Saltwater boats docked at a marina

I have written previously about the boating industry’s push towards “green energy” solutions in recent years.  For example, Mercury has introduced a line of small electric outboards called Avator, and Yamaha displayed a hydrogen outboard prototype at the 2024 Miami International Boat Show.  I don’t believe these efforts are driven by consumer demand.  Instead, I suspect it is more out of regulatory concerns (standard gas-powered engines eventually being prohibited or limited) and/or the desire to innovate and not get left behind.   In this article, I share some facts regarding recreational boating’s contribution to greenhouse gases (GHG) and touch on some of the alternative solutions being considered to create greener and more sustainable forms of marine propulsion.

What are Green-House Gases (GHG)?

Greenhouse Gases are simply gases that trap heat in the atmosphere by thickening the earth’s atmospheric blanketOur atmosphere has always retained heat and this is essential for our survival.  The only issue is how much.

Since the industrial revolution and proliferation of combustion engines (that consume fossil fuels), the amount of greenhouse gases in the atmosphere have increased considerably.  This, in turn, is believed by some to be responsible for the warming of ocean waters, the melting of polar ice caps, and perhaps even the proliferation of more violet weather events.   Hence the term “climate change”.

Boating’s Contribution to Greenhouse Gases

Recreational boating is responsible for 0.7% of CO2 emissions emitted by the transport sector in the U.S.

Recreational boating is responsible for less than 0.1% of global greenhouse gas emissions or 0.7% of carbon dioxide (CO2) emissions emitted by the transport sector in the US.  By comparison, passenger vehicles are estimated to be responsible for around 60%, while commercial trucks and buses account for approximately 25% of CO2 emissions from transportation.

Despite this low percentage, the marine industry is determined to reduce its carbon output even further.  And considerable progress has already been made.  Over the last two decades, the introduction of four-stroke and direct fuel injection outboards and the addition of catalytic converters for inboards and sterndrives has reduced marine engine emissions by over 90% and improved fuel efficiency by around 40% in the process.

But reducing this level even further will be a bit of a challenge.  This is because some of the solutions that have evolved in the automotive industry such as electric vehicles might not be the best fit for boating given its differing usage cycle.  While automobiles are used frequently and often rack up 10,000 to 20,000 miles annually, the average recreational boat is only used 35 to 50 hours per year.  Further, according to a recent study from the U.K. research firm Ricardo, only 40% of the carbon impact from recreational boats is from their operation.  50% is from manufacturing and 10% comes at the vessels end of life.  By comparison, the proportion of the carbon impact from the operation of automobiles is more than double (85%).  This means that the “payback” time for solutions such as electric propulsion, which tends to have a higher upfront carbon impact during the manufacturing process (vs. traditional internal combustion engines), is much longer for boats vs. cars.  In fact, the total emissions might even be higher than that of a traditional gas-powered engine over the entire life cycle of the vessel.

Comparison chart showing the carbon impact for boats vs. automobiles over its life cycle

Alternative Solutions

There are a number of alternative solutions put forth to potentially reduce the carbon impact from recreational boating.  The following is a brief overview of some of the more prominent methods.

Electric

As in the case with automobiles, electric propulsion for boating offers some compelling benefits.  Namely, they are very clean to operate, are extremely quiet, and they produce torque very quickly (good hole shot).  But, with current battery technologies, the range is fairly short and the recharge time is substantial.  Part of the reason why the range is so low is because it takes more energy to push a boat through water than to keep a vehicle rolling down the road.  That is why some electric boat manufacturers have been experimenting with “foiling”.  By lifting the hull out of the water, you reduce the friction making it easier to propel the boat forward.

Electric boats are also much more expensive than those with traditional combustion engines.  However, the difference should diminish somewhat over time as more are produced.

From a climate perspective, they might not be ecologically advantageous considering the higher front-end carbon impact in the manufacturing process and because much of our electricity today is produced from fossil fuels.  Until this is addressed, there really is no compelling reason to offer electric outboards at present if the objective is to lower our carbon footprint.

Hydrogen

There are two primary types of hydrogen engines – fuel cells and hydrogen internal combustion engines (HICE).  The former uses hydrogen to produce electricity to drive and electric motor.  The later uses hydrogen gas to combust in an engine that is very similar to what is used today (see article on Yamaha’s hydrogen engine prototype).

Regardless of the approach, the advantages of hydrogen are that it is much cleaner to “consume” than gas or diesel and you can refill very quickly – similar to how long it takes today.  Plus, the ingredients to produce hydrogen are abundant and renewable.

However, the availability of hydrogen fueling stations is extremely limited at present and the fuel price is considerably higher than that of gas or diesel.  Plus, hydrogen is less energy-dense on a volumetric basis.  To account for this, hydrogen fuel is typically stored in fairly large, pressurized tanks which also introduces some space and safety concerns.

