Propeller and rudder together: the evolutive effects
See it in 3D
Try a short engine burst, then watch the stern, bow and pivot point. Forces are simplified for this 11 m teaching yacht; practise on your own boat in safe open water.
Propeller–rudder effects
Select a cell to play a six-second trial in the sandbox.
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| Helm | Engine ahead | Engine astern | ||||
|---|---|---|---|---|---|---|
| At rest | Headway | Sternway | At rest | Headway | Sternway | |
| Amidships | ||||||
| Port | ||||||
| Starboard | ||||||
| Rudder effectiveness | Rudder effectiveness: 2 of 3 | Rudder effectiveness: 3 of 3 | Rudder effectiveness: 2 of 3 | Rudder effectiveness: 1 of 3 | Rudder effectiveness: 1 of 3 | Rudder effectiveness: 1 of 3 |
Engine astern · At rest · helm Amidships
Right-handed propeller, seen from above, bow up
Thin arrow = small swing; thick arrow = strong swing; ? = uncertain; red dot = pivot point.
Table after Prof. Ramon Curto
1The lesson in plain words
What the boat does depends on three things at once: engine ahead or astern, the boat's way, and the helm.
Three questions
Before any manoeuvre in a single-screw boat, ask: is the engine going ahead or astern? Is the boat at rest, making headway (moving forward) or sternway (moving backwards)? Where is the helm: amidships, to port or to starboard? The answers tell you where the bow and the stern will go. This lesson assumes a right-handed propeller.
Engine ahead
The propeller washes water over the rudder, so the rudder works well, best with headway. With the helm amidships at rest, walk pushes the stern a little to starboard and the bow falls slightly to port. Helm to port swings the bow strongly to port, because rudder and walk work together. Helm to starboard swings the bow to starboard, but less from rest, because walk works against it.
Engine astern
The wash goes forward, under the stern, so from rest the rudder does almost nothing at first and walk pushes the stern to port. While you still have headway, the rudder keeps working as going ahead; below about 2 knots the walk takes over. Once sternway builds, water from astern and the propeller's suction reach the rudder: helm to port adds to the walk, so the stern goes decidedly to port; helm to starboard opposes it, and the result is weak and unclear.
Pivot point
While she has headway, the turning point is usually forward and the stern swings wide. As she gathers sternway, it moves aft and the bow swings wide. Motion and forces determine it, not the engine order. Check clearance at both ends.
Use the table
It shows typical tendencies for a small single-screw yacht in calm water a few seconds after the order. Astern rows are initially walk-dominated; the rudder acts as sternway builds. Pivot marks describe established motion. Strengths are teaching estimates: predict, act, then check the boat.
2Key data
- Rudder effect, engine ahead
- Good at rest, best with headway
- Rudder effect, engine astern
- Nil from rest; weakened steering as sternway builds
- Initially astern from rest (RH)
- Walk dominates: stern to port, bow to starboard
- Ahead at rest, helm amidships
- Stern slightly to starboard
- Pivot point
- Usually forward with headway, aft with sternway
3Key points
- 1
Three inputs decide the result: engine, way and helm.
- 2
Engine ahead: the rudder leads, strongest with headway.
- 3
Engine astern from rest: walk initially leads, stern to port (RH).
- 4
With sternway, port helm adds to walk; starboard helm opposes and may overcome it.
- 5
Predict, act, then check what the boat really does.
4Frequent mistakes
Wrong: Astern, putting the helm to starboard will swing the stern to starboard straight away.
Right: From rest, rudder force is nil and walk leads. As sternway builds, starboard helm opposes walk and may eventually overcome it.
Wrong: With the engine ahead and the helm amidships, the boat always starts dead straight.
Right: From rest, walk nudges the stern to starboard. With headway she runs nearly straight; a little compensating helm may be needed.
Wrong: The rudder works the same ahead and astern.
Right: Ahead wash helps it work. Astern from rest, force is nil; with sternway it works, weakened. Retained headway can still provide flow.
5Practice case & exam keys
Your yacht has a right-handed propeller. You are at rest and need the stern to go to port to reach a berth astern.
Show the answer
Select astern: walk initially pushes the stern to port. Port helm adds only as sternway develops. Use short bursts and watch both ends; the bow swings to starboard. Do not expect immediate rudder force from rest.
Exam keys
- Engine ahead: rudder effect good at rest, best with headway; walk ahead puts the stern slightly to starboard.
- Astern from rest (RH): walk initially takes stern to port. With sternway, port helm adds; starboard helm opposes and may overcome it.
- Pivot usually forward with headway, moving aft as sternway builds; engine order alone does not decide it.
Finish this unit
When you've read it, answer three questions. Get all three right to finish the unit; opening the page alone doesn't count.
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