Deep Hole Drilling for Marine Propeller Shaft Boring and Stern Tube Retrofit: In-Situ Shaft Manufacturing and Bearing Boss Modifications at Sea

A vessel with stern tube oil leaks required in-situ drilling of 4 holes of 20 mm diameter x 1030 mm depth through the aft bearing boss (positioned on a 710 mm PCD) and 4 holes of 20 mm x 400 mm depth through the forward boss. A magnetic drill rig clamped to the shaft flange drilled horizontally through 1030 mm of cast iron bearing boss inside the confined stern tube tunnel, achieving positional accuracy within plus/minus 0.5 degrees on the PCD.

New-Build vs In-Situ Drilling Methods ​

Marine propeller shaft deep hole drilling encompasses two fundamentally different applications: manufacturing new hollow propeller shafts in a factory environment, and drilling holes into existing stern tube bearing bosses while the vessel is in service for retrofit modifications. The requirements, equipment, and quality standards differ substantially between these two applications, as shown in the following comparison table.

New-build hollow shaft manufacturing is performed on a horizontal deep hole drilling machine with the shaft rotated by a headstock and supported by steady rests along its length. The shaft is rough bored from both ends using BTA drilling with a 6-10 mm oversize allowance, meeting in the middle. The centring accuracy of the BTA drilling must be within 0.1-0.3 mm at the meeting point. After rough boring, the shaft is finished by pulling a floating reamer through the full bore length from one end while the shaft rotates at 20-50 rpm. The floating reamer self-centres within the bore, correcting any deviation from the rough BTA drilling and achieving a straightness of 0.02-0.05 mm/m. The final finishing step is skiving and roller burnishing (or roller burnishing alone), which produces a surface finish of Ra 0.2-0.4 microns and imparts a compressive residual stress at the bore surface that significantly improves the shaft's fatigue life under torsional loading. In-situ retrofit drilling is performed using portable magnetic drill rigs that clamp to the shaft flange face or to a mounting frame bolted to the stern tube face. The magnetic clamp provides typically 10-20 kN of holding force. The drill is positioned using a laser alignment system that references the shaft centre line, and the alignment is verified by a digital inclinometer. The hole is drilled using carbide-tipped twist drills for diameters up to 32 mm, or diamond core drills for larger diameters. The coolant is a water-miscible emulsion at 10-20 bar, and chip evacuation relies on the drill flutes. Operators must manually remove chips from the confined space after drilling to prevent contamination of the stern tube bearing oil system.

Classification Society Requirements and Quality Standards ​

Propeller shaft drilling and stern tube modifications are subject to approval by the vessel's classification society, which sets the quality standards for materials, workmanship, and inspection. The five major classification societies have broadly similar requirements but differ in specific details. The following table compares the key requirements for propeller shaft deep hole drilling across the major classification societies.

The classification society requirements for new-build hollow shafts focus on material traceability, dimensional accuracy, and non-destructive testing. The shaft material must be supplied with a mill certificate that specifies the chemical composition, mechanical properties, and heat treatment condition. The forging must be ultrasonically tested before drilling to verify internal soundness. During drilling, the classification society surveyor typically witnesses the rough boring operation (to verify the drilling setup and chip condition) and the final inspection (dimensional measurement, surface finish measurement, and hydrostatic test). For in-situ retrofit drilling, the classification society requirements are less stringent because the holes are not structural (they are clearance holes for bolts or lubrication passages). However, the surveyor still must approve the repair procedure before work begins and witness the final positional verification. The positional tolerances for in-situ retrofit holes are typically specified in angular terms: the hole position on the pitch circle diameter must be within +/- 0.5 degrees, and the hole axis alignment must be within +/- 0.5 degrees of the radial direction. These tolerances are verified using a digital inclinometer for the axis alignment and a digital calliper referenced to the shaft centre line for the PCD position. The inspection records are submitted to the classification society surveyor for final approval before the vessel can return to service.

