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Do you want to save 15 per cent on fuel? High fuel prices have lead to a great focus on efficiency increase. Conversion from open propeller to a ducted type with a high efficiency nozzle, can lead to an efficiency increase of more than 15 per cent, with an attractive ROI.

Propulsion improvement – fuel saving by means of upgrading ship propulsion

By Ir. A.A.M. Voermans
Hydrodynamic Engineer Propulsion Service
Wärtsilä Propulsion The Netherlands BV
anton.voermans@wartsila.com

The fuel prices have risen considerably in the last years, resulting in an increase of ship operating costs. To maintain the economic profitability of the vessel, a large focus is nowadays on fuel saving devices in the broadest sense of the word.
A number of options are available to improve the efficiency of the propulsion system, depending on the type of propeller and vessel. This article explains the state of the art of propulsion improvement by means of retrofits.

	Figure 1. Example of increase in fuel prices.

Besides the gain in propulsion efficiency, the investment required is also determining whether a propulsion upgrade is profitable or not. Therefore, this article will also outline the economic effects by means of the Return On Investment (ROI) (see figure 1).

Hydrodynamics
First some hydrodynamic principles are explained, whereas later on the basics of the propulsion improvement devices will be outlined.
The total propulsion efficiency of a propeller varies between 50 per cent and 70 per cent, depending on the type of application. High-speed ferries can have propellers exceeding even 70 per cent efficiency, whereas tugs or other low speed vessels can have propellers with efficiency even below 50 per cent. This is mainly related to the hydrodynamic background and not to the quality of the propeller design.

With a propulsion efficiency of 60 per cent, the losses for an average propeller can be traced to three physical phenomena;

• Axial losses
• Frictional losses
• Rotational losses

(See figure 2)



Figure 2. Example of type of losses, general ship propeller.

Figure 3. Visualization of axial losses.

A propeller generates thrust, due to the acceleration of the incoming water. Behind the vessel, the outgoing flow mixes with the environmental flow. Due to turbulence energy will be lost, we call this axial loss (see figure 3). The larger the propeller diameter, the smaller the acceleration of mass needs to be, in order to generate the same thrust. Consequently, a large propeller will have lower axial losses and therefore a higher efficiency.

Axial losses contribute for a major part to the total losses of a ship propeller.
Water in contact with the propeller blade surface causes friction, and thus losses. The total blade surface, speed of rotation and surface roughness are the dominating factors concerning friction losses. A small blade area and a low propeller speed lead to higher propulsion efficiency.

	Figure 4. Full-scale flow observation of a large fixed pitch propeller on a container ship.

Rotation of the blade introduces a rotation in the wake too; consequently this energy is lost to generate a thrust in axial direction. Locally, around the tip and the hub, additional energy dissipation due to rotation effects is introduced. The picture shown in figure 4 shows the rotation in the flow.

Summarized, we can state that improvement of propulsion efficiency needs to be based on reduction of one or more of above described losses.

Propulsion improvement
Fuel saving devices, as described in the next chapters, are categorized on percentage of efficiency improvement, up to five per cent, up to ten per cent, and up to 15 per cent.

The working principle will be discussed as well as the field of application. A propeller surrounded by a nozzle, for instance, cannot be applied for a high-speed ferry. The economic effects by means of the Return On Investment will also be considered.

Two stroke engines usually operate with heavy fuel oil (HFO); the market price in 2005 was approximately EUR 250/ton. Four stroke engines can be operated with heavy fuel oil, marine fuel oil and gas oil. A market price of 450 euro/ton for gas oil was not uncommon in 2005. The calculation of the Return On Investment is based on the specific vessel and fuel type.

Two options are available to improve efficiency up to five per cent:

• Propeller polishing and/or repair of edge damage
• Modern design propeller

Propeller polishing and repair
of edge damage
Maintenance of the propulsor pays off. After several months in service the roughness of the blades already starts to increase, either due to impact damage of sand/ debris or by marine growth. Figure 5 shows the relation between surface roughness and loss in propeller efficiency.


Figure 5. Efficiency losses due to roughness for an average propeller.

A roughness increase from IS class 1 to class 3 is rather common. The graph shows that this equals with an efficiency loss of approximately two per cent.
Blade edge damage varies from small scratches or bend tips to cut out pieces. The propulsor of vessels operating in harbours or harsh environments like dredgers are especially vulnerable for edge damage. Degradation of the propeller’s operating performance is the result.

Propeller polishing in combination with repair of edge damage easily increases the propeller’s operating efficiency with two to four per cent. It will be clear that proper maintenance is of importance for all vessel and propeller types.
The Return On Investment (ROI) period is quite short. After one to three months, the maintenance costs are already recovered for repair in dry-dock. Underwater grinding and repair is also executed, the pay back time then becomes five to six months.

