Chapter 3 / The Propeller

Ch3. Propeller

-Ahead movement-Astern movement-Transverse thrust

Ch3. Pitch of the propeller

Ch3. Right handed propeller

Ch3. Ahead / Direct Transverse thrust

•Helical discharge from propeller creates a larger pressure on port side of rudder

•Slight upward flow from the hull into propeller puts more pressure onto the down sweeping propeller blades

•Speed of water into the propeller is eneven in velocity

Result: tendency to give a swing to port

Ch3. Ahead / Indirect transverse thrust

Ch3. Ahead / Indirect transverse thrust

Effect of propeller flow on the rudder: due to helical dischargeFrom propeller pressure of water more regular on left side ofRudder

Result: increase the swing to port when running ahead

Ch.3. Ahead / Skin friction effect

• Ship drags water along with it due to skin friction: reduction in flow effects a big portion of propeller disc.

• Variation of flow velocity changes the relative angle of incidence to the rotating blades and creates an inbalance of drag forces in upper and lower sections of propeller disc

Result: the ship turns to starboard

Ch3. Ahead / Transverse thrust

• Direct effect: helical flow tends to turn the ship to port

• Indirect effect: the upward flow on the propeller disc tends to turn the ship to port

• The variation of velocity into the propeller disc tends to turn the ship to starboard

• Resultant: the transverse thrust causes a gentle turn to Port

Ship AheadPropeller AheadRudder Amidships

Shiphandling: Single Screw Ships

Ch3. Astern / Transverse thrust

Direct Effect•Water enters propeller disc at uniform velocity and direction•Weak transverse force generated by difference of pressure on upper and lower propeller blades

ResultGentle turn to starboard

Ch3. Astern / Transverse thrust

Indirect effect • Helical flow of propeller wash strikes after body of hull with inward component on Ps and outward component on Sb: Result is a higher pressure on Sb pushes stern to Ps.• Reverse flow over rudder and rudder effect reversed but weaker

Ch3. Astern / Transverse thrust

Ship AsternPropeller AsternRudder Amidships

Ship follows the rudder:Ship will tend into the wind:Ship will tend to port very easilyShip does not tend to starboard easily

Shiphandling: Single Screw Ships

Conclusion:

•pronounced turn to Sb when engine is going astern•Similar effect with headway, sternway of vessel stopped

Ch3. Astern / Transverse thrust

Ship AheadPropeller AsternRudder Amidships

Shiphandling: Single Screw Ships

Crash Stop manœuvre:• In deep water, pronounced turn to Sb• In shallow water, trun less pronounced to the restriction of

transverse components of propeller flow due to small UKC

Ch3. Interaction between propeller and rudder Propellers / Rudders

• Primary means of controlling the stern

Thrust

Side Force

Rudder Force

Controllable Forces

Engine ahead:

Propeller flow strikes rudderand increases the ruddereffect.

Action of propeller flow on rudder more pronouncedwhen vessel is stopped or with sternway.

Ch3. Interaction between propeller and rudder

Engine astern and Rudder amidships: the vessel isSwinging to Starbard.

Ch3. Interaction between propeller and rudder

• Engine astern and Rudder to Port: reverse effect on the rudder and increased swing of vessel to starboard.• Effect more pronounced with vessel stopped or with sterway

Ch3. Interaction between propeller and rudder

• Engine astern and rudder to Sb: rudder effect opposes transverse thrust• Vessel may swing to Port (rudder action bigger) or keep a straight course or swing gently to Sb

Ch3. Interaction between propeller and rudder

Headway + engine astern + Sb. Rudder: • as long as the vessel keeps some headway: vessel turn to Sb due to rudder + propeller effects• when vessel gets strenway, it may turn to port if rudder effect greater than propeller effect.

Ch3. Interaction between propeller and rudder

Kick ahead manoeuver to regain control of a vessel with sternway:Rudder is put hard to port with engine ahead : turn to Sb due to effectof propeller astern is stopped.

