CARE OF PROPELLERS

Inspection by ship's engineer. Whenever a ship is docked, one of the ship's engineers should examine the propellers, and the result of the examination should be entered in the engineering log.

Inspection by yard force. As soon as practicable after docking a vessel, the yard force should make a careful examination of the propellers, and any repairs found necessary should be immediately undertaken, so that the undocking of the vessel will not be delayed. (For sample inspec­tion data requirements, see Fig. 15A, B & C.)

Data on propeller hubs. The following data should be clearly stamped in a conspicuous place other than the blades:

A. B. S. Approval Manufacturer's Number

Name of Ship

Builder

Owner's Approval

Plan Number

Hull Number

Finished Weight, lb.

"Port" or "Starboard" if twin screw

Diameter

Face Pitch (at 0.7 rad.)

Also the number of each blade should be stamped on the hub next to that blade.

Verify data. The heat number and builder's name stamped on the hub should be verified and recorded upon each removal at docking. Adjust­ments made, general conditions found and work performed should be recorded in the engineering records of the vessel.

When the propeller is removed, the condition of the propeller nut and locking device should be inspected and should be checked again before the fairwater cap is replaced.

Make a report of all inspection, repairs to or changes of propellers installed on ships, hub data, pitch measurements and balancing data.

Cleaning propellers. Fouling or even slight damage to a propeller has a marked influence on the frictional resistance, and results in an increase of horsepower over that necessary to deliver the same number of revolutions with a clean or undamaged propeller.

Propellers may be cleaned by the ship's force when the vessel is in dry

 

 

   

Fig. 15A. Sample inspection data sheet                        Fig. 15C. Inspection data sheet continued

dock or on a marine railway, or by divers when the vessel is in the water. When the fouling rate may have been appreciable due to inactivity of the vessel or to water conditions, propellers should be cleaned prior to exten­sive operations.

 

After docking it may be necessary to remove barnacles by scraping and wire brushing.

After cleaning bronze propellers, give them a final polishing with a fine abrasive. Then give them a coat of lubricating oil. Do not apply paint.

Faulty and damaged sections, of bronze propellers, if not too extensive, may be cut out and then rebuilt by use of bronze welding.

Pits and rough portions of cast-iron propellers or cast-iron fairwater caps should be thoroughly cleaned and filled with iron cement before being painted. Apply the same quality of paint and number of coats that are used in painting the ship's bottom.

Care of shaft taper. Most failures of tail shafts occur at the large end of the taper just abaft the bronze shaft sleeve, under the propeller hub. To make sure that salt water will not enter this dangerous region, rubber rings are provided in the propeller gland, and the space aft of the rubber rings is filled with a mixture of white and red lead and linseed oil. When the propeller is removed the shaft taper, including the keyway, should be carefully examined for corrosion or cracks. If the rubber rings have deteriorated they should be replaced. The propeller may not be re­placed without the approval of the American Bureau Inspector.

PRECAUTIONS    TO    MAINTAIN    PROPER    PITCH    OF PROPELLER   AND   DIMENSIONAL  CHARAC­TERISTICS   OF   BLADES

Pitch. To preserve the pitch of a propeller the shape and setting of blades must always correspond to the specifications for the propeller.

Square mark.

All removable blades have a square mark to register with a corresponding mark on the hub. Coincidence of these marks proves a blade to be properly set and the pitch to be correct unless the blade has been bent. Should it be desired to prove the pitch of a propeller, a pitch-ometer is used. Repair yards are equipped with instruments suitable for the purpose.

Pitchometer.

 Should it be necessary to improvise a pitchometer, pro­ceed as follows:

If the pitchometer is to be used with the propeller in place, remove the cap and the propeller nut, and make a block with counterbore to fit neatly over the threads of the propeller shaft. If it is to be used after the pro­peller has been removed, provide a wooden plug to fit the bore of the propeller. Pivot a stiff, straight, wooden batten to the after end of the block at a point corresponding to the center of the shaft. The batten must revolve in a plane at right angles to the axis of the shaft. At intervals of 6 or 12 in. along the batten nail small wooden strips for guiding a grad­uated pointer. These strips must be secured as nearly as possible parallel to the axis of the shaft. A pointer graduated in inches and fractions thereof and a paper protractor graduated in increments of 5° complete the ap­paratus. The paper protractor is pasted on the after end of the block.

Obtaining the pitch.

In obtaining the pitch of a propeller three measurements are necessary:

The radius at which the pitch is measured.

