ONDECK
Academy
OOW Oral Prep
Module 21 — OOW 3000 GT Oral Preparation
Bridge Equipment, VTS, Damage Stability & Engineering Watch
Echo sounder · Course recorder · NAVTEX · VTS · Damage stability · Flooding trim · Engineering watch · Advanced anchoring
Bridge Equipment — Echo Sounder
| Aspect | Detail |
|---|---|
| Principle | Transducer transmits an acoustic pulse downward; the echo is received after bouncing off the seabed. Time × speed of sound in water (approx 1500 m/s) ÷ 2 = depth |
| Transducer position | Located on the hull — typically at the bilge keel or keel. The displayed depth may be depth below the transducer (not below the keel). Know your vessel's transducer position: add keel-to-transducer distance to readings to get true depth below keel |
| Speed of sound correction | Speed of sound varies with water temperature, salinity, and pressure. Echo sounders are calibrated for a standard value. In very shallow fresh water vs deep salt water the error is typically small — but note the limitation |
| False bottom | A second trace below the true bottom — caused by the echo bouncing off a thermal layer (thermocline) or a dense fish shoal. Can also appear as a result of internal reflections. The OOW must identify the correct bottom trace |
| Limitations | Cannot detect objects between the vessel and the bottom (wrecks, rocks) — it shows depth to the first strong reflector, usually the bottom. Minimal range at high speed in a seaway due to air bubbles under the hull. Requires UKC calculation to be meaningful |
| UKC calculation | Displayed depth − vessel draft = UKC. Example: echo sounder reads 18m, draft 4.5m = UKC 13.5m. Apply any tidal height correction if using chart datum |
Bridge Equipment — Course Recorder
The course recorder (heading recorder) produces a continuous paper trace of the vessel's heading over time. Required under SOLAS V/18 for vessels over 150 GT on international voyages.
- The trace shows every helm movement — a straight line means steady course; zigzag shows alteration of course, steering corrections, or evasive action
- In the event of an incident (collision, allision, grounding), the course recorder trace is primary evidence — preserve the paper roll and do not wind it on. Inform the Master. The trace will be requested by MAIB investigators and insurers
- The recorder must be checked at the start of every watch: confirm it is running, the paper roll has sufficient paper, the pen is writing (ink level), and the time is correct
- Log any interruptions to the recorder in the deck log — a gap in the trace will need to be explained
Bridge Equipment — NAVTEX Receiver
NAVTEX (Navigational Telex) is the primary GMDSS MSI (Maritime Safety Information) broadcast system for coastal waters within 200–400 nm. Operates on 518 kHz (international) and 490 kHz (national language broadcasts).
| Message Type Character | Content | Selectable (can be rejected)? |
|---|---|---|
| A | Navigational warnings | No — must be accepted |
| B | Meteorological warnings (gale warnings) | No — must be accepted |
| D | SAR information, piracy warnings | No — must be accepted |
| G | NAVTEX system test transmissions | Yes |
| H | Loran C / positioning system messages | Yes |
| J | Satellite system (GNSS) messages | Yes |
| L | Navigational warnings (additional) | No — must be accepted |
| U–Y (even) | Meteorological forecasts | Yes |
OOW duty with NAVTEX: Check the printer at the start of every watch. All NAVTEX messages must be read and actioned if relevant to the passage. Gale warnings require informing the Master. Navigational warnings must be plotted on the chart or electronic chart if they affect the vessel's route. Never reject message types A, B, or D — these cannot be filtered out.
Vessel Traffic Services (VTS)
VTS is a service implemented by a competent authority (usually a port or coastal state) to improve the safety and efficiency of vessel traffic and protect the environment. Governed by IMO Resolution A.857(20). VTS is mandatory in many ports and straits.
| VTS Service Type | Function |
|---|---|
| Information Service | Provides vessels with navigational information — traffic, hazards, weather, depths. One-way broadcast or available on request. The vessel acts on the information but makes its own decisions |
| Traffic Organising Service | Sequences vessel movements — provides instructions on when to enter, leave, or transit a port or fairway. Compliance is mandatory when a vessel enters a mandatory VTS area |
| Navigational Assistance Service | Active assistance to a vessel in difficulty — e.g. the vessel has radar failure and requests VTS guidance through a fairway |
OOW Duties in a VTS Area
- Contact the VTS authority on the designated VHF channel as instructed in the port guide / ALRS Vol 6 — typically before or on entering the VTS zone
- Report: vessel name, call sign, MMSI, position, course, speed, destination, ETA, dangerous cargo if any
- Maintain a continuous listening watch on the VTS working channel throughout transit
- Acknowledge and comply with all VTS instructions — in mandatory VTS, non-compliance is a breach of port regulations
- Report any change in ETA of more than 15 minutes, any change in vessel status (e.g. draught, propulsion issues)
- When departing: obtain VTS clearance before getting underway in mandatory areas
VTS vs. Pilot
- VTS provides information and may give instructions — it does not assume responsibility for the safe navigation of the vessel
- A VTS instruction does not override the Master's responsibility under SOLAS and the COLREGS
- The Master and OOW retain full responsibility at all times — VTS is an additional resource, not an authority that takes command
- A pilot on board takes over navigation — but the Master still retains responsibility and the pilot's advice may be declined (though this has commercial and legal consequences)
- ALRS Vol 6 (NP286) contains VTS area descriptions, frequencies, and reporting requirements for ports worldwide
Damage Stability — Flooding a Compartment
When a compartment is flooded, the vessel's stability, trim, and freeboard change immediately. The OOW must understand the effects and the emergency response.