Fatigue Life Considerations for Drilled Shafts ​

The fatigue life of a hollow propeller shaft under torsional loading is influenced by the bore surface condition, the residual stress state, and the presence of stress concentration features such as cross-drilled holes or keyways. The following table compares the fatigue life of different bore finishing methods based on rotating bending fatigue tests of AISI 4340 steel shafts.

The fatigue life improvement from skiving and roller burnishing (SB) is primarily due to the compressive residual stress induced at the bore surface. The SB process uses a tool with a skiving blade (PCD or CBN) that removes a thin layer (0.03-0.08 mm per side) to achieve the dimensional tolerance, followed immediately by 4-6 carbide rollers that cold-work the freshly machined surface at a roller pressure of 2-5 kN. The cold working induces a compressive residual stress of -400 to -600 MPa at the surface, extending to a depth of 0.1-0.3 mm. This compressive stress reduces the net tensile stress experienced at the bore surface during torsional loading, increasing the fatigue life. For a propeller shaft that experiences cyclic torsional loading from engine torque fluctuations (typically 5-15 percent of the mean torque), the fatigue life improvement translates to an extended inspection interval or a longer service life. The DNV rules for propeller shafts require a fatigue analysis for shafts with cross-drilled holes (such as oil distribution holes or pitch adjustment mechanism holes). The analysis must show that the fatigue life exceeds 20 years of operation at the specified loading spectrum. Cross-drilled holes create a stress concentration that reduces the fatigue strength by a factor of 2-3 compared to the plain shaft bore. The stress concentration factor depends on the ratio of the cross-hole diameter to the shaft bore diameter, the edge condition of the cross-hole (sharp edge vs radiused), and the proximity of the cross-hole to other features. A radius of 0.5-1.0 mm at the intersection of the cross-hole and the main bore significantly reduces the stress concentration and improves the fatigue life.

Frequently Asked Questions ​

What is the reverse pulling floating hole technique? ​

The reverse pulling floating hole technique is a precision finishing method for manufacturing long hollow propeller shafts, covered by Chinese patent CN106002120A. The process begins with rough boring the shaft from both ends using a BTA drilling system, which produces a bore that is 6-10 mm undersize and has some degree of deviation (typically 0.1-0.3 mm per metre of length). The rough BTA drills meet at the centre of the shaft, and the alignment at the meeting point must be within 0.3 mm for the subsequent finishing operation to succeed. The finishing operation uses a floating reamer: a reaming head with carbide or PCD cutting edges that is not rigidly fixed to the pulling rod but is free to float (self-centre) within the bore. The floating reamer is attached to a pulling rod that is inserted through the full length of the rough bore and connected to a pulling head at the opposite end. The pulling rod pulls the reamer through the bore at a feed rate of 50-100 mm/min while the shaft rotates at 20-50 rpm. Cutting fluid is pumped through the annulus between the pulling rod and the bore wall to lubricate the reamer and flush the chips. The floating reamer self-centres within the bore by following the existing bore path rather than forcing a new path. This means it corrects for deviation by following the natural curve of the bore, which results in a smooth, continuous bore path rather than a perfectly straight but misaligned bore. The technique achieves a straightness of 0.02-0.05 mm/m and a surface finish of Ra 0.8-1.6 microns. After floating reaming, the bore is typically finished by skiving and roller burnishing or by roller burnishing alone, which achieves the final surface finish of Ra 0.2-0.4 microns and the required compressive residual stress. The reverse pulling technique is particularly advantageous for shafts over 10 m in length, where push-reaming (pushing the reamer from one end) would cause the reamer to deviate due to column buckling of the reamer bar.

