1. CARE OF PROPELLERS
The bronze propeller is an expensive and scientifically designed ship part which requires considerable care and attention. The propeller in use suffers wear like any other moving part and its life, aside from direct physical damage, is determined by its rate of wastage, a corrosive and erosive process. As far as corrosion alone is concerned, bronze propellers in still or moving sea water may lose 0.05 to 0.10 mm (0.002 - 0.004 in.) of surface metal every year. The wastage by corrosion over the blade tip area of a clean undamaged propeller working at its designed revolutions may increase by 4 to 5 times if the blade surfaces are rough. However, wastage by erosion may be much greater than wastage by corrosion. Highly loaded manganese bronze propeller blades have shown wastages up to 1.27 mm (0.050 in.) a year. These high erosion losses have lead to the development of the improved, more resistant alloys. The newer propeller alloys, though more costly, are stronger and much more resistant to wastage.
Since propeller efficiency is largely dependent on drag, propeller surfaces should be polished smooth. The effect on drag of roughness on the suction face of the blade is considerably larger than the effect of a corresponding degree of roughness on the pressure face. Consequently it is extremely important that the suction faces be very smooth not merely painted, as is sometimes done. Since the propeller material itself is much more resistant to the scrubbing action of sea water than is any paint, the right way of getting a smooth surface for highest efficiency is by machine polishing.
Whereas elimination of surface roughness pays measurable dividends in fuel oil savings, care must be exercised to avoid introducing humps and follows. For this reason only cup wheels should be used when grinding with stones (wheel type polishers which grind with the rim should be prohibited). Final polishing may be done with discs. During all polishing the machine should be kept moving to avoid making local low spots.
Propeller Protection During Dry-docking
Physical damage to propeller blades should receive prompt attention For example; a bent leading edge creates a condition of disturbed flow for a considerable distance on the blade surface which can cause serious cavitations erosion damage. While minor damage may be too slight to have any observable effect on the performance of the vessel, quite small edge deformation (nicks, bends, etc.), particularly at the leading edge, can result in an important increase in the normal rate of erosion wastage of the blade surface.
Propeller damage requiring repair falls into either or both of the following general Classes:
1. Physical damage in the form of breaks, tears or bends in the blades resulting from impact with foreign bodies in the water or from unsatisfactory repairs.
2. Propeller damage in the form of wear, as the result of erosion and/or corrosion. Wear damage is usual the result of one or more of the following:
- Normal scrubbing action of the water. Local cavitations caused by physical damage.
- Cavitations due to design characteristics.
- Hull obstructions in the slip-stream ahead of the propeller, such as poorly placed anodes or lifting eyes. These may cause local water turbulence and result in cavitations damage.
- Electro-chemical attack resulting in wastage and/or dezincification of the blade surface. (Dezincification sometimes occurs on manganese and nickel manganese bronze propellers, producing a copper colored surface).
- Attack by chemicals contained in harbor water.
- Abrasive action of sand or other bottom solids when operating in shallow water.
Water temperature may also be a significant factor influencing the rate of erosion, particularly for highly-loaded propellers.
2. GENERAL REPAIR PRECAUTIONS
1. No repair of a propeller can be performed in place in drydock as satisfactorily or as safely as it can be done under controlled conditions in a propeller repair shop.
2. Only very minor straightening and no welding should be done while the Ship is in the water or the propeller is still on the shaft in drydock.
3. Welding repairs should never be made to the propeller unless the work is done by qualified personnel under informed supervision.
4. Many repair yards are not equipped with either qualified personnel or proper equipment necessary for satisfactory propeller repair.
5. Welding on the hub or near the blade root should not be attempted in the drydock even with the properly removed from the shaft because of the difficulty of obtaining sufficient sustained heat far proper stress relief. Also, heal in the hub area, if improperly applied, can distort the bore and spoil the propeller fit on the shaft taper.
6. Frequently, on the first few minor repairs made on the propeller, the cost can be held down by simply trimming back the blade edges to sound metal as they become ragged and torn. This obviously can be done only a few times before the vessel’s performance suffers. Excessive edge trimming leads to poor maneuvering ability, lower backing power, increased fuel consumption and cavitations erosion. It can also lead to propeller imbalance. When the diameter is trimmed, the power absorption and fuel consumption of the propeller suffer even more sharply. The more trimming done, the more costly will be the eventual repair to restore the blades lo design size.
