Stern Tube Seal Inspection Singapore — Underwater Survey of Stern Tube Seals, Rope Guard & Boss

What Is a Stern Tube Seal Inspection?

A stern tube seal inspection is an underwater, afloat examination of the after seal assembly and the aft running gear around it — the point where the propeller shaft leaves the hull. Where the shaft exits, it passes through the stern tube, and the aft seal closes the annular gap between the rotating shaft and the fixed tube. Our divers and ROV inspect that seal, the rope guard fitted just forward of the propeller, the propeller boss and fairwater cone, and the shaft-to-stern-tube interface, looking for oil leakage, water ingress and rope or net fouling.

This work forms part of the underwater inspection scope owners commission when they want current evidence of aft-gear condition without lifting the vessel. Because the seal and its housing sit in a congested, low-visibility zone right at the propeller, they reward a close, deliberate look rather than a quick swim-past. A diver can position at the seal face and hold station while a video is recorded; an ROV can do the same where diving is restricted or the water is deep.

The output is a documented, video-backed record of seal and aft-gear condition that fits the format a class surveyor or superintendent expects — the same reporting used for an in-water survey (IWS) or a UWILD survey. That lets the seal be assessed alongside the propeller, rudder and hull in one consolidated picture.

Why the Stern Tube Seal Matters

The aft stern tube seal does two jobs at once: it keeps the stern-tube lubricant in and it keeps seawater out. On a traditional oil-lubricated stern tube, the seal separates the oil bath around the white-metal bearing from the sea. If the aft seal wears or is damaged, it can fail in either direction, and both failure modes carry consequences that build quietly until they force the ship off-hire.

An outward failure lets stern-tube oil escape into the sea. Beyond the running cost of continually topping up, that is a pollution matter — oil discharge from a leaking stern tube is exactly the kind of operational release that MARPOL and port-state control take seriously. It is one reason many operators have moved to water-lubricated or biodegradable-oil (bio-oil) stern-tube systems, and why even those operators still want the seal condition monitored underwater, because a compromised seal on any system tells the same story about the running gear.

An inward failure is quieter but no less serious. Seawater and the fine abrasive it carries reach the stern-tube bearing, contaminate the lubricant and accelerate bearing wear. Left unaddressed, that can score the shaft liner and lead to bearing failure — a repair that means an emergency drydocking. Rope and fishing net wrapped around the boss compounds all of this: as the shaft turns, entanglement drags against the seal lip and rope guard, cutting into components that are only ever meant to see clean seawater. Spotting a weeping seal, a distressed rope guard or the early stages of fouling underwater lets the owner plan a repair on their terms instead of reacting to a failure at sea.

What a Stern Tube Seal Inspection Assesses

The survey is organised around the components that make up the aft running gear. Each is inspected at close range, described, and captured on video for the class surveyor and superintendent.

How the Underwater Stern Tube Seal Survey Is Done

The inspection is carried out afloat, with the vessel at anchor or alongside in Singapore anchorages and terminals. A diver — or an ROV where diving is restricted — approaches the aft running gear and works methodically from the rope guard inward to the seal face and boss.

Close visual inspection is the core of the work. The inspector holds station at each component and records continuous, well-lit video, calling out condition against the survey items. Where the seal is masked by light marine growth or a wisp of rope, minor fouling is cleared by hand so the seal face and rope guard can actually be seen and assessed — clearance and inspection go together in the same attendance.

Leak assessment is done by observation at the seal. On an oil-lubricated tube a weeping aft seal typically shows as a fine oil sheen or a milky oil-and-water emulsion rising slowly from the seal, often with staining on the boss below. That behaviour is recorded on video and noted in the report so the owner and surveyor can judge whether the seal is holding, weeping or failing.

• Afloat, no drydock — the seal, rope guard and boss are surveyed in the water, so there is no need to lift the vessel just to check condition.
• Diver or ROV — full-capability attendance; we deploy divers for hands-on clearance and an ROV where access, depth or safety calls for it.
• Fouling cleared as we go — minor rope and growth are removed from the seal area so the components can be properly inspected, with before-and-after records.
• Video evidence — every finding is captured on video and still images, geo-referenced to the vessel and dated for the survey file.

Because the diver or ROV is already at the aft gear, the seal survey is normally run as one job with propeller polishing and a rudder inspection, and it slots naturally into a wider underwater hull inspection. One mobilisation, one consolidated report.

Standards and Survey Context

Stern-tube and shaft arrangements sit within the shafting and propulsion surveys defined by the classification societies — ABS, DNV, Lloyd's Register, Bureau Veritas and ClassNK among them — which set the periodicity and scope of tailshaft and stern-tube examinations. An underwater seal inspection gives the owner and the attending surveyor current condition evidence to feed into those surveys and into decisions on whether the seal can run to the next scheduled opening.

The reporting follows the in-water survey and UWILD conventions the societies recognise, so the seal, boss, rope guard and anode findings drop straight into the survey record. On the pollution side, oil leakage from a stern tube is read against the MARPOL framework on operational oil discharge, which is why the leak assessment is treated as a specific, recorded item rather than a passing note. Our diving is conducted to recognised commercial diving practice, consistent with IMCA guidance.

We describe these standards to show the context our reporting is built for. We do not claim any specific class approval, certificate or endorsement on this page — the survey delivers the underwater evidence, and the class surveyor and the owner's superintendent make the formal assessment.

What We Inspect

Every stern tube seal attendance covers the full aft-gear picture, not just the seal in isolation.

Why Choose SG Drone Inspections

We are a full-capability underwater inspection provider in Singapore, running divers and ROV so the aft running gear can be surveyed the right way for each vessel and situation.

Frequently Asked Questions