How is positional accuracy maintained during in-situ drilling? ​

Positional accuracy during in-situ stern tube drilling is maintained through a systematic process of reference establishment, tool alignment, and verification. The first step is establishing a reference point on the shaft centre line, typically using a laser alignment system. A laser transmitter is mounted on the shaft centre line at one end, and a target receiver is mounted at the opposite end. The laser beam defines the shaft centre line, and all drilling positions are referenced to this line. The second step is positioning the magnetic drill rig: the rig is clamped to the shaft flange face or to a mounting frame that is bolted to the stern tube face. The mounting frame must be rigid enough to support the drill rig without deflection under the drilling load (typically 1-3 kN axial force for a 20 mm drill). The drill rig position is adjusted using the laser reference: the drill axis must intersect the shaft centre line at the correct angular position on the pitch circle diameter (PCD). The alignment is verified by measuring the distance from the drill axis to the shaft centre line using a digital calliper. The third step is verifying the drill axis alignment: a digital inclinometer is placed on the drill rig mounting plate (or on the drill body), and the inclination is measured in two planes (vertical and horizontal). The axis must be within +/- 0.5 degrees of the radial direction (pointing toward the shaft centre line). The fourth step is drilling pilot holes: before drilling the full 1030 mm depth, a short pilot hole (10-20 mm deep) is drilled and the position is verified by inserting a pin and measuring the position relative to the PCD reference marks. If the pilot hole position is out of tolerance, the drill rig is repositioned and a new pilot hole is drilled adjacent to the first one. Once the pilot holes for all positions are verified, the full-depth drilling proceeds. During drilling, the operator monitors the drill rig for any movement or vibration that could cause the drill to deviate from the intended path. The completed holes are inspected by: positional measurement (the edge distance from the hole to the PCD reference mark is measured at the entry and exit of each hole), alignment verification (a pin of the same diameter as the hole is inserted, and the pin angle is measured with the digital inclinometer), and borescope inspection (the internal condition of the hole is inspected for any surface defects or debris).

What are the classification society requirements for propeller shaft drilling? ​

The classification society requirements for propeller shaft deep hole drilling are specified in the rules of each society (Lloyd's Register, DNV, ClassNK, ABS, AMSA, and others). While the specific requirements differ slightly between societies, the general requirements are: (1) Material traceability — the shaft forging must be supplied with a mill certificate showing chemical composition, mechanical properties, and heat treatment. The material must be manufactured to an approved standard (e.g., LR Rules Chapter 4, Section 4 for Lloyd's). (2) Non-destructive testing before drilling — the forging must be ultrasonically tested to verify internal soundness, with acceptance criteria typically allowing no indications larger than a 3 mm diameter flat-bottom hole equivalent. (3) Dimensional tolerances after boring — the bore diameter tolerance is typically H8 (plus 0.063 mm for a 150 mm bore), the bore concentricity to the shaft outside diameter must be within 0.1 mm TIR, the wall thickness variation must not exceed 5 percent of the nominal wall thickness, and the bore surface finish must be Ra less than 0.8 microns (for LR and DNV) or Ra less than 1.0 microns (for ClassNK). (4) Non-destructive testing after boring — the bore surface must be inspected by magnetic particle inspection (MPI) or eddy current testing at 100 percent coverage to detect surface cracks or seams. The acceptance criterion is no linear indications longer than 1 mm and no rounded indications larger than 3 mm. (5) Hydrostatic testing — the finished shaft must be hydrostatically tested to 1.25-1.5 times the maximum working pressure (depending on the society) for 30 minutes with no leakage or permanent deformation. (6) Surveyor witness — the classification society surveyor must witness the final dimensional inspection, the NDT of the bore, and the hydrostatic test. Some societies (LR, ABS) also require witness of the rough boring operation. The surveyor reviews the inspection records and, if satisfied, stamps the shaft with the society's mark and issues a certificate of approval. For in-situ retrofit drilling, the requirements are less stringent but still require surveyor approval of the repair procedure and witness of the final positional verification.