7. It is recommended that the Owner maintains a record of condition surveys and of all repairs for each of his propellers. It is particularly important to record each blade edge trimming and diameter reduction and their later restoration. It also will be of value to note each straightening, weld repair, stress relief or other heating and the procedures followed. Records of minor repairs made “in-place” are especially useful when evaluating later major damage.
8. The necessity for stress relief when required cannot be over-emphasized. Not applying proper stress relief treatment to a repair presents a greater danger of the propeller than that presented by a poor weld.
The poor weld merely offers the possibility of failure lo the repair but the lack of proper stress relief can result in cracks forming in any part of the heat affected zone, such cracks can be expected to grow rapidly and can lead lo condemnation of the propeller.
3. PROPELLER REMOVAL
It is of utmost importance that proper care be taken when removing the propeller from the shaft. Propellers of manganese bronze or nickel manganese bronze compositions are quite susceptible to stress-corrosion cracking. Cases are on record of manganese and nickel manganese bronze propeller hubs having cracked because of improper application of heal used to facilitate removal of the propeller from the shaft. Stress-corrosion cracking of this type is easily caused by high temperature locally applied flames, of which oxyacetylene and oxy-propane are most dangerous.
If it is necessary to apply heat when removing a manganese or nickel manganese bronze propeller from the shaft, it is important to use a constantly moving low-heat flame in order to avoid setting up excessive stress.
(Note: As an alternate, the use of solidified CO2 (“dry ice”) held in place by layers of packing around the exposed shaft, fore and aft of and insulated from the hub, is a sate and sometimes effective way of loosening a propeller).
Heating temperatures should be evenly distributed and checked with “Tempilsticks” or a surface contact pyrometer, taking care not to exceed 200 ºC (392 ºF) at any time during the heating operation.
Uniform heating and cooling in way of the hub is essential to avoid major bore distortion. When carried out carefully under controlled conditions stress-corrosion cracking will rarely occur.
Since stress relief treatment is troublesome, time consuming and difficult to control, it should be obvious that when removing a propeller from the shaft every possible effort should be made to distribute the heat evenly and not exceed the maximum temperature of 200 ºC, so that the need for post-hest treatment may be avoided.
4. BLADE EDGE DAMAGE
Blade edge damage usually takes the form of cracks, bends, or breaks and may include the loss of a small section of the blade.
Minor bends or tears can sometimes be repaired without removing the propeller from the shaft.
Very slight distortions along the blade edge can usually be straightened cold by hammering carefully. After straightening however, the area worked should be dye-penetrant examined for cracks which may have resulted from the hammering.A bend which may appear to the eye to be confined to the edge actually may extend a considerable distance into the blade. If there is any question as to the extent of the bend, the plan should be examined and blade checked with a pitchometer and/or gages. When this is not possible, a straightedge may be applied to surface radially and compared either with the drawing or with the undamaged blades to assist in estimating extend of the bend. Bends extending more than 100 mm (4 in.) in from the blade edge should be considered to be a shop job.
Cracks require careful scrutiny to ascertain their extent and it must be decided if an in-place repair will suffice or if a shop job is necessary. Cracks are dangerous and are potential fatigue nuclei for a major break and possible loss of that section of the blade should that area receiving a heavy blow. The permanent repair of cracks requires complete removal of the crack, followed by welding using the prescribed welding procedure for the particular metal.
Edge damage, which is impossible to repair with the propeller in-place, but not requiring shop repair, requires proper positioning of the blade on the drydock floor.
When large areas of the blade edges or tips have been broken off, it is necessary to “burn-·on” or weld-on a cast piece to replace the lost section.
This procedure requires equipment and melting facilities not usually available at ship repair yards, therefore it is recommended that this type of repair be done by a propeller manufacturer.
