Building Yachts to Create a Zero-Carbon Future

Chief Engineer John Pierce shares his vision on how yachting can become more sustainable.

Yachting is an industry that consumes a vast amount of natural resources in comparison to the number of people it serves and is therefore largely seen by society as being incompatible with sustainability. From my experience on board yachts, I know this is also the view of a lot of yacht crew. For example, I recall having a barbecue after completing an Atlantic crossing and one of the deckhands smoked a cigarette and threw the butt into the water. The captain reprimanded him, and his reply was, “We have just burned a hundred tonnes of diesel getting here and you are worried about a cigarette butt.” Of course, wanton littering is absolutely reprehensible, but the deckhand’s comment warrants some thought.

Future Survival of the Planet

For yachting to survive into the future and be a sustainable industry in a world that’s becoming ever more aware of the damage that humanity is causing to the environment, it will have to evolve and adapt greener technologies. As an engineer, I am interested in looking at yachting’s power and propulsion greenhouse gas (GHG) footprint and how it can be reduced. I am sure that yacht crew are already aware of how to reduce the daily environmental impact from things like throwaway plastics.

[Yachting] can be the leader in developing new greener technologies and thereby help to justify its existence in environmental terms.

Yachting, like commercial shipping, has to abide by the current environmental rules and regulations, such as the latest MARPOL Tier III limits, but as an industry with access to a lot of money, it should make the effort to go beyond the requirements. It can be the leader in developing new greener technologies and thereby help to justify its existence in environmental terms. Also, the health and cleanliness of the oceans and coastal areas are what make yachting an enjoyable pursuit in the first place, so it’s in the interest of yacht owners to make the extra effort to preserve the marine environment.

The International Maritime Organization (IMO) aims to reduce global shipping’s greenhouse gas emissions by 50 percent by the year 2050, as compared to 2008. The UK is more ambitious, and the UK shipping industry is currently lobbying the government for a £1 billion investment in order to achieve the government plans for zero-carbon shipping by 2050. Whatever new rules and regulations are implemented in the future to bring these plans to fruition will directly impact yachting.

Eco Power Options & Fuel

Burning fossil fuels produces greenhouse gases, mainly carbon dioxide (CO2), and these GHGs are causing the average temperature of the earth’s surface and atmosphere to increase, with extremely destructive consequences, including vast wildfires, diminishing of the Arctic and Antarctic wildlife habitats, and increasingly more powerful tropical typhoons/hurricanes. It is interesting to note that the CO2 concentration in the atmosphere was at 280 parts per million (PPM) for many centuries of human history until the beginning of the Industrial Revolution around the year 1760, after which the large scale burning of fossil fuels increased the CO2 concentration to the current value of about 412 PPM. That is a 47 percent increase. The global maritime fleet, of which yachting is a part, accounts for 2.2 percent of global CO2 emissions.

Most of the CO2 a yacht produces comes from burning diesel in the main propulsion engine and electrical generators. In order to reduce this, propulsion efficiency has to be increased so less fuel is burned each trip. Running the electrical generators has to be kept to a minimum. There is also the option of using lower-carbon fuel like LNG or having a different method of powering the vessel altogether.

My last yacht was a 58-meter sailing yacht and the fuel storage capacity was only about a quarter of that of a motor yacht of the same length. Since the sails were utilized for propelling the vessel as much as possible, the carbon footprint was only a fraction of that of the latter. Some sailing yachts, such as Black Pearl and Ethereal, go even further by having the propeller rotate in the wake water as the yacht sails, driving an electrical generator via the propeller shaft to supply the hotel load or charge batteries. However, many owners do not like sailing yachts. But during a motor yacht build, it may be possible to convince an owner to install some level of wind assist such as Flettner rotors, a rotating cylindrical column that stands vertically on the vessel’s deck and generates forward thrust from a side wind. Note that the rotor has to be rotated by a power source such as an electric motor. UK company ANEMOI is currently installing these rotors on commercial ships and claiming 10 percent reduction in fuel consumption. Of course, this all depends on the wind conditions. Another option would be the use of rigid wing sails.

Apart from wind assist, the yachting industry could follow the automotive industry and look increasingly towards electric or hybrid drive. The problem with a full electric drive, i.e. charge up in port and use electricity alone to propel the yacht between ports, is that the batteries would be unfeasibly large. In order for such yachts to exist, the energy density of batteries will have to improve a lot. For example, say a 50-meter private yacht that runs on batteries and an electric motor and mainly cruises the Med needs 1500 kW of propulsion power for five days continuously in order to satisfy the owner’s requirements. The batteries will have to store: 1,500 kW x 24 hours/day x 5 days = 180,000 kWh of energy.

The next generation of Tesla batteries will apparently have an energy density of 0.4 kWh per kilogram. Dividing 0.4 into 180,000 gives us 450,000 kg of batteries required, which is 450 tonnes, obviously an unfeasible number for a 50-meter yacht. However, a hybrid drive, i.e. partly battery powered and partly diesel powered, could reduce the yacht’s carbon footprint. The batteries can be charged in port using renewable energy sources like wind and solar, and an electric motor can assist the diesel or diesel-electric drive, thus reducing the amount of fuel burned. This would also allow installation of a smaller diesel engine, which would improve efficiency as the engine would be running at a higher load percentage than a bigger engine would at lower cruising speeds and during maneuvering. Diesels run more efficiently when run at higher loads, near their design point.

However, a hybrid drive, i.e. partly battery powered and partly diesel powered, could reduce the yacht’s carbon footprint. The batteries can be charged in port using renewable energy sources like wind and solar, and an electric motor can assist the diesel or diesel-electric drive, thus reducing the amount of fuel burned. 