The angle or part of the complete circumference corresponding to the distance on the blade between the two points between which the pitch is required.

Fig. 16.

The advance parallel to the shaft line, corresponding to this part of a complete revolution. (Note: The advance in inches for an arc of 30° is equal to the total pitch in feet.)

These measurements are obtained by means of the pitchometer as follows:

With a radius EA' = R on batten ED draw the arc OP, using graduated pointer AG. (See Fig. 16.) Select two points, A and C, such that the angle

AEC = 0 is any multiple of 5° measured on the protractor F. By means of the graduated pointer measure the distances AA' and CC. The differ­ence between these measurements (BC) is the advance corresponding to the angle 6. Since in a complete revolution the point A travels a distance equal to the whole pitch, and for the angle 0, a measured distance equal to CC" — AA', we have 360° whole pitch (P) :: angle 6 : fraction of pitch (CC - AA'). Therefore

 

The number of points measured depends on the requirements of the occasion.

Rough check on the pitch. A check on the pitch of an installed pro­peller may be obtained by the following rough but fairly accurate method:

Turn the propeller to bring one of the blades horizontal. Select the radius R at which the pitch is desired and hang a cord with two weights as shown in Fig. 17. Care must be taken that the two points, A and B, at which the cord touches the edges of the blade, are at the same distance from the center of the propeller. It is then readily seen that the points A and B of Fig. 17 correspond to the points A and C of Fig. 16, except that in Fig. 17 they are necessarily taken on the extreme edges of the blade. Level a batten PQ and measure the distances AC and BC. The pitch for radius R is:

 

Balancing.

Propellers should not be dynamically balanced unless spe­cifically required. The material to be removed to effect static balance may be taken from either the after (pressure) or forward (suction) faces of the blades, but in those cases where the material is to be removed from forward faces, special care should be exercised to maintain the designed curvature of. the blade section and to avoid humps and hollows. In those cases where metal is removed from the after faces for the purpose of balancing after the establishment of the finished surface for this face, the areas from which the metal may be removed shall not approach the blade edges within 25% of the blade width, care being exercised to avoid radical changes in thickness of sections. The unbalance tolerance shall be based on a centrifugal force equal to 1% of the weight of the propeller when revolving at its rated r.p.m. (The reason for balancing propeller is to aid in eliminating vibration.)

Average pitch.

For repaired propellers the average pitch of any one blade shall not vary more than plus or minus 1% from the average pitch of all blades, and the average pitch of all blades shall not vary more than plus or minus 11\2 % from the designed pitch. No individual element of the blade should vary over 2% from the designed pitch.

Propeller blade thicknesses, and leading and trailing edge details, shall be accurate within a tolerance limit of plus or minus 2% of all variations in excess of 1/32 in. from the designed dimensions. This is for propeller efficiency and to avoid vibration.

Fig. 17.

For checking the accuracy of the blade surfaces a straight edge can be applied to the after face of constant pitch propellers and variable pitch propellers, if the pitch variation is uniform along the blade generatrices. The uniformity of the forward face can be checked by the use of a flexible steel straight edge which can be sprung across the surface at various loca­tions to detect humps and hollows.

Repair guide list.

When repairs are undertaken in dock, it is essential that a thorough inspection be made so that all defects may be found and remedied. Therefore a repair guide list is suggested for use. Should an item be not done it should be so checked on the list, as often information of a negative character is found valuable. Upon completion of work the repair guide list should be preserved as a record of conditions found, data taken, and work done. It will then be available for examination when the next docking takes place, and the series of guide lists will form a complete history, with minimum clerical effort.

Removal of wing propellers.  

To take off a wing propeller proceed as follows:

a) Remove propeller nut.

b) Place a draw-bolt clamp or strong back over the end of the shaft.

c)  Interpose a piece of soft metal between damp and shaft end, if there is danger of upsetting threads on shaft.

Fig. 18A. Pad eye for lifting propeller. (154" size—eye 2", pad 7" x 10" x 54").

d)Insert draw bolts in tapped holes in hub face if provided. Otherwise hook the bolts over the blade edges or screw them into pieces of metal laid across the forward edges of the blade roots.

  e) Arrange propeller rigging.

f)Set up on clamp nuts until propeller starts, or stress is as great as clamp will stand.

 Use a maul or a button set judiciously.

g) If propeller will not start, heat must be applied to expand the hub.

 

Removal of single-screw propellers.   

