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Metalock Engineering (2024) completed an in-situ stern tube repair drilling 4 x 20 mm x 1030 mm deep holes through the aft bearing boss and 4 x 20 mm x 400 mm holes through the forward bearing boss, plus 24 tapped M20 holes on a 710 mm PCD. A magnetic drill rig clamped to the shaft flange with laser alignment (within 0.5 degrees radial) was used. The CPP hub contains gun-drilled oil passages (6-15 mm diameter, 500-2000 mm length) for hydraulic blade pitch adjustment.
Controllable Pitch Propeller Hub Oil Passage Gun Drilling
The controllable pitch propeller (CPP) hub is a complex mechanical assembly that allows the propeller blade pitch angle to be adjusted during operation, optimising propeller efficiency across different operating conditions -- free sailing, towing, and manoeuvring. The hub contains a hydraulic piston connected to a crosshead mechanism that rotates the blade roots to change pitch. The hydraulic oil is supplied through gun-drilled passages in the propeller shaft and hub: typically 6-15 mm diameter, 500-2000 mm long, drilled from the shaft flange face through the shaft length and into the hub body to intersect the piston chamber. The passages are drilled using PCD-tipped or carbide gun drills with counter-rotational technique -- the shaft rotates at 10-30 rpm while the gun drill rotates at 500-2000 rpm. The oil passage intersection with the piston chamber must be free of burrs to prevent seal damage, verified by borescope after drilling.
| Parameter | CPP Hub Oil Passage | Azimuth Thruster Shaft Bore | Waterjet Impeller Shaft Bore | Stern Tube Boss (In-Situ) | Stern Tube Lubrication Gallery |
|---|---|---|---|---|---|
| Material | AISI 4140/4340 (32-38 HRC) | AISI 4130/4140 | Duplex stainless steel (2507) | Cast iron / steel boss | Cast iron / bronze |
| Bore diameter | 6-15 mm | 40-100 mm | 20-50 mm | 20-25 mm | 10-20 mm |
| Length / depth | 500-2000 mm | 1-3 m | 500-2000 mm | 400-1030 mm | 200-500 mm |
| Drilling method | Counter-rotational gun drill | Counter-rotational BTA | Counter-rotational gun drill | Magnetic drill rig (in-situ) | Twist drill or gun drill |
| Straightness | 0.1 mm/m | 0.1 mm/m | 0.05 mm/m | N/A (short in boss) | 0.2 mm/m |
| Surface finish (Ra) | < 1.6 microns | < 1.6 microns | < 0.8 microns | < 3.2 microns | < 3.2 microns |
| Cutting speed | 60-90 m/min | 60-80 m/min | 40-60 m/min | 15-25 m/min (magnetic) | 50-70 m/min |
| Coolant pressure | 50-80 bar oil | 40-60 bar oil | 50-80 bar oil | Mist / manual | 30-50 bar oil |
In-Situ Stern Tube Bearing Boss Drilling (Metalock Case Study)
The stern tube bearing boss drilling is an in-situ repair operation performed on vessels that develop stern tube seal leaks. In the Metalock Engineering project, the bearing boss was drilled from the interior of the stern tube tunnel using a magnetic drill press clamped to the shaft flange. The drill was aligned by a laser sight projecting a beam along the radial direction from the shaft centre. The hole positions were on a precise pitch circle diameter (PCD of 710 mm) matching the seal housing bolt pattern. The drilling was performed without the drill breaking through into the oil or water galleries surrounding the bearing housing -- the operator monitored the coolant return for signs of oil or water breakthrough. If oil or water appeared in the coolant, the hole had intersected a gallery and was plugged and re-drilled at a slightly different angle. The 24 tapped M20 holes (M20 x 2.5 thread) were then used to bolt a new anti-pollution seal housing to the bearing boss, providing a second seal barrier that prevents oil leaks into the water.
| Parameter | Aft Boss Holes | Forward Boss Holes | Seal Housing Tapped Holes |
|---|---|---|---|
| Hole diameter | 20 mm | 20 mm | 18 mm pre-tap (M20 tap) |
| Number of holes | 4 | 4 | 24 |
| Hole depth | 1030 mm | 400 mm | 60-80 mm (tap depth) |
| Drill type | 20 mm carbide twist (magnetic) | 20 mm carbide twist (magnetic) | 18 mm carbide twist + M20 tap |
| Alignment method | Laser sight to shaft centre | Laser sight to shaft centre | Laser sight plus template |
| PCD | 710 mm | 710 mm | 710 mm |
| Drilling orientation | Radial outward from shaft axis | Radial outward from shaft axis | Radial outward from shaft axis |
| Coolant monitoring | Watch for oil/water in return | Watch for oil/water in return | N/A (short, tapped) |
Azimuth Thruster and Waterjet Impeller Shaft Drilling
Azimuth thruster drive shafts (vertical shafts transmitting power from the motor to the propeller in a podded drive) require centre bores for hydraulic lines that control the propeller pitch and steering mechanism. The shaft is typically 200-500 mm diameter, 1-3 m length, requiring a 40-100 mm centre bore. The bore is BTA-drilled in two passes (rough and finish) with the shaft rotating (counter-rotational technique) and the BTA head stationary. Waterjet impeller shafts require a smaller centre bore (20-50 mm diameter, 500-2000 mm length) for control rods that adjust the jet deflector and reverse bucket. The bore is gun-drilled in duplex stainless steel (2507), which requires PCD-tipped tools and careful parameter selection (Vc = 40-60 m/min, f = 0.02-0.05 mm/rev) to avoid work hardening.
