Electric Propulsion for Coastal Ships

According to many specialists electric propulsion is the best to have but worst to buy, with the argument that the investment is too high and the system does not pay itself back. Therefore a challenge and an opportunity for development to prove otherwise. Former investigations show an ongoing misunderstanding between shipbuilders, diesel engine suppliers, propeller suppliers and suppliers of electrical equipment towards electric propulsion. The history of shipbuilding put the suppliers of diesel engines in a prominent place in ships propulsion. Diesel engines became the most economic solution for driving the propeller. The diesel engine manufactures put a lot of effort in optimization of their engines to create higher available power and efficiency. Propeller manufacturers base their selection criteria on diesel engines. A diesel engine still is a stable value for the most of them. The main problem however, was the disappointing attitude of the suppliers of electrical equipment for propulsion.

They didn't put up the same amount of effort as the diesel engine manufacturers did. Following the lead of the shipbuilding industry, they offered installations which where overdone to virtually eliminate risk. The consequences were high prices and low efficiency. Van der Leun BV has long been active in shipbuilding, and automation, electrical drive systems and electric installations onboard of ship are the company's main skill. In evaluating the process of propulsion of a ship, the company concluded that things had to change. The ship, the propeller and its driving system should work together boosting new possibilities. Therefore the point of view should be changed; parameters had to be revealed. Van der Leun BV started off putting the electric drive system there were the diesel engine used to be. Now the optimization could be seen from the electrical point of view, not hindered by the existing boundaries of the diesel engine.

Parameters of propellers are mostly derived from computer based analytical models. These models are often the result of a matrix series of tank test and therefore close to truth. So far there is nothing to worry about in relation to electric propulsion. The problems starts when the academic view on propellers is gone and selection guides of manufacturers are based on the restrictions of diesel engines. Of course there are restrictions on electric propulsion as well, but these restrictions are far less compared to diesel engines.

Acknowledging the extra possibilities, we designed a specific guideline to overcome the standard selection guides of propeller manufacturers. This specific guideline is based on the new electric propulsion concept of Van der Leun BV. The electric propulsion concept of Van der Leun BV offers a very special feature called "active slip compensation." Due to this the installed power for propulsion can be reduced without affecting the desired thrust capability of the propeller. A reduction of approximately 20 percent in power (not in thrust) can be achieved in every type of ship with a regular speed. The reduction on high speed vessels is even higher. The desired power is derived from smaller gensets. This means that besides the smaller investment in diesel engine power, savings can be achieved in installation and auxiliaries. This is often forgotten while comparing direct driven propulsion with electric propulsion. Finally the engines are running in optimal load conditions causing less fuel consumption and less "wear and tear." Specific fuel consumption is higher in the lower load conditions of engines.

Looking at the improvability of the hull, our conclusion was not a surprise. Nowadays shipyards try to sell as many copies as possible keeping the overall costs down. Creating a new hull form is time consuming and expensive. A lot of new drawings need to be made and cutting the steel needs to be programmed. All this is not necessary using an earlier hull form. Shipyards also tend to stay away from the unknown electric propulsion as long as possible. Ship owners specific demand is nearly always the final reason to make the step. And then they hit the above problems.

Electric propulsion, however, creates possibilities for optimizing the hull form. The components are relatively small and most of them are not dedicated to a certain position. The improved natural stability of the ship together with the improved water tlow will lower hull resistance boosting efficiency even up to 115 percent compared to direct driven propulsion hull form. As the electric installation is the sole responsibility of the supplier.

he gets to be responsible for a huge part of the propulsion as well. Van der Leun BV acknowledges the position it has to take: a position in the process: right where the diesel engine supplier was before. That is the only way to achieve an integral adoption of the 'new' system in the ship. As an eye-opener we have taken a simple but actual example of the use of electric propulsion.

A harsh example for electric propulsion because ships with no high auxiliary power demand were considered to be a no go zone. The ship has two 1,000 kW thrusters according to the normal propeller selection guidelines. Electric propulsion is, according to the Van der Leun concept, using active slip compensation.

Hull improvement efficiency is kept on a low 105 percent for electric propulsion. This value is easy to get with small modifications on existing hull forms. As a voyage consists of various conditions, we separated 100 percent (power) sailing, 50 percent (power) sailing and cargo operation. The amount of voyages with the conditions can be found in the table.

Electric propulsion is an interesting feature for a lot of ships. The example shows the efficiency of electric propulsion being 91 percent compared to direct diesel engine. This might even get higher when large electric consumers for e.g. cargo handling are on board.

Honesty, however, forces us to point out some disadvantages as well. Some advantages and disadvantages as an addition to the ones you all ready know: Advantages • In many cases more efficient • Less service • Natural stability possibilities • Less vibrations in the ship • Less complex hull forms possible • Ready for fuel cell technology • More redundancy possible Disadvantages • Return on investment sometimes too long • More complex systems on board • Unknown by a lot of people • Not much evidence from the market (due to above reasons) • Still too few parties with necessary overall knowledge There certainly is an economic basis for selecting electric propulsion, even in regular ships. Electric propulsion is central to environmental issues, such as C 0 2 reduction. Still, profitability is the driver in the end. and electric propulsion can make it easier. When electric propulsion is used to replace the existing way of propulsion for your vessel without adding fancy features, a lot of possibilities are there all ready. If you consider the use of electric propulsion for any of your ships, find yourself a solid supplier of electric systems who is familiar with of electric propulsion. Provide him with actual figures about the normal voyages of your ship and he should be able to show you if electric propulsion is a serious opportunity. Most of them will initially help you for free. I am sure you will be surprised!

The preceding was authored by ./. van Tilborg, Van der Leun B V