To remove the propeller of a single screw ship with a balanced rudder, remove rudder and then treat like a twin-screw ship. Otherwise the following procedure will be necessary.

a) Break tail shaft and aftermost line shaft coupling, and loosen up the
stern-tube glands.

b) Clear aftermost line shaft section.

 

 

Fig.  18B.  Location  of  pad  eyes  for lifting propeller and rudder.

c)Remove propeller fairwater cap.

 d)Loosen and back off propeller nut.

e)Place strong back in place and insert draw bolts, placing a piece of copper or brass between the strong back and the shaft.

f) Adjust rigging.

g) Set up on the strong back until either the propeller starts or the stress on the strong back is as great as desirable.  Use a maul judiciously.

h) If the propeller does not start, heat will have to be applied to the hub. Care should be taken that steam is not formed in the cored hole in the hub, as this has caused serious accidents.

i) After the propeller starts the weight of the propeller is taken by chain hoists of adequate size, and the tail shaft drawn inboard until clear.

Fig. 19A. Installation of propeller, using wedges.

Partial straightening of bent blades can be easily done. The method used depends on the extent of the damage. Bends near the edge can be handled by sledges and clamps. For more extensive bends special heavy clamps must be rigged to take a strain from a chain fall. With a heavy strain on a clamp a light blow on the blade at the proper place will frequently do considerable straightening.

How to apply heat.

Heat is applied to a propeller by using preferably two large burners, one on each side of the hub. If only one burner is avail­able, work the flame around the hub to heat and expand the metal uni­formly. The largest propeller can usually be heated sufficiently in about an hour with two burners.

Before applying heat, cover exposed parts of shaft near the propeller with asbestos. Remove filling hole plugs in the hub and hub cap and loosen all cover plates to permit escape of vapor.

In removing or replacing a propeller nut, it is good practice to fit a shore from the bottom of the dock to a propeller blade near the root, so that it will take the shock of driving the propeller nut and prevent the shock of the blows being transmitted to reduction gearing, rotors, or crank shaft.

Method of installing propeller of single-screw ship.

The wheel and tail shaft should be carefully checked. A thin coat of white lead and lubricating oil or Houghton's Rust Veto should be applied to the tail shaft taper. The propeller is set in position and held by a screw jack against the rudder post. The tail shaft is pushed into the hub taper, using chain falls in the shaft alley. After the wheel is installed on the shaft as far as it will go, the propeller nut should also be driven as far as it will go, always allowing room for stub wedges to drive wheel up in place. When wheel is close up to counterbore, red lead putty is inserted in the ( counterbore and then the wheel is driven up as far as it wiH go with the use of stub wedges. After the wheel is up as far as it will go, using stub wedges, they should be removed and box wedges installed. The box wedges should be driven up until the wedge sings. Then the boxes should be reset so there is plenty of movement to drive the wedges (wedging up tail shaft from inside the ship).

After the wedges are again set and in a singing position the old fit mark should be observed to see how far the propeller should go up. After checking this mark and seeing that everything is well protected, a S-minute heat should be thrown on the wheel with two Hawk torches, one on each side of the hub. After taking any chill out that may be in the wheel, the heat should be stopped and orders given to the men to stand by the box wedges. When wedges are again singing, a careful check should be made

Fig. 19C. Propeller wedge assembly and details.

to determine whether they are past the marks installed on the shaft, in which case a new mark should be made. The boxes are now removed, the thread cleaned and the nut tightened, using chain falls. The keeper is installed and the hub cap fitted. The hub cap and the interior of the pro­peller hub are filled with tallow, and the pipe plugs fitted and then prick-punched to prevent backing out. Then the last section of the line shaft is installed.

The rigging supporting the propeller while it is being fitted should be carefully inspected and should be thoroughly adequate.

Extensive repairs.

Propellers, if appreciably damaged, should be repaired in a shop rather than in place on the ship. When extensive pro­peller repairs are contemplated or when repairs are of a nature which require machining or chipping to restore to designed dimensions, a yard estimate should be obtained before proceeding with the work, and ap­proval of the A.B.S. and the Marine Inspection Service of the U. S. Coast Guard must be had.

If a propeller blade is bent to the extent that it must be repaired quickly so as to keep the vessel in service, and it is not possible to return it to a qualified propeller manufacturer for repair, great care should be exercised in the application of heat. At no time should the repaired sec­tion be worked at a temperature higher than cherry red. As soon as this condition changes to black heat, work should stop and the worked portion reheated.