Thursday, 21 June 2012

Increasing propulsive efficiency

Kort nozzle
Increasing propulsive efficiency

Kort nozzle

The action of the screw within a nozzle with a small clearance between the tips of the prop blade and the walls of the nozzle ( clearance about 1/100 of prop diameter) is to eliminate or reduce trailing vortices which cause loss of efficiency and cavitation at the blade tips The propeller increases efficiency by about 0.4% and their is additional reduction in vibration, cavitation and erosion. It is particularly effective for tugs where they provide an initial thrust although they can cause problems with manoeuvring

From an operational point of view there are problems with cavitation around the inner section of the nozzles although from what I have seen this is not particularly severe. It is important to ensure contant clearance betwen the blades and nozzle inner circumference. this is particularly important say when fitting a new blade to a CPP. Failure to do so can lead to increased vibration, noise and cavitation

Contra-rotating propellers

contra rotating propeller layout

Whenever a propeller turns it exerts a thrust but it also rotates the water and this gives a loss of energy. If a second propeller os fitted immediately down stream but rotating in the opposite direction the second propeller exerts a rotating force on the water which opposes that of the upstream propeller.
water flow is axial and the drive more efficient the costs involved are high due to complicated shafting and gearing that is required to drive the two shafts from a single source. In order to minimise problems of vibration the downstream propeller usually has more blades than the upstream blade but the thrust from each is designed to be the same.

Costa bulb (Propulsion bulb)

 Limited to single screw vessels (usually) the bulb is a simple but effective device for recovering energy from losses aft of the propeller. It consists of a fabricated stream lined steel shell, manufactured in two halves and welded onto the rudder immediately aft of the propeller boss with its centre line continuous with the tail shaft.

effect of fitting a costa bulb
The bulb eliminates vortici created due to turbulent flow and sudden contraction of the water which trails from the boss. Thsi contraction is caused by the suden release of the very large volumes of air released under normal operational conditions when the water passes through the prop
It has a tranquillising effect on the flow of water behind a propeller. Reduces prop vibration, stiffens rudder, increases buoyancy and improves steering. Increase in propulsive efficiency is about 0.5%

Grim wheel

The grim wheel is a free turning propeller mounted after the main propeller. It use the rotational energy of the main prop wash that would otherwise be lost to provide increased propulsive force. The inner section up to the diameter of the main propeller acts as the turbine section. The area outside this is the propulsive section, thus the grim wheel must be larger than the diameter of the main wheel
Initial design had the grim wheel mounted on the main propeller boss. Severe problems including entire loss occurred. The more modern approach is to mount the wheel on the rudder horn. This having the added advantage of allowing a dedicated lube oil supply and reducing main prop shaft and stern tube bearing loading

Alternate design

Shown below is a design of unknown origin but has appeared for some time in this site so the source is lost. I am unsure as to its correctness. It is unsual that the propulsive section should be fitted in the vortex produced by the main prop hub. Grim wheel
The Grim wheel is mounted on roller bearing and is therefore free to windmill on the end of the propeller boss. The Grim wheel is of two parts. The outer section acts as a turbine driven by the wake of the main propeller blading. This turns the inner section which is the propulsion section and provides extra thrust. Increases efficiency by 5%

Highly skewed blades

- are used to lower vibration

Prop Boss Cap Fins

Fins of opposite hand to the main blades are mounted on the prop coss cap. These correct the prop hub vortex and recover rotational energy that would otherwise be lost. Fuel savings up to 5% are claimed

Contracted Loaded Tip

This is a modern design trend under heavy investigation adn yielding good results. They are screw propellers fitted with end plates at the blade tips. The plates are deigned to give minimum resitance to flow.

Half duct

Sometimes refered to as half kort
One or two nozzles may be fitted just ford of the propeller. There purpose is to steer the flow of water to enter the propeller with minimum shock. Efficiemcy claims are up to 3%. The advantage of this system over the full duct or kort nozzle is that it does not suffer from the same cavitation damage on the inner surface. A simplified version of this is two fins welded to the hull at a slight angle to the shaft centre line

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