ONDECK
Academy
OOW Oral Prep
Module 13 — OOW 3000 GT Oral Preparation
Manoeuvring
Propeller effects · Helm orders · Wind & current · Squat & interaction · Anchoring · Berthing · Mooring lines
Propeller Effects — Single Screw
Understanding propeller effects is essential for predicting how the vessel will behave — especially at low speed when rudder effectiveness is reduced.
| Effect | Right-hand propeller ahead | Right-hand propeller astern |
|---|---|---|
| Paddlewheel effect (transverse thrust) | Stern swings to starboard (bow to port) | Stern swings to port (bow to starboard) — stronger astern than ahead |
| Helical discharge current (wash) | Water discharged aft and to port — adds to paddlewheel effect | Water discharged forward — strikes starboard side of rudder, pushing stern to port |
| Rudder effect | Effective when vessel has headway | Very limited astern — rudder in turbulent backwash |
Right-hand single screw vessel going astern: The stern will kick to port regardless of helm position. Use this to your advantage when berthing — swing the stern into the berth by going astern with helm to starboard.
Twin Screw Handling
Advantages
- Can turn in own length — ahead on one screw, astern on the other
- Transverse thrust effects largely cancel out — predictable astern movement
- Better manoeuvring at low speed
- Redundancy — can manoeuvre on one engine if one fails
Turning short round
- Port screw ahead, starboard screw astern — vessel turns to starboard
- Starboard screw ahead, port screw astern — vessel turns to port
- Rudder amidships for clean rotation
- Use thrusters if fitted to assist the turn
Effects of Wind
| Situation | Effect |
|---|---|
| Wind on the bow | Slows the vessel — increases stopping distance. Bow tends to pay off downwind as speed decreases |
| Wind on the quarter / astern | Vessel tends to yaw — more rudder required to maintain course. Following sea can cause broaching |
| Wind on the beam | Vessel makes leeway — set to leeward. High freeboard (superyachts) increases windage significantly |
| Berthing with wind off the berth | Most difficult — vessel blown away from the berth. Use a bow line first, let the wind close the stern, or use a spring |
| Berthing with wind onto the berth | Easier — wind helps close the vessel. Control approach speed carefully to avoid coming in too fast |
Squat and Interaction
Squat
- In shallow water, the flow of water under and around the vessel is restricted — creating a low-pressure zone that draws the vessel down
- Vessel sinks bodily and may trim by the bow or stern
- Effect increases with the square of speed — double the speed, four times the squat
- Can significantly reduce UKC without warning — reduce speed well before entering shallow water
- Single screw vessels typically trim by the stern; twin screw by the bow
Interaction
- When two vessels pass in close proximity, the bow wave and stern suction of each vessel affects the other
- In narrow channels: vessels attract at the bow and repel at the stern — creates a tendency to sheer toward each other
- Passing alongside a moored vessel: suction effect can pull the moored vessel into the passing vessel
- Overtaking: the overtaking vessel's stern suction draws the slower vessel toward it
- Reduce speed when passing in confined waters — reduces interaction forces
Anchoring Procedure
| Step | Action |
|---|---|
| 1. Anchor plan | Position selected — shelter, depth, holding ground, swinging room, clear of traffic. Master approves. Anchor party briefed |
| 2. Approach | Approach into wind or current (whichever is dominant). Slow to bare steerage way before letting go |
| 3. Let go | At the chosen position — anchor let go, cable veered out under control as vessel drifts back |
| 4. Scope | 3× the depth at high water minimum for good holding. More in heavy weather. Deckhand counts shackles and reports to bridge (each shackle = 27.5m) |
| 5. Confirm holding | Take cross-bearings immediately. Monitor for drag. Vessel should face into wind/current and lie steady |
| 6. Exhibit lights/shapes | Anchor ball by day, anchor light by night |
Mooring Lines — Purpose and Names
| Line | Position | Purpose |
|---|---|---|
| Head line | Forward, leads ahead from bow | Holds bow to the berth |
| Forward breast | Forward, leads at right angles to the vessel | Holds bow close to the berth — no fore/aft control |
| Forward spring | Forward, leads aft from bow to a shore point aft | Prevents vessel moving forward along the berth |
| After spring | Aft, leads forward from stern to a shore point forward | Prevents vessel moving aft along the berth |
| After breast | Aft, leads at right angles | Holds stern close to the berth |
| Stern line | Aft, leads astern from stern | Holds stern to the berth |
SPRINGS ARE THE WORKING LINES: In a marina or alongside berth, the springs take the load and prevent the vessel surging fore and aft. Breastlines keep the vessel close to the berth but carry no fore/aft load. Double up springs and headlines in exposed berths or deteriorating conditions.