FAQ
What laser alignment method is used for in-situ stern tube boss drilling?
A laser sight is mounted on a magnetic base at the shaft centreline, projecting a visible red beam radially outward along the intended drilling axis. The drill rig (a magnetic drill press clamped to the shaft flange) is positioned so that the drill axis is aligned with the laser beam to within 0.5 degrees. This is verified by placing a target cross-hair on the drill chuck and adjusting the drill position until the laser beam passes through the cross-hair centre. For the Metalock case, the drill was aligned to the laser beam at both the start and the end of each hole (the drill was retracted periodically and the alignment re-verified). The laser alignment compensates for the curvature of the shaft flange surface and ensures that the holes are drilled along true radial lines from the shaft centre.
How is the counter-rotational drilling technique implemented for CPP shaft oil passages?
In counter-rotational drilling, the workpiece (the propeller shaft) rotates slowly in the lathe chuck (10-30 rpm) while the gun drill rotates at its normal cutting speed (500-2000 rpm) in the opposite direction. The counter-rotation cancels the relative rotational speed at the drill-workpiece interface for a specific point on the shaft circumference, allowing the drill to cut a straight axial bore even on a long, slender shaft. The shaft rotation also helps to centre the drill: if the drill deflects toward one side, the rotating shaft carries the drill back to centre as the shaft rotates. The coolant is introduced through a rotary union at the tailstock end of the lathe, and the coolant pressure (50-80 bar) also helps to centre the drill hydrodynamically.
What causes a stern tube bearing boss to require in-situ drilling?
A stern tube bearing boss requires in-situ drilling when the original seal arrangement has failed and a new anti-pollution seal system must be retrofitted. The original seal (typically a forward and aft lip seal arrangement) wears over time -- seal lip cracks allow seawater to enter the stern tube and lubricating oil to leak out, causing an environmental pollution incident. The repair involves drilling and tapping the existing bearing boss to accept a new seal housing that adds an additional seal barrier. The drilling must be performed in-situ because removing the propeller shaft and stern tube bearing for workshop modification would require dry-docking the vessel for 2-4 weeks, whereas the in-situ drilling can be completed in 3-5 days with the vessel afloat.
What inspection is required for intersecting oil passages in a CPP hub?
The intersection of the gun-drilled oil passage with the hydraulic piston chamber must be inspected by borescope (a flexible fibre-optic or video borescope with a 360-degree articulating tip, typically 4-6 mm diameter). The inspector looks for: (1) Burrs at the intersection point -- any burr can be sheared off during operation and carried into the piston seal, causing seal damage and oil leakage. If burrs are found, they are removed by a small deburring tool inserted through the oil passage or by electrochemical deburring. (2) Drill breakthrough location -- the passage must intersect the piston chamber within +/-2 mm of the design position; if the intersection is too high or too low, the hydraulic oil cannot flow freely into the chamber. (3) Surface finish inside the passage -- if the finish is rough (Ra > 3.2 microns), the oil flow may be restricted, and the passage should be reamed or honed.
What tool material is required for gun drilling duplex stainless steel waterjet shafts?
Duplex stainless steel (UNS S32750 / 2507, 30-35 HRC) is difficult to gun drill due to its high strength (700-900 MPa yield), low thermal conductivity (approximately 14 W/mK, half that of carbon steel), and tendency to work-harden. PCD-tipped gun drills are required for production drilling, as carbide drills wear to VB > 0.3 mm within 5-10 holes. The PCD tip should be fine-grain (2-10 micron diamond particle size) for a sharper cutting edge. The drilling parameters must be controlled carefully: Vc = 40-60 m/min (if Vc exceeds 70 m/min, the heat at the cutting edge causes the duplex steel to work-harden to 40-45 HRC, rapidly destroying the drill), feed f = 0.02-0.05 mm/rev, and oil coolant at 50-80 bar with a minimum flow rate of 30 litres/min. The peck depth should be 2-5 mm to clear the work-hardened chips.
Data are based on published research and industry experience as of 2026. Always consult your equipment manufacturer and applicable marine classification society rules (Lloyd's Register, DNV GL, ABS, Bureau Veritas) for specific application requirements.