Modern design propeller
A redesign of the current propeller, based on the state of the art, without optimizing the boundary conditions like propeller speed and diameter, can lead to an improvement of approximately five per cent.

	Figure 6.

Tip rake, similar to tip wings of airplanes, reduces the local rotation around the tip. For large fixed pitch propellers of bulkers, tankers and container ships, the positive effect can be up to three per cent (see figure 6). The ROI is limited to four–five years, since a new propeller is required.

With the help of modern hydrodynamic software, more reliable cavitation predictions can be made. This way higher power densities can be allowed, and thus a lower blade area can be applied, which results in a reduction in frictional losses. This reduction in frictional losses will improve the efficiency up to three per cent. Smaller blade area’s can be applied to almost any propeller type. The pay back period is again limited, since a new propeller is required.

A combination of the two, however, is possible. Fuel savings up to five per cent are realized. The pay back period also becomes more interesting; approximately three years.

Improvements of over five per cent can be realized by application of a modern design in case the diameter and the speed can be optimized. Also an efficiency rudder can reduce fuel consumption. Finally, curing heavy running fixed pitch propellers can lead to a better performance overall.

Modern design propeller with increased diameter and low speed
In general, a larger propeller diameter in combination with a low rotational speed leads to an improvement in efficiency (see figure 7). The axial losses will be reduced. Basically we can apply this for all ship/propeller types.

Figure 7. Example of efficiency related to propeller diameter and rotational speed.

It will be clear that the engine and shaft line lay out need to be checked, for instance for strength and torsional vibration. The gearbox or a power take off can be a restriction to adapt the shaft speed.

A larger propeller diameter can have a negative effect on the propeller/hull interaction, since the tip clearance becomes smaller. The increase in pressure pulse level and radiated energy needs to be evaluated.

In case the diameter cannot be enlarged, sometimes increase in the number of blades (five or six) provides a solution for further optimization.
The pay back period is rather interesting; we have references showing ROI’s of 1.5 to 2.5 years.

	Figure 8. Example of an efficiency rudder.

Efficiency rudder
The efficiency rudder is a successful new development. An example is given in figure 8. Axial and rotational losses in the slipstream of the hub are eliminated by the torpedo, which is fitted in between the propeller and the rudder. Highly loaded propellers and ships with speeds exceeding 15–20 knots can benefit from an efficiency rudder. Ro-ros, container ships and reefers is the field of application.

The efficiency rudder is less interesting for the retrofit market, since the required investment is rather large related to the efficiency improvement, a new propeller and rudder with appendices is required. For new building vessels it can be interesting since a rudder and propeller is needed anyway.

The return on investment for the retrofit market exceeds a period of five years.

Propeller/engine interaction
Vessels equipped with a fixed pitch propeller can suffer from a so-called heavy running propeller. The terminology is somewhat misleading; the propeller loading and engine characteristic are not matching. Normally, a propeller absorbs about 85 per cent MCR at 100 per cent engine speed, meaning a margin of 15 per cent power is available as sea margin/for heavy weather.

In case the resistance of the vessel increases during the years, or the engine performance goes down in combination with roughening of the propeller, the mismatch is there. The propeller curve crosses the load limit of the engine before full power is reached. As a consequence the engine is overloaded, leading to increased fuel consumption as well as increased wear of internal engine parts (see figure 9).

Figure 9. Power absorption diagram heavy running propeller.

A pitch modification of the propeller can be conducted. In this way the same power is absorbed at a higher rpm, and the engine operation is shifted out of the “red” area. Figure 10 shows a drawing of details of the trailing edge blade modification. Wärtsilä Propulsion conducts about 15 trailing edge modifications a year, with positive results.

Fuel savings of six to eight per cent are reported after trailing edge cutting. In relation with the limited costs, the pay back time is rather short, about two months.

By conversion of an open propeller to a ducted propeller, efficiency improvement up to 15 per cent is established.

The idea of surrounding a propeller by a nozzle is already very old. The first applications, in the early 1930’s, dealt with bollard pull. Obviously, specific ships do benefit from the ducted propellers, such as tugs, icebreakers and trawlers, push boats and trawlers. However, also ships sailing at low to moderate ship speeds can benefit from the use of nozzles. To illustrate this: today about 25 per cent of all Lips controllable pitch propellers are running in a nozzle. It is for this reason that Wärtsilä Propulsion has contributed to the development and design of ducted propellers.



Figure 11. Working principle of ducted propeller.

