Hybrid diesel-electric propulsion systems

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Hybrid diesel-electric marine propulsion systems

Tim Hunt

Beam Reach Fall 2007

Sustainability Report

The world of hybrid diesel-electric marine propulsion systems is a very large one, and dominates in the yacht/ship industry where efficiency is very important. It is a complex and technical industry and so the purpose of this impact reduction report is to take a small part of this world and briefly describe how a hybrid diesel-electric system works and the benefits of using such a system. The report will use the Gato Verde catamaran as an example of such a system, and then explain/propose a similar bio-diesel system that could be used on smaller, more manoeuvrable vessels that are used in the whale watch industry of the Salish Sea. The purpose of such a proposal is to potentially minimise the impact that these vessels have on the environment (both in terms of underwater noise impact and polluting carbon emissions into the atmosphere).

The hybrid/diesel electric system

In simple terms, a hybrid diesel-electric system (HDES) is comprised of three parts; a generator, a battery, and an electric motor. The generator is fuelled by diesel, which charges the battery to run the electric motor. In a marine propulsion system, these electric motors power propellers that enable movement through the water.

Advantages of a HDES:

• Diesel fuel can be substituted with bio-diesel fuel that is more environmentally friendly as it produces less harmful emissions

• Electric motors do not use fuel and therefore produce no polluting emissions

• Electric motors are silent and thus have minimal noise impact on the underwater environment

• Diesel-electric propulsion systems have an outstanding low-RPM torque characteristic in their electric motors, which means that the electric motor speed can be matched to the propeller’s optimum speed, even with a large slow-turning propeller (something that may not be possible with a conventional engine and transmission combination)

• Constant advancement in battery technology will allow for increased efficiency in future electric motors

Disadvantages of a HDES:

• Cost. These systems are initially much more expensive to purchase/outfit compared to that of a standard petroleum engine and transmission combination

The Gato Verde

The Gato Verde (’Green Cat’) is a 42ft long, 23ft wide catamaran weighing 20,000lbs. It operates on a hybrid bio-diesel electric plug-in system (i.e. can plug in at dock to charge batteries without burning fuel). The system is comprised of 8 x 12V (volt) batteries in series, and strings (2 x 4 batteries of 100 Amp Hours [AH]) in parallel. Having the batteries in series (+ to -) enables the voltage to be added together and having the batteries in parallel strings enables the AH to be added. Therefore the system is comprised of 2 x 48V batteries with a 200AH total capacity. The generator operating on bio-diesel is connected to the battery and 2 x 9000W electric motors (one in each hull). The generator is able to monitor the batteries state (i.e. the charge) and adjust its output accordingly. If the batteries are fully charged, the generator will run the electric motors directly. The electric motors have a maximum HP (horse power) of around 20, and in calm conditions the vessel can reach a maximum speed of about 7.2 knots. When the Gato Verde (GV) runs solely on electric power it is able to travel at an average speed of 4-6 knots for almost 2 hours. Having this hybrid system enables the GV to motor silently under electric power and have hydrophones deployed and recording and still able to maintain speed with the whales when they are travelling 4-6 knots. The kind of propulsion system on the GV is very fuel efficient, has minimal environmental impact (in terms of underwater noise and emissions), and is able to maintain a speed suitable for a cruising catamaran. What if we were to put a similar system on a 5-10m mono-hull vessel used primarily for whale watching?

