Larger Propeller = Larger Savings?

The drive for ship owners to measure the fuel consumption for the propulsion of their ships to the widest possible extent at any load has resulted in the adoption of slow or even super slow steaming as standard practice.

This slow steaming drive results in the operation of ships at lower than normal service ship speeds compared to the ascribed Normal Continuous Rating (NCR) speed, as per engine type, make or model, which can result in reduced propulsion power utilisation. The design ship speed at NCR, including 15% sea margin, used to be as high as 14.0 knots. Today, the ship speed may be expected to be lower, possibly 13 knots, or even lower.

In light of regulatory pressures, many operators are also adopting slow steaming to ensure their ships are adhering to enforcements.  Through the practice of slow steaming, the inherent CO2 design index of a newbuild ship (the Energy Efficiency Design Index (EEDI)), will be reduced. Under the EEDI regulations, based on an average reference CO2 emission from existing ships, the CO2 emission from new ships in gram per dwt per nautical mile must be equal to or lower than the reference emission figures valid for the specific ship.

However, although slow or super slow steaming can be and is seen as a quick fix that can drastically reduce fuel consumption and aligns ships within EEDI enforcements, running ships engines at rates much slower than originally designed can and will create issues.

Man Diesel & Turbo, recently released a technical report that shed light on how the issues associated with engines and slow steaming can be overcome through engine de-rating and new engine installation, but also the possibility of using larger propellers with a view to using engines with even lower speeds.

The enlightening study that focussed on the small tanker ship types shared insight on how operators can save fuel and allow ships to cruise at the same speed (NCR speed) using less power.

Larger Propeller = Larger Savings?

In their report, Man Diesel & Turbo described that a more technically advanced development drive is to optimise the aftbody and hull lines of the ship – including bulbous bow, also considering operation in ballast condition. This makes it possible to install propellers with a larger propeller diameter and, thereby, obtaining higher propeller efficiency, but at a reduced optimum propeller speed, i.e. using less power for the same ship speed.

The report detailed that as the two-stroke main engine is directly coupled with the propeller, the introduction of the super long stroke engines, such as the Man Diesel & Turbo S30ME-B9.3 engine, with an even-lower-than-usual shaft speed will meet this goal.

Small Tankers and Slow Steaming

Traditionally, long stroke L-type engines, with relatively high engine speeds, have been applied as prime movers in very small tankers.

Following the efficiency optimisation trends in the market, the possibility of using even larger propellers has been thoroughly evaluated with a view to using engines with even lower speeds for propulsion of particularly small tankers and bulk carriers.

The report concluded that small tankers and bulk carriers may be compatible with propellers with larger propeller diameters than the current designs, and thus high efficiencies following an adaptation of the aft hull design to accommodate the larger propeller, together with optimised hull lines and bulbous bow, considering operation in ballast conditions.

The paper indicated, depending on the propeller diameter used, an overall efficiency increase of 3-7% when using Man Diesel & Turbo's S30ME-B9.3 engine type (their new and super long stroke engine type), compared with the old main engine type the Man Diesel & Turbo L35MC6.1 engine applied so far.

In general, the highest possible propulsive efficiency required to provide a given ship speed is obtained with the largest possible propeller diameter d, in combination with the corresponding, optimum pitch/diameter ratio p/d.

The small tanker case study, detailed at the end of this spotlight, showed that with regards to propulsion, it will always be an advantage to choose the largest possible propeller diameter, even though the optimum pitch/diameter ratio would involve a too low propeller speed (in relation to the required main engine speed). Thus, when using a somewhat lower pitch/diameter ratio, compared with the optimum ratio, the propeller/engine speed may be increased and will only cause a minor extra power increase.

The efficiency of a two-stroke main engine particularly depends on the ratio of the maximum (firing) pressure and the mean effective pressure. The higher the ratio, the higher the engine efficiency, i.e. the lower the Specific Fuel Oil Consumption (SFOC).

Furthermore, the higher the stroke/bore ratio of a two-stroke engine, the higher the engine efficiency. This means, for example, that a super long stroke engine type, may have a higher efficiency compared with a shorter stroke engine type.

Small Tanker Case Study

In one case study detailed within the Man Diesel & Turbo report an 8,000 dwt small tanker with a service ship speed of 14 knots is studied. The existing propeller diameter of 3.5 m may have the optimum pitch/diameter ratio of 0.72, and the lowest possible SMCR (Specified Maximum Continuous Rating) shaft power of about 3,625 kW at about 219 r/min. If a bigger propeller diameter of 3.9 m is possible, the necessary SMCR shaft power will be reduced to about 3,425 kW at about 179 r/min, i.e. the bigger the propeller, the lower the optimum propeller speed. If the pitch for this diameter is changed, the propulsive efficiency will be reduced, i.e. the necessary SMCR shaft power will increase.

A copy of the technical report entitled 'Propulsion of 7,000-10,000 dwt Small Tanker' , written by Birger Jacobsen, senior research engineer, MAN Diesel & Turbo is available online via www.mandieselturbo.com.