Energy-Efficient Running of Ships

The propulsion efficiency of a ship does not depend only on the propeller design. The flow of water approaching and leaving the propeller is equally important. Unfavourable inflow, uneven velocity distribution, vortex formation and rotational energy in the propeller slipstream result in energy losses, even when the propeller itself is well designed.

To reduce these hydrodynamic losses, Energy Saving Devices (ESDs) are fitted around the propeller–hull interface. These devices guide, straighten or deflect the water flow so that the propeller can convert more of the available engine power into useful thrust.

ESDs are particularly useful for existing ships, where replacing the complete propulsion system may not be technically or economically practical. Depending on the type of device and the ship's operating profile, they can provide a measurable improvement in propulsion efficiency and reduction in fuel consumption.

Common devices include:

1. Propeller Nozzle

A propeller nozzle is an annular hydrodynamic structure fitted around the propeller. It guides and directs the water flow through the propeller and improves the inflow conditions.

The shape and position of the nozzle help convert a greater portion of the propeller-generated impulse into useful axial thrust.

The benefit is particularly significant at low ship speeds and high propeller loading, where an open propeller is comparatively less efficient.

Advantages:

• Increased thrust at low speed and heavy load.
• Improved propeller efficiency.
• Useful during manoeuvring and operation against currents.
• Particularly suitable for tugs, dredgers and workboats.
• Provides better handling and working capability in laden conditions.

2. Guiding Fins / Stators

Guiding fins, also called stators, are generally fitted ahead of the propeller. They modify the incoming water flow by aligning and redistributing it, reducing swirl and making the velocity distribution over the propeller disc more uniform.

As a result, water reaches the propeller blades at more favourable angles of attack, improving the hydrodynamic performance of the propeller.

Advantages:

• More uniform water inflow.
• More even loading of propeller blades.
• Better utilisation of available shaft power.
• Reduced local blade overloading.
• Reduced vibration and pressure pulses.
• Reduced possibility of cavitation.
• Lower fuel consumption.
• Reduced stress and wear on the propeller, shaft line and bearings.

3. Propeller Boss Cap Fins (PBCF)

Behind a conventional propeller hub, a concentrated rotating flow called a hub vortex is normally formed. This vortex contains kinetic energy that does not contribute to useful propulsion and is therefore lost as vortex energy and turbulence in the propeller wake.

The hub vortex may also cause:

• Additional energy losses.
• Increased turbulence in the wake.
• Pressure pulses and vibration.
• Adverse interaction with the rudder and other stern components.

Propeller Boss Cap Fins (PBCF) are fitted to the propeller boss cap to reduce the strength of the hub vortex. By recovering part of the rotational energy and improving the flow leaving the propeller, they can increase propulsion efficiency and reduce energy losses.

Variable Frequency Drives (VFDs) are used to control the speed of electric motors driving auxiliary machinery such as centrifugal pumps, fans, blowers and compressors.

In conventional systems, an electric motor often runs at a constant speed, while the required flow or pressure is controlled using valves, dampers or bypass arrangements. This wastes energy because the motor continues to operate at full speed even when the actual demand is low.

With a VFD, the frequency and voltage supplied to the motor are varied according to the required load. Therefore, the motor speed can be adjusted to match the actual demand of the auxiliary machinery.

Working Principle

AC supply → VFD → Variable-frequency/variable-speed motor → Auxiliary machinery

The VFD changes the frequency supplied to the motor:

Frequency ↓ → Motor speed ↓ → Flow ↓ → Power consumption ↓

When demand increases:

Frequency ↑ → Motor speed ↑ → Flow ↑ → Power consumption ↑

For centrifugal pumps and fans, the affinity laws show that:

• Flow ∝ Speed
• Pressure/Head ∝ Speed²
• Power ∝ Speed³

Therefore, even a small reduction in motor speed can produce a large reduction in power consumption.

Applications on Ships

VFDs can be used for:

• Sea-water and fresh-water cooling pumps.
• Boiler feed-water and circulation pumps.
• Ventilation and engine-room fans.
• Air-conditioning and chilled-water pumps.
• Fuel and oil circulation systems, where applicable.
• Other variable-load auxiliary machinery.

Advantages of VFDs

1. Reduced electrical power consumption by matching motor speed to actual demand.
2. Reduced fuel consumption, because less electrical power is generated by the ship's generators.
3. Better control of flow and pressure without excessive throttling or bypassing.
4. Reduced mechanical wear due to smooth starting and stopping.
5. Reduced starting current and mechanical shock.
6. Improved operating efficiency during part-load conditions.
7. Reduced running hours/load on diesel generators, helping optimise generator operation.
8. Overall improvement in the ship's energy efficiency and operating cost.

Example

Consider a cooling-water pump operating at full speed when only 70% flow is required. Instead of keeping the pump at full speed and throttling the discharge valve, the VFD reduces the motor speed to approximately the required level.

Because pump power varies approximately with the cube of speed, a reduction in speed can result in a significant reduction in electrical power consumption.