Advances in superyacht propulsion

Increasing propulsion system efficiencies to cut fuel consumption and decrease emissions has been a growing trend in recent years in the superyacht industry. But it will soon go from being optional to being required as upcoming regulations from the International Maritime Organization (IMO) will necessitate major changes to a yacht’s engine room.

The IMO has amended the International Convention for the Prevention of Pollution on Ships (MARPOL), putting new, much more stringent limits on nitrogen oxides (NOx) and sulphur oxides (SOx) emissions. When these new regulations come into full effect in 2016, all yachts over 24m (78 feet, nine inches) will be affected.

SOx can be reduced by cutting sulphur levels in fuel, which require yachts to use the more expensive low-sulphur fuel.

NOx, on the other hand, is the tricky part. As it’s an exhaust by-product, reducing NOx levels will require the use of bulky Selective Catalytic Reduction (SCR) units in the engine compartment.

‘To our understanding of the technology available today, the only (reliable) way to meet [the amended MARPOL NOx emission limits] is through SCR technology,’ says Udo Kleinitz, technical manager at the International Council of Marine Industry Associations (ICOMIA).

This will have an enormous impact on production yachts as they are made in existing tooling that predates upcoming regulations. ‘Adding an SCR unit to an existing hull can be very difficult and may affect many other parts of the yacht,’ says Kleinitz.

With the current state of SCR technology, an SCR reactor could be as large as 30 per cent of the engine weight and size. Plus, the yacht will need urea tanks, piping and an atomizer, which, based on today’s technology, could be up to about 1.5m long, maintenance accessibility notwithstanding. In addition, the engine will have higher backpressure.

On the positive side, engines today are often tuned to minimize emissions through in-engine solutions that increase fuel consumption, but with the SCR treating the emissions, the engine could be tuned to minimize fuel consumption.

In addition, US-flagged yachts with engine installations above 600 kilowatts (800 horsepower) and falling under commercial vessel regulations will be subject to even more stringent controls from the US Environmental Protection Agency (EPA).

These match MARPOL’s NOx limits and add limits for other pollutants such as hydrocarbons and particulates. Meeting this EPA requirement is even more challenging and will require the use of clean fuels, which could mean applying a technology other than SCR or in addition to SCR (eg particulate filters and oxidation catalysts).

With these regulations on the horizon, manufacturers will have to look for space savings in the engine compartment and more efficiencies in their drive trains, as any improvement in efficiency will help to reduce exhaust emissions.

There are several ways to improve a yacht’s efficiency; the biggest contender is to improve the yacht’s propulsion system. Several new systems have shown promising results.

New fuels

The most major change could be in the type of fuel burned. Liquefied natural gas (LNG) burns far cleaner than diesel and is coming into use aboard commercial ships as dual-fuel vessels, able to seamlessly transfer between running on diesel and LNG.

Trinity Yachts is currently constructing two such offshore support vessels, and the yacht industry seems poised to join the new trend, shown by a recently proposed 99m yacht project, conceived by builders Fincantieri, designer Stefano Pastrovich, and engine manufacturer Wärtsilä.

However, LNG requires a large fuel tank (far larger than typical yacht tanks), has a lower output in terms of propulsive power per cubic foot than diesel, and it does not have the infrastructure needed to enable a yacht to refill at the nearest marina. Until a large infrastructure is developed and large tank storage methods are refined, dual-fuel engines are unlikely to be used aboard yachts.

This leaves us with improving efficiencies in the driving end of the engine and transmission.

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