Ecologically, the production of hydrogen energy is quite “dirty” in most cases.  This is because large amounts of natural gas are used (in a method called Steam Methane Reforming) which emits a considerable amount of CO2 in the process.  While there are cleaner methods to produce hydrogen, they are much more expensive and therefore seldom used at present.

Sustainable (Drop-in) Fuels

A third method to reduce carbon emissions involves using sustainable fuels such as biobutanol and hydrotreated vegetable oil that can be combusted in standard internal combustion engines.  This approach has received relatively little attention within the industry thus far but, according to the environmental research firm Ricardo, it might offer the best potential to reduce carbon emissions for most recreational boating applications.

What are sustainable fuels?  According to the U.S. Department of Energy, it is a type of fuel that is “produced from biomass sources through a variety of biological, thermal, and chemical processes. The fuel is chemically identical to petroleum gasoline.”

The main advantage of this approach is that it can be used with existing marine engines without sacrificing range like most other alternatives.  This is why they are often referred to as “drop in” fuels.

While sustainable fuels have the added advantage of being renewable, the degree to which they can reduce carbon emissions isn’t evident to me.  As with fossil fuels, combusting these alternative fuels does create CO2 emissions (the largest component of greenhouse gases).  However, much of this can be theoretically offset by the farming of biomass materials to produce it (since plants absorb CO2).  Further complicating matters is the degree to which fossil fuels are used in the production process (which would obviously lower the net environmental benefit).

On the whole, it is believed that these sustainable biomass fuels produce fewer greenhouse gases but the extent of the improvement is up for debate and depends, in part, on how the fuel is made.  Further, these alternative fuels are not readily available at present and are not expected to be in abundant supply anytime soon.

Hybrids

Hybrid propulsion systems combine traditional internal combustion engines with electric motors to provide both range and clean, quiet operation.  Typically, a diesel engine is used for the main propulsion, but an auxiliary electric motor is also used to provide power in slow speed situations (e.g., docking and maneuvering in the harbor) or to provided added thrust when needed.  Very little attention has been devoted to hybrids in boating.  Ironically, I was actually hired to conduct research with a hybrid prototype for Brunswick back in 2010.  However, I have seen very little activity with hybrids by any engine manufacturer since that time.  Because having a hybrid system will add considerable cost, complexity and space, it is likely only suitable for larger vessels at present.

Final Thoughts

Though few consumers are clamoring for these “green propulsion” solutions at present, manufacturers have no choice but to pursue them.  Given the emission reduction targets that have been placed on automobiles, recreational boats can’t be far behind.  But the timeline to develop these new technologies is lengthy and poses considerable risk to manufacturers.  I lived through this firsthand in the mid 1990s as Director of Marketing at Mercury.  Because of the emission reduction targets established at that time, the company was forced to expand into new technological areas – including the development of a “clean” two-stroke outboard which we later named OptiMax.  This is something that had never been done before and there definitely were no guarantees that it would work.  Fortunately, our solution, which used an air compressor to inject a pressurized fuel/air mix into the cylinders, succeeded.  OMC’s approach for Evinrude engines which used compression from the engine did not and ultimately led to the company’s demise.

Making the situation even more challenging is the fact that the volume of boats is much less than that of automobiles and the breadth of engines that needs to be developed is much more expansive – from around 2.5 HP all the way up to 500 HP in the case of outboards.  This leads to a huge capital investment to extend any new technologies across the product line-up while, at the same time, the economies of scale and the potential to reap the benefits (in terms of sales) are much less.   The lack of “scale” is largely while boats and marine propulsion in particular are so expensive.

So, against this backdrop, manufacturers will continue to push forth with innovative new products that are designed to potentially lower carbon emissions.  It is unclear at this time as to which approach (if any of these) will ultimately win out.  And, as the authors of the Ricardo study suggests, it will likely take a multitude of approaches given the diverse nature of recreational boating (from dinghies and small fishing boats, to PWCs, performance-oriented bass boats, offshore fishing vessels, cruisers and yachts).  The key thing to watch will be how various manufacturers strive to address the weaknesses or limitations of the alternative propulsion systems (electric, hydrogen, sustainable fuels or hybrids) to deliver a satisfying and affordable ownership experience.

Jerry Mona - BoaterInput

About the author

Jerry Mona is an avid boater and angler and long-time boating industry insider. With over three decades of experience, he is often considered to be the leading research expert with boaters and has helped numerous manufacturers and trade associations to understand the needs, wants, attitudes and behaviors of boaters. He now shares many of his insights about boats and boaters for free on his BoaterInput.com website.

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