Miscellaneous

Propellers

PILGRIM NUT


Assembly

Propeller bedded to tailshaft and jacked up to usual shop mark. The Pilgrim nut is then screwed on the shaft with the loading ring against the prop boss. With the lever operated, high pressure grease gun, grease is pumped into the inner tube inside the nut at around 600 bar, ( w.p. stamped on nut, not to be exceeded), the prop will be pushed sufficiently up the taper to give the required frictional grip. The pressure is then released and the nut is rotated until it is hard up against the aft face of the prop hub and locked, fair water cone then fitted.

Removal

After removal of fair water cone and the locking plate, the pilgrim nut is removed, reversed and together with a loose shock ring is screwed back onto the shaft. A strong back is fitted and secured with studs to the prop boss. Grease is now inserted to the system expanding the inner tube forcing the loading ring, strongback, withdrawal studs and prop aft.

Advantages


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


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.



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.



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 and yielding good results. They are screw propellers fitted with end plates at the blade tips. The plates are deigned to give minimum resistance to flow.

Half duct

Sometimes referred 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


Securing propellers


HYDRAULIC FLOATING OF PROPELLERS

Used for keyless propellers, ensures the correct interference fit using measured oil pressures for expanding the boss and hydraulic jacks or a Pilgrim nut for pushing it up or down the tailshaft taper. No heavy slogging required and low shock loads are applied, quick and safe.

A disadvantage is loss of bearing area due to oil grooves which means that propeller must be longer or greater in Dia to give sufficient area to transmit the torque.

To remove, nut just slackened back. Oil injection applied to expand the boss which allows propeller to move off the taper.

Another disadvantage of wet fitting over dry fitting is that wet boss expansion stress is 30% greater than dry fitted which means that boss must be thicker




Sleeved propeller




Usually fitted on large diameter shafting. Usually hydraulically floated and keyless. Difficult to bed large props to taper, easier to bed sleeve. Also each time a prop is refitted, prop bore becomes larger, this is accentuated in large bore dia props. Hence, after a few refits the prop moves to far up the shaft, more economical to replace the sleeve than the whole prop.

Pearlitic cast iron used to mate with forged mild steel shaft because this combination offers the greatest resistance to fretting which can be caused by prop excited vibration


Molecular fretting can occur internally, generally from the center outward due to molecular rubbing together. Surface fretting occurs at the surface due to two items moving over each other due to vibration.When fitting or removing, heat not to be used since it may effect the mechanical properties of the resin. Wedging or withdrawal systems should not be used since this would cause shearing of the araldite.

Traditional method

Shaft turned to top centre, i.e. when key is on top, convenient for key way inspection and prop slinging. Shaft locked, and prop nut just slacked.

Coupling bolts at tail end flange removed, if lignum vitae bearing- stuffing bearing removed and tailed flange shored up against aft peak bulkhead.

Secured lifting gear to propeller, then wedge off prop using box wedges between stern tube nut and ford face of prop.

If tail end and bronze liner are to be inspected then it must be brought inboard which requires the removal of one or two lengths of intermediate shafting.



Bolted

Controllable pitch propellers require a hollow prop shaft for the oil and feed back tubes to pass through. non of the above methods are suited to this.

Instead the propeller is bolted to a flange, the other end of the propshaft must therefore be parallel to allow removal from the stern bearing.

The prop shaft is attached to the intermediate shaft by a 'muff' coupling. Once the bolts have been tightened they are secured by tack welding locking bars across the heads.



Controllable Pitch Propellers


Advantages

Disadvantages

The CPP consists of a flange mounted hub inside which a piston arrangement is moved fore and aft to rotate the blades by a crank arrangement.The piston is moved by hydraulic oil applied at high pressure (typically 140 bar) via an Oil transfer tube (OT tube) This tube has and inner and outer pipe through which Ahead and astern oil passes. The tube is ported at either end to allow oil flow and segregated by seals.