The hybrid biodiesel-electric whale watch vessel

It is understood that although configuring a vessel to a hybrid diesel-electric propulsion system is initially quite expensive, it is much more fuel efficient and environmentally friendly. As stated above, HDESs are common in the yacht/ship industry, but what would need to be considered to configure a vessel that was used primarily for whale watching trips of 3 hour duration? Imagine a Zodiac, an inflatable rigid plaining hull twin outboard (300-400HP total) vessel capable of carrying 12 passengers and reaching speeds of up to 25 knots when travelling to or from a whale watching site. Although very fast and manoeuvrable with a good power to weight ratio, this propulsion system operates at a very high RPM as they have smaller propellers and are thus less fuel efficient (operate on petroleum gasoline) and “louder” underwater (in terms of dB) at high speeds (>20 knots) compared to other propulsion systems such as inboard, inboard-outboard, jet drives and surface piercing. Therefore, without going into detail of the different propulsion types, it was decided that for the purpose of outfitting a hybrid biodiesel-electric system to an existing vessel, the inboard motor would be the simplest and most cost effective. It is cheaper to maintain and the engines have more torque at lower RPM (c.f. outboard). Jet drives and surface piercing propulsion systems, although much quieter underwater, are very expensive. An inboard motor could be fitted with a battery and an electric motor (EM), by attaching the EM between the transmission and the propeller, and attaching a battery to the EM. By running the engine you would turn the electric motor which in turn would charge the battery. There are however a few things to consider:

• The EM must be able to handle the high RPM of the engine

• Would need a charged controller between the battery and the EM to control the charge going to the battery so as not to overcharge

• The battery would have to have a large capacity to able to run the vessel for an hour-hour and a half at moderate speed (5-10 knots). #NB. There would be no point in trying to run the EM at full speed as you would need an insanely large battery to generate enough power to reach those speeds

• The vessel would need to be a plaining hull (compared to a displacement hull)

• The size and weight of the batteries (this is the biggest concerning factor)

• Manoeuvrability of inboard vessels is limited, therefore to overcome this you would need dual engines, transmissions, propellers and batteries

Although the above stated seems to hinder the prospect of converting an inboard motor to a hybrid electric, it is possible. It will be an expensive conversion, but as technology and demand for these kinds of systems increase, prices will decrease. The main impact reduction idea behind this system is as follows: The whale watch industry in the Salish Sea, along with commercial and private vessel traffic, makes these waters some of the busiest in the world. Killer whales use sound to navigate, communicate and detect prey, and increased boat traffic may hinder their ability to do so effectively. If whale watch boats were to convert their propulsion to HDESs, then they would still be able to travel to and from whale watching sites at high speeds, but when in the vicinity of whales they could switch to the electric motors and motor silently, thus minimising the noise impact on these creatures. The beauty of this system is that travelling to and from the whale watching sites has less environmental CO2 emission impact as the fuel used is bio-diesel and therefore emissions are recycled back into the atmosphere. Standard diesel engines can easily be fuelled by biodiesel with minimum adjustment. Furthermore, this travel to and from the whale watch sites would charge the batteries, so there would be no unnecessary operation of the engine just to charge. Furthermore, from personal experience, when whale watch boats turn on their ignition and engage the transmission, it makes quite a loud “clunk” underwater (particularly for outboard motors). By operating under electric motors, when a group of whales leaves an area vessels can either motor silently with the whales, or motor silently a good distance away from the whales then engage their engine to either catch up with the whales or travel back to port. Having 20+ vessels all engage their transmission at once would create quite a loud series of “clunks” underwater, which may or may not affect the whales. This is something that should be tested. As we step towards the future, the combination of increased bio-fuel efficiency and batteries, as well as just general technological advancement, outfitting whale watching vessels with HEDSs seems to be a logical, environmentally friendly step. All it takes is for someone to pioneer this system, and I believe there will be a strong push by the general public to choose those whale watch operators that are more “eco-friendly”. Thus the other operators will ultimately need to follow if they wish to be competitive in this ever increasing environmentally-conscious society.


Information from the above report taken from the following websites:

• <http://www.nordhavn.com/constr_con/>, accessed Friday 19th October, 2007.

• <http://vetusmarine.com/elec_propulsion/elec_propulsion.cfm>, accessed Friday 19th October, 2007

• <http://www2.sea.siemens.com/NR/rdonlyres/109BE3FD-E3C0-4D15-A637-F45EEBB4D299/0/ELFA_Technical_Description.pdf>, accessed Friday 19th October, 2007

Other information by personal communication of Todd Shuster, captain of the Gato Verde catamaran, and pioneer of a hybrid bio-diesel/electric plug-in propulsion system on a commercial vessel in the Pacific North-West.