New technology for greener yachting

21 January 2015 • Written by Dag Pike
Arcadia‘s superstructure makes extensive use of krypton-filled double-glazing with solar cells incorporated into the glass.

Every boat show sees new motor yachts coming on to the market claiming to have green credentials. The variety of designs being promoted highlights the many different ways that there are to go green and there are a lot of alternative technologies on offer that point the way to greener yachting.

As with many new technologies there are claims and counterclaims and for many owners it is a confusing picture. Here we look at these technologies and analyse just where the green market is heading and what it is trying to achieve.

There are various ways to cut the pollutants yacht engines emit, but they fall into two categories: prevention and removal.

First of all, it is important to decide just what we are trying to achieve when we want to go green.

There can be no doubt that trying to reduce the amount of harmful emissions that come out of engine exhausts is a worthy aim. This has led to two different approaches to the green challenge: reducing the amount of fuel that is burnt by the engines, and reducing the amount of harmful gases that come out of the exhaust itself. To counteract emissions such as particulates and nitrous and sulphur oxides there is a need to clean up the exhaust from the engine with both filters and scrubbing devices, such as a Selective catalytic reduction (SCR) system.

While emissions are being tackled by the engine manufacturers, the general reduction of CO2 and other emissions is also in the hands of the yacht designer.

SCR systems add a urea solution into the exhaust flow. This creates ammonia which turns nitrogen oxides into harmless nitrogen and water vapour.
There is a lot of green innovation in this new design from Baia, the 24m Sevolution

Hybrid systems

Much of the focus of so-called green yachts is on hybrid system. In most hybrid systems there is a combined generator and electric motor included in the drive train between the engine and the propeller. This can generate electric power when the diesel engine is running and this power is fed to a large bank of batteries that stores the electric power until needed. The batteries supply electric power back to that generator/motor when it is needed for propulsion so that the diesel engine can be switched off and you have wonderful silent propulsion with no emissions.

However, although the propulsion is emission free, the diesel engine used to generate the power creates emissions. In fact by using a hybrid system you are actually producing more emissions than you would if a straight diesel drive was used. This is because every time you use diesel fuel to generate electricity for the batteries and feed it from there to an electric motor there are considerable losses, perhaps around 10 per cent with every step. That can amount to around 40 per cent of efficiency loss by the time the power gets to the propeller.

A false economy

Engine manufacturer MTU is developing a hybrid propulsion system based on its experience with train propulsion. It is working with Sunseeker to develop a hybrid motor yacht and it claims it may be able to make fuel savings of 5 to 10 per cent. The MTU system uses a motor/generator coupled directly to the engine but with a clutch. The savings are created when one engine provides power to the two electric motors, so it is only possible at slower speeds.

Against this saving, the extra weight of the battery bank required for the system and the efficiency losses incurred when diesel power is translated into electrical power have to be taken into consideration.

Fuel cells – such as the proton exchange membrane, or polymer electrolyte membrane fuel cell (PEMFC) – are now viable for transport applications.

Of course, the yacht salesman will tell you that you can enter harbour with no emissions if you use electric propulsion, but emissions have to be produced somewhere, and they are just as harmful out at sea. You might think you are doing your bit by charging the batteries up from shore power but again that electrical power has to be generated somewhere. So think carefully about using hybrid systems if you want a clean, green conscience.

Sure, you will get quiet propulsion when you want some peace, and electric power can make harbour manoeuvring a more precise operation – but never forget that no matter what power base you are using, you will still be generating emissions somewhere along the line.

With a hybrid system adding perhaps around 10 per cent or more to the overall cost of the yacht you are paying a lot and achieving very little in terms of emissions reduction.

With this in mind, it is time to look at the factors that affect fuel consumption to give perhaps a more positive way of reducing emissions.

The Ferretti Mochi Long Range was one of the first green yachts.

Speed and hull design

The faster you go, the more fuel you will burn and the higher your emissions. So one of the easiest ways to reduce emissions is to reduce speed.

There is a whole new genre of motor yacht coming to the market with models that are designed to operate at slower speeds. The maximum speeds might be in the region of 20 knots and they are designed to be fuel efficient at 10 to 12 knots. These yachts are based mainly on semi-displacement hulls that can be efficient at lower speeds but if you put the power on, the fuel consumption increases rapidly.

For example, the 26.22m Custom Line Navetta 26 we tested in February 2011 would burn 450 litres per hour at 16 knots, but ease back to 10 knots and the consumption drops to a mere 80 litres per hour. The Navetta is a heavy yacht but a lighter yacht such as the Vicem 78 burns 240 litres per hour at 16 knots dropping to 100 litres per hour at 10 knots.

The important thing if you want to go green is to choose a hull form designed to operate efficiently at the sort of speed that you want to cruise at.

A full displacement hull will cruise at a very miserly consumption if you aim to cruise at 8 knots and the speed will top out at probably around 12 knots using double the fuel.

If you feel the need to go faster on occasions – perhaps to catch a tide or to get into harbour before dark – then a semi-displacement hull could be your choice. This should give you speeds up to 20 knots plus.

The Mochi Long Range exploits long, thin hull technology with its single hull extending forward and its trimaran form running aft

Then there are planing hulls based on a moderate or deep V – you get top speeds of 30 knots or more but they are unlikely to be economical at lower speeds because the hull is not optimised for these.

Weight is a determining factor for speed and hybrid yachts will suffer increased fuel consumption because of the extra weight of the batteries. One semi-displacement yacht I tested recently also had more than six tonnes of marble on board – quite a penalty when you want to go faster.

The demand for economical solutions is leading designers and builders to consider some novel hull forms. Mochi started the trend with its 23m Long Range design (a combination of trimaran/catamaran hulls) and Azimut’s Magellano range has a rounded V hull with wide chines.

Bray Yacht Design and Research in Canada has spent years refining and tank testing displacement hull designs to improve efficiency. Its concepts focus on the bulbous bows, side pods and stern skegs and it claims improvements of 15 per cent or more over conventional designs.

Hull materials also need to be considered. Composite hulls are not eco-friendly and are largely non-recyclable so there is a new focus on using composites made from vegetable materials.

Azimut is exploring this possibility and it is also focusing on reducing power demands on board by using anti-UV glass and LED lighting.

While green technology – like Arcadia‘s solar panelling – can reduce a yacht’s environmental impact, they’re expensive and end results are limited.


Ignoring for a moment the complications and technical resources needed to build a green custom superyacht, going green is not easy and it is not cheap even at the production boat level.

To a certain extent yachts may forced to go green in the future because the next generation of clean diesel engines will be required by law.

Most of the green options can only realistically be fitted to new yachts; owners of existing yachts will have to operate their yachts at a slower speed to reduce their carbon footprint. Reduce the speed and the fuel consumption drops along with all types of emissions, so it comes as no surprise that about 50 per cent of the new production yachts coming on to the market in the larger sizes are based on slower displacement or semi-displacement hulls with modest engine power.

Hybrid systems may be less efficient than a straight diesel drive but can offer a greener image simply because the yacht is going slower.

Perhaps that is the key. Slow speeds are a way for all owners to do their bit to save the planet and when you think about it, that is exactly what yachting should be about – living life at a slower pace.

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