Oil is transfered to the tube via ports on the shaft circumference over which is mounted the OT box. This sits on the shaft on bearings and is prevented from rotation my a peg. The inner bore of the box is seperated into three sections. The ahead and astern and also an oil drain which is also attached to the hydraulic oil header to ensure that positive pressure exists in the hub and prevents oil or air ingress

The OT tube is rigidly attached to the piston, as the piston moves fore and aft so the entire length of the tube is moved in the same way. A feedback mechanism is attached to the tube, this also allows for checking of blade pitch position from within the engineroom.

Operation Modes

There are two main methods of operation of a veseel with a CPP.

Combinator-For varying demand signals both the engine revs and the pitch are adjusted to give optimum performance both in terms of manouevrability and response, and also economy and emissions.
Constant speed- The engine operates at continuous revs ( normally design normal max working revs), demand signals vary CPP pitch only. This is particularly seen in engines operating PTO generator systems

Emergency running

In the event of CPP system hydraulic failure an arrangement is fitted to allow for mechanical locking of the CPP into a fixed ahead pitch position. This generally takes the form of a mechanical lock which secures the OT tube. Either hand or small auxiliary electric hydraulic pump is available for moving the pitch to the correct position


Thrusters

Tunnel thruster


Thrusters are designed to increase the manoeuvring ability of a ship and may make the use of tugs unnecessary. They also be used to impart side thrust to a ship at a berth in order to counteract wind effect thus minimising stress on mooring wires. At slow speeds the effectiveness of a rudder is reduced and so thruster units are very useful, at higher speeds thruster become less and less effective.

Multiple thruster units are fitted when the fitting of a single large unit is impractical. It is necessary to penetrate the hull on order to provide ducting for bow and stern thruster units and arrangements must be made to ensure the integrity of the hull remains. There is a small increase in hull drag and therefore a slight increase in fuel consumption. Grids protect the propeller from debris.

The thruster is mounted as low in the hull as practical to ensure a reasonable head of water. If this is insufficient it is possible that air from the surface might be drawn into the ducting thus reducing the thrust effect and causing cavitation of the blades

Shown above is one arrangement called a tunnel thruster. A prime mover, normally a constant speed electric motor a drive via a bevel gear a rotating element carrying the blade. The pitch of the blades is variable to allow thrust to be controlled in both direction. An alternative to this is to have fixed blades with a variable speed motor. In some cases with gill jet azimuth thrusters a diesel engine is used allowing it to act as emergency propulsion.

An alternative to the tunnel thruster is the gill jet which can take two forms



Gill jet-bottom suction


Water is drawn from the bottom of the ship and directed to either side by means of hydraulically actuated vanes. The angle of the vanes may be varied thereby allowing the water to be directed forward or aft thus providing forward and aft movement as well as sideways. A unidirectional constant speed motor provides the drive. As with the other system described the motor is vertically mounted and so a form of thrust bearing must be incorporated. This will usually be of the roller type and wear down must be checked periodically.



Gill jet-side suction



Podded Drives


Introduced relatively recently podded drive systems have become more and more common especially in the passenger ship market
They offer combined propulsive and steering ability thereby simplifying the stern arrangement. In addition they may be fitted relatively late is the ship build and remove the need for shaft alignment.

The plant layout may be optimised for the vessel type.

Construction

The pod is allowed to rotate on a rolling element slew bearing driven by either an electric or more commonly hydraulic motor via a gear ring. Electric power si supplied via slip rings to an electric motor mounted in the pod. This electric motor drives one or two fixed pitch propellers.
manouevring is a achieved by modulation of the power supply.

More recent developments have seen the use of single propellers arranged so that in normal running they are in pulling mode sitting ahead of the pod. This has been shown to increase propulsion efficiency.

Operational Difficulties

The removal of gearing from these units allows for increased operational reliability. They are air cooled and therefore require large volumes of air to cool. This can introduce sea water and salt laden air into the windings and lead to overheating and corrosion.

Slip rings must be properly maintained otherwise can be a potential source of failure.

The units are relatively exposed and may be susceptible to impact damage