CLT propellers – Advantages

Energy saving is a primary objective in the design of marine propellers; the increase of oil price and the more strict regulations in terms of air pollution and NOx/Sox emissions require more and more efficient designs. At the same time, requirements about radiated noise and vibration emissions in order to reduce the negative effects on marine life and the hydroacoustic signature of new commercial and navy vessels are the primary aims of any newbuilding.

CLT propellers represent a further opportunity to increase efficiency, reduce the risk of cavitation (and radiated noise as a consequence) without the employment of completely different geometries and propulsive solutions (ducted, contrarotating, etc.)

CLT propellers, thanks to the higher efficiency, help reducing the fuel consumption, the emissions and then both the Energy Efficiency Design Index (EEDI) and the Energy Efficiency Operational Index (EEOI) without requesting any modification to the vessel.

The main advantages of CLT propellers can be summarised as follows:

1. HIGHER EFFICIENCY
2. HIGHER SHIP SPEED
3. LOWER FUEL CONSUMPTION
4. REDUCTION IN CO2 EMISSIONS
5. HIGHER RANGE
6. BETTER MANOEUVRABILITY CHARACTERISTICS
7. LOWER NOISE AND VIBRATION LEVELS
8. LOWER OPTIMUM DIAMETER
9. MUCH BETTER PERFORMANCE FOR CP BLADES IN OFF-DESIGN
10. HIGHER CAVITATION INCEPTION SPEED
11. HIGHER KT MARGIN AGAINST FACE CAVITATION

The underpressure on the suction side caused by a CLT propeller is much lower than for an equivalent conventional propeller, while the overpressure on the downstream side is much higher.

Theory demonstrates that the open water efficiency of the propeller increases with an increase in pressure differential between the overpressure aft of the screw and the underpressure forward of it.

The pressure drop on the suction side of CLT propellers is less than for the conventional equivalent and therefore the extent of the cavitation developed on the suction side is lower and hence the pressure forces that a CLT propeller exerts on the stern hull structure are lower. Additionally, CLT propellers have a reduced tip vortex because of the existence of the end plates.

The combination of these circumstances means that the pressure forces exerted by a CLT propeller on the stern structure are of a lower magnitude than for conventional propellers, and so in turn the induced hull vibration and noise levels on board are lower.The noise radiated to the water is also lower for CLT propellers.

As the blade area of the CLT propeller is more efficiently used for supplying thrust, the optimum diameter is lower than for an equivalent conventional propeller.

CLT propellers offer higher efficiency which may be used to achieve fuel savings at constant ship speed or alternatively ship speed increase at constant fuel consumption.

Trawlers and tugs achieve an increase of the pulling force.

The downstream overpressure produced by a CLT propeller is higher than for an equivalent conventional propeller. This increases the pressure on the rudder and so increases the ship’s response to rudder action, leading to a substantial reduction of the turning circle for any given rudder angle; a reduction of both time and distance required to stop the ship in a crash stop manoeuvre and an increase of the ability to mantain a rectilinear course.

Further to the above, CLT-CP blades have additional advantages when CP blades operate at constant rpm because the CLT efficiency, the cavitation performance and the noise and vibration levels in off-design conditions are much better than those of the alternative conventional blades.

The reason is that for conventional blades the radial pitch distribution at design condition is unloaded at the tip to avoid high levels of pressure pulses; when the blade operates in off-design conditions with pitch setting below the design pitch, the blade tip is too much unloaded and a negative pitch is achieved at blade tip. Under these circumstances the upper sections of the blade produce a negative thrust while the lower sections produce a positive thrust and as a consequence the propeller efficiency decreases and the pressure pulse levels increase, because a face cavitation is developed at the pressure side of the conventional blade leading to a broad band energy spectra and the noise and vibration level increases in spite of the reduced power delivered to the propeller in off-design condition.

The situation is not the same for CLT blades because the radial pitch distribution at design conditions bears an important load at the tip due to the existence of the end plates and therefore when the blade operates in off design conditions with pitch setting below the design pitch, the load at the blade tip decreases but still is positive for a wide range of variation of the geometrical pitch angle and so the propeller efficiency remains almost constant and the pressure pulses levels are lower than those of design condition because the power is lower.

The percentage of improvement on efficiency over an alternative optimum conventional propeller depends on the type of vessel, being larger for slow vessels and high block coefficient as tankers, bulkcarriers, etc. for which the thrust coefficient based on advance speed is higher.