How does deep drilling affect shaft fatigue life? ​

Deep drilling of a propeller shaft to create a hollow bore reduces the shaft's torsional stiffness (proportional to the polar moment of inertia, which decreases with the fourth power of the bore diameter) but does not necessarily reduce the fatigue life if the bore is properly finished. In fact, a properly finished hollow shaft can have a longer fatigue life than an equally sized solid shaft when both are subjected to the same torsional loading, because the material at the bore surface (where the maximum shear stress occurs) is in compression rather than tension. The fatigue life of a drilled shaft is determined by: (1) The surface finish of the bore — a rough bore (Ra > 3.2 microns) has micro-notches that act as stress concentrators and reduce the fatigue strength by up to 40 percent compared to a smooth bore (Ra < 0.4 microns). (2) The residual stress state at the bore surface — a compressive residual stress of -400 to -600 MPa from skiving and roller burnishing can increase the fatigue strength by up to 80 percent compared to a bore with tensile or neutral residual stress. (3) The presence of stress concentration features — cross-drilled holes, keyways, and diameter changes create local stress concentrations that reduce the fatigue strength by a factor of 2-5 compared to a plain bore. (4) The bore concentricity — a bore that is not concentric with the shaft outside diameter creates a bending moment during rotation (due to the imbalance of the offset mass), adding cyclic bending stress to the torsional stress. The classification society rules require that the bore concentricity be within 0.1 mm TIR to limit this bending stress to an acceptable level. For a typical marine propeller shaft operating at 100-200 rpm with engine torque fluctuations of 10 percent of the mean torque, the fatigue life of a properly finished hollow shaft with skive and roller burnished bore exceeds 20 years of continuous operation. A shaft with a rough BTA finish (Ra > 3.2 microns) under the same loading would have a fatigue life of 5-10 years, requiring more frequent inspection and earlier replacement.

What coolant is used for marine shaft drilling? ​

The coolant used for marine propeller shaft drilling depends on the drilling application and the material being drilled. For new-build hollow shaft manufacturing in a factory, the preferred coolant is sulphurised mineral oil (viscosity 10-30 cSt at 40 C), which provides the best combination of lubrication, cooling, and chip evacuation for the high-speed steel and carbide tooling used in BTA drilling of forged steel shafts. The oil must contain extreme-pressure (EP) additives, typically sulphur and phosphorus compounds, that react with the steel surface at the cutting temperature to form a lubricating layer that reduces friction and prevents tool wear. The coolant pressure for BTA drilling of large shafts (80-300 mm diameter) is 20-50 bar, and the flow rate is 200-1000 L/min depending on the bore diameter. The coolant is filtered to 30-50 microns and temperature-controlled to +/- 2 C to maintain consistent viscosity. For in-situ retrofit drilling in the confined space of the stern tube tunnel, the coolant must be chosen with additional considerations: the coolant must be non-flammable (because the stern tube tunnel may contain residual fuel oil or lubricating oil vapour), must not produce toxic fumes in the confined space, and must be compatible with the stern tube bearing oil system (in case of coolant leakage into the bearing oil). The recommended coolant for in-situ drilling is a high-performance water-miscible synthetic emulsion at 5-8 percent concentration. The emulsion provides adequate lubrication for drilling cast iron and cast steel (the typical bearing boss materials), is non-flammable, and is compatible with most mineral oil-based bearing lubricants (the small amount of emulsion that may enter the bearing oil system will evaporate or separate without causing significant contamination). The coolant pressure for in-situ drilling is 10-20 bar, delivered by a portable coolant pump unit connected to the drill rig by high-pressure hoses. The coolant flow rate is 10-30 L/min for a 20-32 mm drill. After drilling is complete, the coolant is recovered (by vacuum or gravity drainage) and disposed of according to local environmental regulations. The coolant must not be discharged into the bilge or overboard because it may contain metal chips and is subject to environmental discharge regulations (MARPOL Annex I for oil-contaminated water).

The information provided in this article is for general informational purposes only. Data and recommendations are based on published research and industry experience as of 2026.