Turning Short Round — The Single-Screw Sequence
Channel one and a half lengths wide, single right-handed screw, no thruster. Turn to STARBOARD — the propeller decides, because every phase then adds rotation the same way:
| Phase | Action | What does the work |
|---|---|---|
| 1 | Kick AHEAD, full starboard helm — short burst | Prop wash over the rudder steps the stern to port, bow to starboard, without gathering real headway |
| 2 | Engine ASTERN, helm amidships | Transverse thrust walks the stern to port — the bow keeps swinging starboard — while taking off the way you made |
| 3 | Repeat, alternating | Each ahead kick with helm, each astern kick with the paddle-wheel — the same swing, fed from both directions |
Watch the pivot point migrate: on the ahead kicks it sits forward, so the STERN sweeps — mind the channel edge behind you; with sternway it moves aft and the BOW becomes the wide end. Use wind and tide if they offer (back into the wind's help), and sound Rule 34 signals throughout — three short every time the engine goes astern in sight of traffic. Turning to port would fight your own propeller on every astern kick.
The Thruster's Honest Envelope
| Limit | Reality |
|---|---|
| Speed | Effective at rest to ~2–3 knots — above that the tunnel flow collapses and the thruster is noise |
| Leverage | Greatest with sternway or stopped (pivot point aft = long arm); weak making way ahead (pivot forward, short arm) |
| Wind | Do the arithmetic once for your vessel: windage area × 25 knots against thruster tonnes-force — most yacht thrusters lose |
| Endurance | Thermal cutouts arrive mid-manoeuvre when you have leaned on it; electrical load may shed other systems |
Plan every berthing to work with helm, engines and lines alone; the thruster polishes the last metres. A plan that REQUIRES the thruster fails the day the breaker trips — and the examiner will ask what you do when it does, mid-manoeuvre.
Berthing in Wind — Two Plans, Not One Technique
Onshore wind
The wind berths you for free; the risk is arriving too fast and being pinned. Approach flat and slow, stop her a beam-width off with everything controlled, and let the wind lay her on — fenders early, lines fast smartly. Plan the DEPARTURE before arriving: spring off stern-first, thruster, or a tug when the forecast builds.
Offshore wind
The high-windage bow blows off the instant way comes off. Come in steeper, carrying way; first line ashore FAST — the fore spring — then power against it to walk her alongside. And know your honest wind limit: above it, the seamanlike call is a tug or a different berth, made out loud before the paint decides.
Why the spring first: it leads along the quay, so slow ahead against it converts your own engine into a mooring winch pinning her alongside while the other lines go out. A square breast line does nothing until tight — and heaving one against 20 knots of beam wind is a parted line and a snap-back zone.
Crash Stop — What Full Astern Actually Does
From full ahead, a crash astern is neither quick nor straight: the vessel carries her way for multiple lengths while the astern power builds, transverse thrust swings the stern (to port for a right-handed screw) so she finishes stopped ACROSS her original track, and steerage way is gone long before the way is. That is why the collision-avoidance answer is usually a bold turn, not the telegraph: a turn keeps steerage and moves the CPA immediately. The crash stop is for when there is no water to turn in — and its distances and behaviour for YOUR vessel are on the manoeuvring data card the examiner expects you to have read.
Additional Question
Twin screw, no thruster: hold her stationary and rotate 180° in her own length.
Engines opposed — starboard ahead, port astern to turn to port (the ahead engine on the outside of the turn) — balanced so ahead and astern thrust cancel and she rotates without making way. Rudders amidships or assisting; small balance corrections with the throttles as wind takes the bow. Transverse thrust largely cancels on a twin-screw vessel, so the direction of turn is my choice — I turn the way that puts the wind's help behind the swing. And I know the failure case: one engine drops out and I am instantly a single-screw vessel, possibly with the wrong-handed prop for the turn I am in — which is why the swing is planned with sea room on the exit side.
Oral Exam Practice Questions
Question 1
What is the paddlewheel effect and how does it affect a right-hand single screw vessel going astern?
The paddlewheel effect is the transverse thrust produced by the propeller blades as they rotate — the blades at the bottom of the arc are in denser water than the top and produce more thrust, causing the stern to swing. A right-hand single screw vessel going astern will have the stern kick to port — this is stronger than the ahead effect because the discharge current also strikes the starboard side of the rudder pushing the stern further to port. This is predictable and can be used — for example, placing the starboard side alongside using astern power will swing the stern into the berth.
Question 2
What is squat and how do you manage it?
Squat is the dynamic sinkage of a vessel in shallow water — as speed increases in confined water, the flow of water under the hull is restricted, creating a low-pressure zone that draws the vessel down and may trim her by the bow or stern. The effect increases with the square of speed — twice the speed produces four times the squat. It can significantly reduce UKC without any visible warning. Manage it by reducing speed well before entering shallow water and by calculating the expected squat at the planned speed and incorporating this into the UKC calculation for the passage plan.