The SSPA shuttle ferry concept

A majority of the world’s largest cities are located in coastal areas. For many of these cities, sea traffic is an integral part of the public transport system, providing important links to busses and rail bound transports and achieving annual passenger volumes ranging from 1 to 5 million people. However, in order to cope with such trends as rapid population growth in urban areas, overloaded roads/rails and the desire to turn former harbor and wharf areas into attractive residential districts, seaways could be used even more in the future as part of the public transport system. In Stockholm, for example, more public transport by sea is already high on the political agenda.

SSPA has been engaged in several projects concerned with public transport by sea and in conjunction with this, we have investigated such things as ship design, environmental impact and risk management, and also conducted simulations. Our experience from performing these assignments, along with our firm belief in the important role of public transport by sea in the future has inspired us to design a passenger ferry suitable for the seaways around Stockholm.

Our goal was to envision a realizable design for an efficient, environmentally sound, modern passenger ferry, with a great deal of focus on accessibility and comfort. In order to meet the environmental constraints, the ship had to comply with the Swedish regulations for ships on inner waterways. A realizable design involved focusing effor ts on incorporating technology readily available on the market.

Why a catamaran?

The final concept was a 27 m catamaran optimized for a 35-minute route between Ekerö Island (located on Lake Mälaren) and the central part of Stockholm.The twin hull concept was selected not only to minimize resistance, but also to reduce the wake wash since much of the route is through sensitive waterways. In order to achieve the desired low resistance and low wake wash the catamaran concept needed to be very light. Accordingly, efforts were made to avoid incorporating equipment or functions that would add too much weight.

Similar catamarans exist in several other major cities, such as Brisbane, London and Amsterdam. In Stockholm, however, they are non-existent. In all likelihood, this has to do with ice considerations. For this particular design project, ice capacity was rejected. The design of an icegoing passenger ferry for the selected route and travel time would have been completely different (most likely a rather heavy, single hull vessel).An ice-going vessel would either be one that had low speed, moderate energy consumption, low wake wash and low emissions, or a higher speed vessel with high energy consumption, severe wake, and comparably high emissions. Both of these concepts were rejected, since many such ships already exist and designing yet another one would add nothing new to either our own skill set or the general debate.

Limitations in the choice of fuels and drive train

Comprehensive research for alternative fuels was also incorporated into the project. Significant effort and investigation was focused on methane based fuels (compressed or condensed), non-fossil based diesel substitutes (like pine oil or Rapeseed Methyl Ester (RME)), batteries and Dimethyl Ether (DME). Several of the identified solutions, such as compressed or liquefied methane, could potentially reduce carbon emissions quite significantly. A number of the solutions also substantially reduce nitrous oxides (NOx) emissions as well. For the selected ferry concept, it was determined that non-fossil based diesel substitutes and DME were the best choices from a technical and practical point of view. One problem with DME, however, is that an off-the-shelf engine with marine certification is not currently available. For the other investigated alternatives, the weight of the fuel and the storage tanks (i.e. not too heavy) was a deciding factor. For the drivetrain and propulsion, several possible solutions were investigated. The one selected is rather straightforward: two controllable pitch propellers on straight shafts, each driven by a diesel cycle engine via a combined gearbox and clutch. This was preferred due to simplicity, propulsive efficiency and weight.

As discussed earlier, the level of installed power is highly dependent on the desired level of onboard comfort and other installations (which also affects displacement). The maximum, however, should not exceed 450 kW for each installed main engine. Hence, the ferry is designed to be equipped with 2 modern 450 kW diesel engines running on biodiesel, providing a service speed of 22 knots.This could be reduced fur ther, depending on the amount of installed equipment and the effort put into weight minimization of the hull and super structure.


As currently designed, the ferry has capacity for approximately 200 passengers (175 seated, 25 standing). On the aft deck, there is space for 20 to 25 bicycles.

Embarking and disembarking

The ferry can be moored with either the stem (e.g. interim stops during off-peak hours) to quay, as well as alongside the dock (e.g. at end stations). Passengers embark and disembark through exits in the stem and the side. Bicycles are mainly transported on the stern deck (with some spaces also available on the front deck). Furthermore, the ship is equipped for disabled persons. In fact, the vessel was designed so that embarking and disembarking would be just as easy as in the Stockholm subway.

One important aspect of the project was to incorporate many features where SSPA has a high level of expertise, such as hull and propeller design/ interaction, harbor planning and hull structural design. We also took great care to consider the requirements of impor tant stakeholders, such as ferry operators and government authorities. It is also impor tant to note that our independence from suppliers ensures an unbiased evaluation of system solutions and selected design decisions.The SSPA Shuttle Ferry is a well elaborated initial design, ready for the next step in the design process. It is an attractive concept and an elegant solution to an urgent transport problem. It also provides an excellent example of what SSPA can provide in terms of comprehensive ship design.

Rapeseed Methyl Ester (RME) – a man-made substitute for diesel oil.
Dimethyl Ether (DME) – in ambient air pressure a gaseous substance possible to burn in a diesel engine. Becomes liquid at roughly 5 atmospheres over pressure.


Future passenger ferry suitable for the seaways around Stockholm.