| Effect | Explanation |
|---|---|
| Loss of buoyancy | The flooded space no longer contributes to the buoyant volume of the hull. The vessel sinks deeper. Draught increases. Freeboard decreases |
| Change in trim | Flooding one compartment shifts the centre of buoyancy. If the flooded compartment is forward, the vessel trims by the head (bow deeper). Aft compartment — trim by the stern. Amidships flooding — deeper draught with no trim change |
| Free surface effect | The flooded compartment initially has liquid with a free surface — this reduces GM. As the compartment fills, the free surface disappears but the added weight and reduced buoyancy lower the vessel deeper |
| Heel (list) | Flooding an off-centre compartment (one side of the vessel) causes a list toward the flooded side as the vessel's centre of gravity shifts. Counter-flooding the opposite side may be necessary |
| Residual buoyancy | The remaining watertight volume must be sufficient to keep the vessel afloat at a safe trim and with positive GM. The damage stability booklet calculates the vessel's ability to survive specified damage scenarios |
Counter-flooding: If the vessel develops a dangerous list after flooding, counter-flooding the opposite side can reduce the list — but this adds weight and reduces freeboard. Do not counter-flood without the Master's authority and a clear understanding of the stability implications. An unchecked list can lead to capsize even when the vessel has survived the initial flooding.
Damage stability booklet: Every SOLAS vessel must have a damage stability booklet on board showing the vessel's stability after defined damage scenarios. The OOW must know where it is and how to use it — the examiner will ask. In a flooding emergency, consulting the booklet for the damaged compartment tells you whether the vessel will survive and what the residual GM is.
| Immediate flooding emergency — OOW actions |
|---|
| Call the Master and engineer immediately. Sound the general alarm |
| Determine the source and rate of flooding — locate the compartment, attempt to identify the breach |
| Activate bilge pumps and emergency pumping — maximise dewatering rate immediately |
| Attempt to plug or reduce the ingress — collision mat, damage control box plugs, wooden wedges |
| Close all watertight doors between the flooded compartment and the rest of the vessel |
| Monitor trim and list — use the inclinometer; report changes to the Master |
| Contact MRCC — transmit a Pan Pan or Mayday as appropriate; give position, nature of flooding, number of persons on board |
| Muster all persons — prepare to abandon ship if situation deteriorates |
Engineering Watch — OOW Principles
The MCA OOW oral tests basic knowledge of the engineering watch. On a yacht, the OOW frequently works in a combined navigating/engineering watch environment. The STCW Code Section A-VIII/2 applies to both the navigational and engineering watches.
| Aspect | The OOW must know |
|---|---|
| UMS (Unattended Machinery Space) | Most modern vessels operate UMS — engine room unmanned for extended periods with automatic alarms activating on the bridge. The OOW receives and responds to all engineering alarms. A high-high bilge alarm or main engine alarm is responded to immediately — call the engineer |
| Bridge alarm system | In UMS operation, the bridge alarm panel shows the status of all monitored engineering systems. The OOW must know which alarms require immediate action (propulsion, steering, flooding) and which are informational (temperature, fuel level) |
| Telegraph / engine room telegraph | The order/answer telegraph confirms engine orders have been received and executed. All telegraph movements must be logged. If the telegraph is not answered, the OOW contacts the engine room directly by phone |
| Communication with the engine room | Before manoeuvring: "Engine room — bridge, standby for manoeuvring" — engines move to immediate readiness. After manoeuvring: "Bridge — engine room, finished with engines" — engines stand down. Always confirm with engineer on passage planning changes |
| Fuel management | The OOW monitors fuel consumption during a watch. A significant increase in fuel burn at the same power setting may indicate hull fouling, weather, or engine problems. Report to engineer |
| Main engine failure on watch | If propulsion is lost: slow down on remaining engine if twin screw; display NUC lights and shapes; broadcast PAN PAN on Ch 16 giving position and nature of defect; call Master and engineer; use anchoring if risk of grounding; continue to monitor situation and report |
Advanced Anchoring — Fouled Anchor, Slipping, Breaking Out
| Situation | Procedure |
|---|---|
| Fouled anchor | The anchor is caught on an obstruction on the seabed (mooring chain, cable, debris). Symptoms: chain does not come in cleanly; cable hangs at odd angle. Actions: heave up slowly — do not force the windlass. Try motoring in a circle around the anchor position to free it. If still fouled, ease cable and drop back, then try a different approach angle. If all else fails: a diver may be needed to clear it. On smaller vessels, attaching a trip line buoy to the anchor crown when anchoring prevents this |
