Advances in superyacht propulsion

21 January 2015 • Written by Roger Marshall
The Fortjes Drive System one of the largest yacht drives available

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.

Volvo Penta’s IPS system have improved performance efficiency on smaller vessels

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.

Cummins’ Zeus propulsion system increases efficiency in smaller yachts.

Many of new drives tout space savings in the engine room, perhaps leaving extra space for an SCR unit and the urea storage tanks needed to reduce NOx emissions.

Whether they will save enough space is doubtful, but these newer drives could change the entire engine room concept by putting gearboxes and propulsion units outside the hull, eliminating steering systems and using the extra space for environmental controls.

Smaller craft also have seen performance efficiencies from various styles of drive trains: Volvo Penta’s IPS system led the way, followed by Zeus and now Fortjes.

The Fortjes system is squarely aimed at larger craft. Rumour has it that the Volvo Penta IPS system also will be available soon for larger vessel’s up to 36.5m.

The Fortjes Drive System

From Reintjes Gears in Germany comes a new pod drive propulsion system. The pod drive has propellers at either end of the unit, one pushing and one pulling, and splits the engine power between both props.

According to Reintjes, this results in a lower blade loading on the prop, less noise, less cavitation and better high-speed thrust. The drives are not rotatable and come with their own rudders.

As expected from a gearbox manufacturer, the gearbox for the drive system is integrated into the housing at the top of the drive, making for easy installation. Reintjes claims it takes about four hours to install.

The thrust line is horizontal, which improves boat speed and handling.

The drives are available for vessels up to 2,000 kilowatts (about 2,800 horsepower) making them one of the largest yacht drives available.

The Voith Linear Drive

Lying somewhere between a ducted propeller (Kort nozzle), with its low-speed, high-towing capability, and a water jet, with its high-speed but lack of low-speed manoeuvrability, is the Voith Linear drive.

By combining the two concepts, Voith has come up with a drive that has the advantages of both a jet drive and a Kort nozzle with low-speed manoeuvrability and high-speed, fuel-efficient potential.

According to Voith, the drive is capable of propelling a vessel at speeds up to 40 knots.

Like a pod drive, it sits outside the hull, saving space that would normally be taken up by the transmission (and maybe freeing up space for an SCR unit). Unlike a conventional ducted propeller, the unit has highly skewed propeller blades contained within a shroud, rather like the jet engine on an aeroplane.

It is steerable and has far higher speed capabilities than a Kort nozzle, plus it has high fuel efficiency. Voith says that the first Voith Linear drive will be placed aboard a wind-powered support vessel later this year.

Wärtsilä Concepts

The engine manufacturer Wärtsilä has suggested that even more changes will be coming to cut fuel consumption and emissions.

Wärtsilä sees lighter vessel construction, the use of interceptor planes for faster craft (the US Navy already has fitted wedge-shaped hull extensions on many frigates and destroyers and cut fuel consumption by several percentage points), counter-rotating propellers (similar to those used on the Fortjes drive system and Volvo Penta IPS), computer-operated pumps to reduce start-up loads and overall consumption, low-wattage lighting throughout the vessel and other systems that reduce house and engine loads.

In other words, Wärtsilä sees a totally integrated house and engine control system designed to cut emissions by keeping energy usage low.

For yachts, this could mean a far greater integration of key systems, multiplexing of all lighting functions and a huge reduction in the yacht’s carbon footprint.

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