makers’ minds,’ Vaux continues. ‘We’re building infrastructure for electric vehicles, but we also need an infrastructure for the supply of sustainable fuels. We also want to challenge innovation, to encourage the industry to use the data-set as a challenge, with a clear focus on what the supply chain carbon impacts are.

‘Our data-set is the majority case, but that leaves room for boat builders to find ways around it, to say, “We’re better than that.”’

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