Lecture 18: Ship Powering & Efficiency Components

Summary

This lecture on Marine Propulsion explores the crucial components involved in ship powering, including brake power, delivered power, effective power, and various efficiencies. The key focus is on understanding the relationships between these power components, particularly how thrust, wake fraction, and hull design influence propulsion performance. The lecturer also walks through an example problem, demonstrating how to calculate propeller RPM, brake power, and effective power using provided data. The session emphasizes the significance of various efficiency terms, such as shafting efficiency and quasi propulsive efficiency, to evaluate a ship's propulsion system.

Takeaways

• 😀 The lecture focuses on ship powering, efficiency components, and the relationships between power components in a ship’s propulsion system.
• 😀 The main engine supplies brake power (P_B), which is transmitted via shafting to the propeller, generating thrust for the ship to move forward.
• 😀 A reduction gear is necessary if the engine speed is higher than the propeller's required RPM, but it can be omitted with slow-speed diesel engines.
• 😀 The delivered power (P_D) is the power absorbed by the propeller, while the effective power (P_E) is the power required for the ship to maintain speed.
• 😀 Shafting losses, due to bearings and gearboxes, typically range from 2 to 4 percent, affecting the relationship between P_B and P_D.
• 😀 The effective power (P_E) is calculated by multiplying the total resistance of the ship by its velocity, while the delivered power (P_D) is based on the rotational speed and torque of the propeller.
• 😀 The quasi propulsive efficiency (η_D) is defined as the ratio of effective power to delivered power, incorporating hull-propeller interaction and hydrodynamics.
• 😀 Hull efficiency (η_H) accounts for the thrust deduction factor and wake fraction, which affect the relationship between thrust power and effective power.
• 😀 Quasi propulsive efficiency is impacted by factors like thrust deduction and wake fraction, which vary between single-screw and twin-screw vessels.
• 😀 The lecture uses a sample problem to calculate propeller RPM, brake power, and effective power for a twin-screw ship, using operational conditions and propulsion coefficients.
• 😀 The final equation for effective power relates brake power (P_B), quasi propulsive efficiency (η_D), and shafting efficiency (η_S) to calculate the total effective power required for the ship's propulsion.

Q & A

What is the main topic covered in the lecture?

The main topic covered in the lecture is Ship Powering and Efficiency Components, which includes the various power components involved in ship propulsion and their efficiencies.

What is the role of the main engine in the ship propulsion system?

The main engine supplies power to the propeller, which is transmitted via a shaft and reduction gear, generating the necessary thrust for the ship to move forward.

Why is a reduction gear necessary in some ships, and not in others?

A reduction gear is necessary when the engine speed is higher than the propeller's required RPM. However, slow-speed diesel engines can eliminate the need for this reduction gear.

What is thrust bearing, and why is it important in the ship propulsion system?

The thrust bearing transmits the thrust generated by the propeller through the shaft to the ship's hull, allowing the ship to move forward.

How is effective power (P_E) calculated in relation to the ship's speed?

Effective power (P_E) is calculated by multiplying the total resistance of the ship (R_T) by the speed of the ship (V_S).

What are the losses that occur between brake power (P_B) and delivered power (P_D)?

The losses between brake power (P_B) and delivered power (P_D) typically occur due to shafting, gearboxes, and bearings. These losses are generally in the range of 2 to 4 percent.

What is the role of shafting efficiency in the ship propulsion system?

Shafting efficiency relates the brake power (P_B) to the delivered power (P_D), accounting for the losses in the shaft, gearboxes, and bearings.

What is the meaning of 'hull efficiency,' and how is it calculated?

Hull efficiency (η_H) is a factor that relates the thrust power of the ship to the effective power, and is expressed as the ratio of (1 - thrust deduction) to (1 - wake fraction). It depends on the ship's hull shape and propeller design.

How is the quasi propulsive efficiency (η_D) expressed, and what factors does it depend on?

Quasi propulsive efficiency (η_D) is expressed as the product of hull efficiency (η_H), relative rotative efficiency (η_R), and open water efficiency (η_O). It depends on the hydrodynamics of the ship-propeller interaction.

How is the effective power of the ship related to the brake power generated by the engine?

The effective power (P_E) can be expressed as the brake power (P_B) multiplied by various efficiencies, including quasi propulsive efficiency, shafting efficiency, hull efficiency, and open water efficiency. The formula is P_E = P_B × η_H × η_R × η_O × η_S.