4.2.3 Maximum propulsion power capacity of a conventional single shaft hull form
A single shaft hull design can accommodate more propulsion power than with only a single propeller when the power can be divided to 2 propellers. This will be especially interesting for fast cargo vessels e.g. large container ships with more than 5000 TEU. The limit of power produced by a single 2-stroke engine is forcing designers to apply twin shaft line ships and to change the single propeller hull form.
4.2.4 Manoeuvrability and dynamic positioning
Manoeuvrability of the vessel in naturally improved as the podded propulsion unit is free to rotate at maneuvering speeds around its vertical axis. Propeller thrust can be directed to the desired direction and the power to achieve the required thrust is lower than what must be supplied to a tunnel thruster to achieve the same thrust. This is due to the larger propeller diameter of the pod and reduced hydrodynamic losses when comparing to the propeller in the aft thruster tunnel.
The use of dynamic positioning systems is common in the offshore industry in e.g. shuttle tankers during buoy loading operations. The use of a podded propulsion unit reduces both fuel consumption and maintenance costs of the main shaft line engines, which would be operating most of the time at very low loads.
4.2.5 General arrangement of the machinery areas
The minimum of additional machinery components required onboard with the podded CRP concept are a frequency converter and the podded propulsion unit with auxiliaries. Other components (generators, main switchboard, and transformers) already exist onboard, although upgrading is usually necessary to meet the requirements of the higher total installed electrical power.
The general arrangement of the vessel is not drastically changed when applying the podded CRP concept to a single shaft line vessel. Additional space is required for the machinery components mentioned above, however changes are small when comparing converting a single shaft line installation to a twin shaft line installation. The case study presented later will present an example of the space requirements.
4.2.6 First cost and installation cost
The first cost of the propulsion machinery is the major factor when shipyards make decisions on which propulsion alternative to choose. Another factor, which is more difficult to calculate accurately, is the installation cost and especially comparison of the installation costs of different concepts, such as electric vs. mechanical or shaft line vs. podded propulsion. The items, which a shipyard has to study in addition to the purchase cost, are costs resulting from the purchasing, designing and manufacturing processes. Also the total building time of the vessel will affect the full cost to the shipyard.
4.2.7 Life cycle cost
The decision criteria of ship owners evaluating new building alternatives are increasingly shifting from first cost evaluation to life cycle cost (LCC) evaluation. The LCC consists of the capital cost, operating cost and resale or scrapping value of the vessel.
Capital costs are directly proportional to first cost. Fuel, maintenance, manning and tug costs are other operating costs of which fuel and tug costs are the most potential sources for savings in the podded CRP concept. Tug assistance requirements vary from port to port and are therefore difficult to estimate. The maneuverability of the podded CRP vessel makes tug assistance from the maneuvering performance point of view unnecessary in almost all conditions. Tug costs can be a major cost and should be included in the LCC.
4.3 Hydrodynamic aspects
Design criteria for optimizing the propellers of the podded propulsion unit can be:
・ Booster power at high speeds
・ Maximum power available also when main propeller is not in operation
・ Dynamic positioning mode
Design of the fixed pitch propeller is optimized according to the set requirements. The characteristics of the electric variable speed drive give good dynamic response to propeller speed commands at all ship speeds even when the propeller is optimized for higher speeds.
The detail design of the propellers and the pod are optimized in model tests to assure best performance and also to avoid harmful cavitation.
4.4 Upgrading the auxiliary power plant
The additional required power of the auxiliary power plant will be small in vessels with an existing high auxiliary load when not using propulsion (shuttle tankers, chemical and product tankers). A larger power increase is required in vessels with a small auxiliary power requirement or a simultaneous high auxiliary power and propulsion power requirement (reefers). The required power can be achieved by installing larger generators sets and/or applying an additional primary shaft generator to the main engine. A shaft generator can be used increase the load on the main engine when the vessel is slow steaming and the maximum power is not used in the main propeller.
4.5 Environmental requirements
In certain geographical areas (e.g. U.S. West Coast and Alaska, Baltic Sea, port areas) strict limits for exhaust gas emission levels are already implemented or can be expected in the near future. Meeting the strictest requirements requires either the use of special measures in diesel engine plants using HFO e.g. SCR (Selective Catalytic Converter) systems or using a higher grade fuel and even gas turbines.
4.6 Safety of navigation
Safety of navigation is a result of the performance of the vessel's crew and functioning of the technical systems. The machinery systems are to be designed for reliable operation minimizing the risk of operator failures. Redundancy is often required to tolerate failures and maintain the capability of safe operations.
4.7 Flexibility in the building process
When a shipyard is studying the podded propulsion system alternative, all processes affected by the new building method should be studied. There are multiple areas where the benefits of the podded propulsion concept, flexibility and modularity, can be maximized.