| Hanging off | The vessel needs to move urgently but the anchor is not yet weighed. Make the cable fast on the compressor at the required scope and attach a buoy to the bitter end — the cable can then be slipped and the anchor and cable recovered later. This is "hanging off and buoying the cable." Only used in genuine emergency — the anchor and cable may be difficult to recover and may be lost |
| Slipping the cable | Emergency — engine room fire, man overboard, collision risk. Knock off the shackle securing the cable at the bitter end and let the whole cable run out. The cable is lost unless a buoy was pre-attached. The Master orders this — not the OOW independently |
| Breaking out a set anchor | Anchor dug in firmly (set). Heave short — veer cable until short stay, then heave in slowly with engine ahead, directing the bow over the anchor using helm and engine to break the suction. In soft mud, long scope with engine power applied gradually works better than windlass force alone |
| Anchor types | Stockless (Hall, Spek) — standard; fits into hawse pipe; poor in hard sand or rock. CQR (plough) — yacht anchor; good holding in mud and sand; not suitable for large commercial vessels. Danforth (fluke) — excellent in mud and sand; stores flat. Bruce/Claw — omnidirectional; good in mixed bottoms. Examination-level: know stockless (what is on a superyacht) and why anchor type matters for holding ground |
Oral Exam Practice Questions
Question 1
You are transiting a busy port approach and the VTS broadcasts a traffic separation instruction. What do you do?
In a mandatory VTS area, VTS instructions must be complied with — this is a condition of entry to the port. I acknowledge the instruction on the designated VTS working channel — name, call sign, and confirmation: "VTS [port name], this is [vessel name], MMSI [number], we confirm your instruction, understood and complying." I then implement the instruction — whether that is a speed reduction, a course alteration, a hold position, or a sequence change. I inform the Master of the VTS instruction and any change to the passage plan it requires. I log the time, content of the instruction, and our acknowledgement in the deck log. The VTS instruction does not override my obligations under the COLREGS — if compliance with the instruction would create a risk of collision with another vessel, I must take the safe action, immediately inform VTS of what I am doing and why, and rectify the conflict once it is safe to do so. VTS does not take command of the vessel — it coordinates traffic. Responsibility for safe navigation remains entirely with the Master and OOW.
Question 2
Your vessel grounds and you believe one forward compartment is flooding. You are the OOW. What are your immediate actions?
Sound the general alarm and call the Master and all crew immediately. Assess the situation rapidly: determine which compartment is affected, estimate the rate of flooding, check the inclinometer for list and the bridge instruments for trim change. Activate all bilge pumps — the engineer must be told immediately to start emergency pumping and to identify the source of flooding. Close all watertight doors and watertight closures between the flooded compartment and the rest of the vessel. Transmit a Pan Pan on Ch 16 — give our position, that we are aground and taking on water, the number of persons on board, and our vessel's name. Contact MRCC and maintain communication. Check the damage stability booklet for the vessel's calculated residual stability in the event of flooding of this compartment — is the vessel expected to remain stable? Muster all persons at their emergency stations and prepare liferafts and LSA for rapid deployment if the situation deteriorates. Post a watch on the bilge level in the affected space and report changes to the Master every few minutes. Do not attempt to refloat the vessel until the flooding is assessed and controlled — attempting to free a grounded vessel that is flooding can make the situation significantly worse.
Question 3
How does the echo sounder work and what are its limitations?
The echo sounder transmits an acoustic pulse from a transducer on the hull. The pulse travels to the seabed, reflects, and returns to the transducer. The time taken for the round trip, divided by two and multiplied by the speed of sound in water (approximately 1500 m/s), gives the depth. The depth displayed is typically depth below the transducer — not depth below the keel. I must know the distance from the keel to the transducer on our vessel and subtract this from the displayed depth to get true UKC. Limitations: the echo sounder does not detect objects between the hull and the seabed — a submerged wreck or a rock pinnacle below the vessel but above the bottom will not appear. It can give a false bottom reading caused by a thermocline (a temperature layer with different acoustic properties) or a dense fish shoal. At high speed in a seaway, air bubbles entrapped under the hull can blank the transducer, giving no reading or erratic readings. The speed of sound in water varies with temperature, salinity and pressure — calibration is approximate. At depths over 200 metres the pulse repetition rate must be reduced to avoid the echo from one pulse overlapping with the transmission of the next.