5. SURVEYS OF CYCLOIDAL PROPELLERS AND CONTROLLABLE PITCH PROPELLERS
Although all propellers are to be externally examined at each Dry-docking Survey, in view of the special configuration of cycloidal propellers it has been decided to credit the machinery of these as part of the Special Survey of Machinery. For the Special Survey crediting, the examination should include at least a functional test and oil Leakage check. At Special Survey No. 2 and alternate Special Surveys thereafter, the machinery and blade assembly may be required to be dismantled, cam lobe and gear teeth condition checked, clearances measured, and the seal rings examined. Any worm or deteriorated parts should be dealt with to the Surveyor's satisfaction by repair or renewal.
Controllable pitch propellers are also to be examined externally al each dry-docking, however the blade pitching mechanism is to be covered under the Tailshaft Survey. All such examinations should include al least functional test and leakage inspection. One or more blades may be required to be removed if leakage, damage, or malfunction is found or suspected, in order to check the seals, seal surfaces, and internal moving parts.
6. Propeller Terms and Definitions
1. Diameter –The diameter of the imaginary circle scribed by the blade tips as the propeller rotates.
2. Radius –The distance from the axis of rotation to the blade tip. The radius multiplied by two is equal to the diameter.
3. Blade Face –Pressure Side, Pitch Side. Aft side of the blade surface facing the stern.
4. Blade Back –Suction Side. Forward side of the blade surface facing the bow.
5. Leading Edge –The edge of the propeller blade adjacent to the forward end of the hub. When viewing the propeller from astern, this edge is furthest away. The leading edge leads into the flow when providing forward thrust
6. Trailing Edge –The edge of the propeller adjacent to the aft end of the hub. When viewing the propeller from astern, this edge is closest the trailing edge retreats from the flow when providing forward thrust.
7. Blade Number –Equal to the number of blades on the propeller.
8. Blade Tip –Maximum reach of the blade from the center of the hub. Separates the leading and trailing edges.
9. Hub –Solid cylinder located at the center of the propeller. Bored to accommodate the engine shaft. Hub shapes include cylindrical, conical, radius & barreled.
10. Blade Root –Fillet area. The region of transition from the blade surfaces, and edges to the hub periphery. The area, where the blade attaches to the hub.
11. Rotation (Right hand shown here) –When viewed from the stern (facing forward): Right-hand propellers rotate clock wise to provide forward thrust. Left-hand propellers rotate counter-clockwise to provide forward thrust.
12. Pitch –The linear distance that a propeller would move in one revolution with no slippage.
13. Cylindrical Section –A cross section of a blade cut by a circular cylinder whose centerline is the propeller
14. Pitch –Reference line Reference line used to establish the geometric pitch angle for the section. This line may pass through the leading and trailing edges of the section and may be equivalent to the chord line.
15. * Geometric Pitch Angle –The angle between the pitch reference line and a line perpendicular to the propeller axis of rotation.
16. * Controllable Pitch Propeller –The propeller blades mount separately on the hub, each on an axis of rotation, allowing a change of itch in the blades and thus the propeller.
17* Fixed Pitch Propeller –The propeller blades are permanently mounted and do not allow a change in the propeller pitch.
18.* Constant Pitch Propeller –The propeller blades have the same value of pitch from root to tip and from leading edge to trailing edge.
19.* Variable Pitch Propeller –The propeller blades have sections designed with varying values of local face pitch on the pitch side or blade face.
20.* Rake –The fore or aft slant of a blade with respect to a line perpendicular to the propeller axis of rotation.
20a. Aft Rake –Positive rake. Blade slant towards aft end of hub.
20b. Forward Rake – Negative rake. Blade slant towards forward end of hub.
21. Track –The absolute difference of the actual individual blade rake distributions to the other blade rake distributions. Always a positive value and represents the spread between individual blade rake distributions.
22.* Skew –The transverse sweeping of a blade such that viewing the blades from fore or aft shows an asymmetrical shape.
22a. Aft Skew –Positive Skew. Blade sweep in direction opposite of rotation.
22b Froward Skew –Negative Skew. Blade sweep in same direction as rotation.
23. Cup –Small radius of curvature located on the trailing edge of blade.
24. DAR. –Developed Area Ratio is blade area expressed as the percentage of a circle shaded by the propeller.