Table of Contents
How do you find the wake fraction?
The wake fraction can also be estimated by making direct velocity measurements behind the hull, with no propeller. Rt = Rsp(1 – t). Often, a propeller will increase the resistance of the vessel by creating low-pressure on its intake side (near the hull), which makes Rsp > Rt.
What is propeller efficiency of a ship?
“In modern ships, only 25 to 35 percent of the energy contained in the fuel is effectively used to propel the ship. The challenge is to reduce the loss factors involved in converting the main engine’s output into propeller thrust and thereby increase overall propulsion efficiency.
What is the thrust deduction fraction T?
the thrust deduction factor, t(= tM = is) is obtained from the difference between the self propulsion thrust and the hull resistance without propeller, corrected if necessary for temperature differences at the time of the separate tests; From: Basic Ship Theory (Fifth Edition), 2001.
How thrust power is determined in ship?
It is equal to the product of the resistance of a ship and the speed of the ship. This power is equal to the Brake Horsepower minus losses due to the gearbox, shafting and propeller, as well as interaction between the propeller and the hull.
What is wake in ship?
What is a ship wake? A moving ship (or any other object moving at or near the water surface) generates a kind of trace on the water surface which is called a wake. Around and directly behind the ship, the wake is rather complex, with so-called bow and stern waves, eddies and currents, and foam.
What is wake propeller?
When the ship is sailing ahead, the friction of the hull will create a boundary layer of water around the hull. All this results in the propeller behind the hull is working in non-uniform water flow called wake-field.
How do I know my propeller torque?
Torque = Force * length [Nm] Power = Force * Velocity = Force * length * angular velocity Power = Torque * angular velocity [Nm/s] When power is given in HP then torque can be found as T = 5252.0 * HP / RPM [ft*lb] = 7121 * HP / RPM [Nm] Page 3 2.016 Hydrodynamics Reading #10 version 3.0 updated 8/30/2005 -3- ©2005 A. …
How is propeller efficiency calculated?
Propeller efficiency is defined as: eff = “K” * Thrust * Speed / Power (where “K” is a constant to account for units).
What is wake distribution?
The wake distribution is measured behind the ship model using pitot tubes or laser-Doppler velocimetry. The results are usually displayed as contour lines of the longitudinal component of the velocity. These data plays an important role in the propeller design.
What factor affects the ship propeller slip?
Propeller slip is affected: by factors related to the engine, the boat and the boat set-up: the type of the hull and its condition, the weight and the aerodynamic design of the boat, the design of the engine’s lower unit, the mounting height of the engine, the loads as well as the design of the propeller itself.
What is the angle of a boat wake?
about 19.5°
A physical explanation for the phenomenon of boat wakes, and for the value of the wake angle, was originally given by William Thomson — better known as Lord Kelvin. He found that the faster the speed of the ship, the faster the wake broadened laterally. Consequently, the wake angle has a constant value of about 19.5°.
What is the wake fraction of a prop?
The propeller is operating in a wake. In practical terms, the wake fraction comes about this way: Suppose the open water thrust of a propeller is known at a given U and np. Behind a vessel moving at speed U , and with the propeller spinning at the same np, the prop creates some extra thrust.
What is the wake fraction coefficient of a ship?
For ships with one propeller, the wake fraction coefficient “w” is normally in the range of 0.20 to 0.45. Ships with a large block coefficient have a large wake fraction coefficient and the distribution of the water velocity around the propeller is very inhomogenous under such condition.
How do you calculate nominal wake fraction?
The nominal wake fraction, w =v−v a v, relates the actual ship speed v to the mean velocity experienced by the propeller vaas operating behind the ship.
Why does the wake vary across the propeller disc area?
The wake will vary across the propeller disc area, being higher close to the hull or behind a structural element such as a shaft bracket arm. Thus the blades operate in a changing velocity field as the propeller rotates leading to a variable angle of incidence.