With regards to fully electric drive, until the energy density of batteries can be vastly improved, the near-future alternative is fuel cells. Fuel cells run on hydrogen or natural gas. The fuel is reacted with oxygen in the fuel cells in the presence of a catalyst to create electricity. Note that natural gas is mainly methane, CH4, and is not carbon-zero. A hydrogen fuel cell, on the other hand, produces only water as a waste product. I am not aware of any large yachts that currently use fuel cells for propulsion power; however, there are rumors that such a yacht is going to be built for Bill Gates. Hynova Boat in France has launched an electric drive prototype 40-foot yacht called New Era that runs on hydrogen fuel cells. Another great advantage of fuel cells is their energy efficiency — 80 percent of the energy in the hydrogen can be converted into useful energy by a fuel cell, whereas an internal combustion engine burning hydrogen would have an energy efficiency of only about 25 percent.

Commercial shipping is beginning a migration to LNG as a fuel. LNG is the cleanest fossil fuel as it is the hydrocarbon with the highest hydrogen/carbon ratio. It produces 20 percent less CO2 than diesel per kWh output. Some large LNG-powered commercial ships, such as container ships and bulk carriers, have been delivered recently by Hyundai Heavy Industries. 

Some conventionally fueled container ships are being retrofitted to run on LNG. Carnival’s brand-new cruise ship Mardi Gras will be LNG fueled. However, the LNG bunkering infrastructure is still not widespread outside some selected ports and an LNG-powered yacht roaming the Mediterranean and Caribbean would have difficulty in finding fuel. The LNG bunker infrastructure is seeing a lot of investment and development, particularly in Northern Europe, and it is only a matter of time before it will become widely available. It is therefore a good idea to “future-proof” yacht new builds by installing main engines that have the capability of running on LNG and having the hull designed to incorporate LNG bunker tanks at a future date. Such yachts will be dual-fuel vessels, i.e. they can run on either LNG or diesel.

The LNG bunker infrastructure is seeing a lot of investment and development, particularly in Northern Europe, and it is only a matter of time before it will become widely available.

Reducing Existing Yachts’ Carbon Footprints

To maximize the propulsion efficiency and reduce the fuel consumption of an existing yacht, there are a few things the captain and chief engineer can implement. The condition of the antifouling below the waterline should be kept in tip-top shape by regular checking and underwater cleaning, with a new coat applied at the recommended intervals to reduce drag resistance. It’s important that new coatings are applied correctly, or excessive roughness will occur, which increases drag resistance. A newly antifouled hull should always be roughness tested — a maximum roughness, say 100 or 120 microns, should be agreed with the coating contractor, above which the job will not be accepted. Propeller roughness is also a cause of excessive fuel consumption and when the propeller is polished, the roughness should likewise be tested.

Modern yachts are designed using the latest Computational Fluid Dynamics (CFD) techniques in order to minimize hull resistance and maximize propeller efficiency, and the margin for improvement on a new yacht is probably small. However, on older yachts it can be worthwhile to get a naval architect to review the hull, rudder, and propeller design with modern techniques and design modifications to improve the efficiency. For example, installation of a bulbous bow can be a relatively cheap way of reducing the wave making resistance.

Some commercial ships reduce drag resistance by blowing compressed air through little holes in the underwater part of the hull at the bow. These bubbles move aft with the flow of water along the hull and provide a resistance-reducing air bubble jacket around it. This system could be worth installing on larger yachts.

If a propeller is noisy, vibrating, or cavitating, it is not running efficiently and is wasting fuel and the reason has to be investigated immediately and rectified. It may be a simple issue such as imbalance or more complex such as having the wrong pitch. I used to work on an old classic 56-meter motor yacht that turned out to have the wrong propellers, which apparently had been taken off a tugboat and obviously had the wrong pitch altogether. Propeller efficiency can also be improved by the addition of propeller boss cap fins (left). These work by breaking up the propeller hub’s vortex.

With regards to yacht electrical load, generator running should be minimized by going on shore power as soon as the yacht arrives in port and not starting the generators too early when leaving. Of course, sufficient time has to be provided for all essential tests.

The yacht' electrical load can be reduced by having more efficient devices on board. For example, Miele produces clothes dryers with heat pumps instead of electrical heating resistors. This reduces heating power consumption by the dryer by about 75 percent. Lights can be replaced by high-efficiency bulbs or LEDs. The air-conditioning load is a major consumer of electrical power and can be reduced by ensuring the yacht interior is insulated properly from the exterior. An audit on the insulation can be carried out and any improvements implemented in the next yard period. Also, old air-conditioning systems can be replaced by more efficient modern ones. Modern AC compressors utilizing modern gases have vastly improved in energy efficiency. It is essential that all apertures between the interior and the outside are kept closed as much as possible to prevent loss of cool air.

The air-conditioning load is a major consumer of electrical power and can be reduced by ensuring the yacht interior is insulated properly from the exterior. 

Yachts over 400 GT, like commercial ships, are required to have a Ship Energy Efficiency Management Plan (SEEMP). It’s important to use the SEEMP as a tool to reduce the yacht’s GHG footprint, rather than see it as an onerous paperwork exercise.

While I wrote this with the intention of increasing awareness of how yachting’s GHG footprint can be reduced, the ideas mentioned are not comprehensive and I encourage officers, crew, and yacht builders to think of and research as many ideas as they can to reduce the GHG impact of the vessels they are building and running. It’s important to keep in mind the ultimate goal of a zero-carbon future.

This article originally ran in the December 2020 issue of Dockwalk.