The working principle of the ducted propeller can be illustrated with figure 11. In an accelerating nozzle, which is the normal used type nozzle, the water speed at the propeller is higher tha n that of the open propeller. The increase in axial velocity reduces the propeller load especially for heavily loaded propellers. This leads to an increase in overall performance of the propeller and nozzle compared to that of a propeller alone. Additionally, the nozzle generates forward thrust caused by the pressure distribution around the nozzle, resulting in a force in the forward direction. The induced nozzle thrust needs to be corrected with the frictional losses of the duct itself. The higher the sailing speed, the larger the frictional losses will be. Beyond a certain vessel speed, the contribution of the duct to the overall propulsion efficiency will be neutral or even negative.
In order to extend the application of ducted propellers towards higher ship speeds in combination with a larger bollard pull, Lips has introduced the high efficiency nozzle (HR-nozzle). The conventional nozzle like the 19A and the 37B, are designed for cost effectiveness, and therefore contain straight parts. Unfortunately, these sections suffer from flow separation. The HR-nozzle has a curved exterior and interior, leading to absence of flow separation and consequently a higher efficiency. (See figure 12)



Figure 12. Advantage of the lips high efficiency nozzle.

When replacing an open propeller with an HR ducted propeller, the bollard pull can be increased with about 25 per cent, while the free running efficiency can be increased with ten per cent to 15 per cent. This mainly depends on the power density of the propeller and the sailing speed of the vessel. In case the propeller is already surrounded by a conventional nozzle, or is replaced by a conventional one, the efficiency increase will be limited.

Figure 13 shows the increase in propeller efficiency for a group of vessels like coasters, bulkers, multipurpose vessels etc, as a function of the sailing speed.
The graph shows that for ship speeds in the range of ten knots, the replacement of an open propeller by an HR nozzle can lead to an efficiency increase of 17 per cent efficiency. At 16 knots, still a considerable improvement can be established.
A retrofit of a small trawler, in the Scandinavian area, from open propeller to a propeller running in an HR nozzle, has lead to an increasing of 29 per cent in bollard pull during trawling, while the free sailing efficiency has gone up with 13 per cent. Although new propeller blades are required in combination with the new HR duct, the pay back period is still short due to the large increase in hydrodynamic performance. The calculated ROI is 1.6 year.

Figure 13. Increase in free running efficiency for retrofit of propeller.

Figure 14 shows pictures of a conversion of a heavy lift vessel, from an open propeller to a ducted propeller of the HR type. After the propeller design was made, successful model tests were conducted. The increase in bollard pull was reported to be 30 per cent, while the increase in free sailing efficiency was measured to be 15 per cent. After conversion, full-scale tests were carried out, confirming the model test results and calculations.

The estimated Return On Investment is 1.0 year.

Figure 14. Conversion of a heavylift vessel, from open type propeller to ducted
propeller HR type.

Economics of retrofits
To judge the profitability of a propulsion improvement, the increase in hydrodynamic efficiency needs to be considered in relation with investment or total costs.

A number of conducted retrofits are reviewed with respect to investment and fuel savings. Data for the fuel consumption are supplied by the ship operator, or otherwise estimated based on the mission profile.

Figure 15 shows the estimated Return On Investment of each discussed propulsion improvement device. Each type of device has a wide field of application, depending on ship type, size, engine, type of fuel etc. Therefore an envelope is sketched in the chart, identifying a range of hydrodynamic improvement and pay back period.

Figure 15. Return On Investments of propulsion improvement devices.

The graph shows that the efficiency rudder and a modern design propeller are not very attractive as a retrofit; the pay back period is more than three years. The investment is too high related to the fuel savings.
Whenever, the boundary conditions like diameter and propeller rotational speed can be optimized, a modern design can be worth considering as an investment to lower the ships operational costs.
Finally, maintenance of the propeller, curing heavy running propellers and the application of ducted propellers are investments that pay off. The pay back periods are short, varying from several months to 2.5 years.
Especially the ducted propellers can decrease the ship operation costs, while the investment is limited taking into account the hydrodynamic improvement.
Summary and conclusion
The fuel prices have risen considerably in the last years, resulting in an increase of ship operating costs. To maintain the economic profitability of the vessel, Wärtsilä Propulsion can provide a number of solutions to save fuel.
With modern design propellers, grinding and repair and modification of heavy running propellers considerable hydrodynamic improvements can be realized, with attractive Return On Investments.
However, through application of ducted propellers, and especially the Lips high efficiency nozzle, an increase of free running propeller efficiency of 15 per cent can be reached, with pay back periods of one to 2.5 years.

Latest update 18-10-2006 8:49

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