Ch3. Rudder counter effect to control propeller effect

1. Rudder to Sb 2. Engine astern3. Put rudder amidships and gradually to Sb4. End with rudder hard to Sb.

Ch.3. Kick ahead manoeuver

To increase significantly the rate of turn of a vessel stopped or nearly stopped : short bursts of engine ahead to increase the rudder effect.

Ch3. Negociating a bend with kick ahead

1. Vessel approaches with reduced speed2. Hard to port3. Half or full ahead4. Rate of turn increases5. Short bursts on the engine to avoid increase of speed6. Reduce or stop the engine

Ch3. Half turn with right handed propeller

Pos 1: Rudder hard to Sb with engine on half/full ahead

Pos 2: Rudder hard to port with engine on half/full astern

Pos 3: Rudder hard to Sb with engine on half/full ahead

Pos 3 : Half turn is completed.

Remark : The wind may modify or even oppose this manœuvre.

Ch3. Half turn with right handed propeller

The previous manœuvre is only possible when the vessel startswith the first turn to Sb. Otherwise will the propeller effect opposethe rudder effect

Ch3. Half turn in heavy wind condition

Pos 1 : Engine half/full astern – the stern comes into the windPos 2 : Rudder hard to port and engine half/full aheadPos 3 : Half turn completed

Ch3. Twin propellers

Handling characteristics depends of several factors:• Rudder configuration• Effect of torque• Transverse thrust• Pivot point• Turning ability

Ch3. Twin propellers / Rudder configuration

Single rudder is situated on the center line between the two propellers: even with hard over is rudder partially or wholly out of propeller helicaldischarge.Very poor single rudder response at very slow speeds.

Ch3. Twin propellers / Torque effect

Torque effect: turning effect created by one engine astern and one engine ahead or only one engine used.

• poor effect with engines too close together (for exemple on narrow beamed ships) – better to use the propellers together with rudder as for a single screw ship.

Ch3. Twin propellers – Torque effectPropellers Split

Shiphandling: Twin Screw Ships

Ch3. Parallel propeller shafts

Best configuration for handling capacity

Ch3. Convergent propeller shafts

Medium handling capacity

Ch3. Divergent propeller shafts

• Poor handling capacity • no turning moment if shafts converge in the pivot point.

Ch3. Twin propellers / Outward turning

Outward turning fixed pitch

The blades are outward turningIn the upper half of the circle of rotation when viewed from astern

If Sb propeller is put astern it willbe rotating in the opposite direction

Ch3. Twin propellers / Transverse thrust

Outward turning fixed pitchpropellers(Sb ahead & Ps astern):

Helical discharge of Ps propellerdeflected up and onto Sb quarterof the ship.

Transverse thrust is assisting thetorque effect and rudders to turn the vessel to port.

Remark:Transverse thrust is a poor force compared to rudder force.

Ch3. Twin propellers / Transverse thrust

Inward turning fixed pitch propellers

If the ship is turning to port and the port propeller is put astern, it will be rotating in the opposite direction and is then acting as a left handed propeller on a single screw ship: part of the helical discharge will be deflected up and towards the starboard quarter.

The transverse thrust attempt to turn the bow to starboard in the opposite direction of the desired turn, working against the rudders and the torque effect.

Twin propellers / Transverse thrust

Inward turning (handed) fixed pitch propellers

The transverse thrust effect can be extremely severe

And render the vessel totally uncontrollable.

It is better to stop one engine and work the vessel as a single crew ship.

This configuration gives a better economical performance in terms of fuel consumption.

Ch3. Transverse thrust / Variable Pitch propellers

Inward turning:

The best configuration for CP (controllable pitch) propellers:

the inside propeller during a turn gives transverse thrust on the appropriate quarter of the ship andincrease the effects of rudders andtorque.

Transverse thrust / Various configurations

1. Fixed outward turn 2. CP inward turn 3. CP outward turn

Pivot point position

Engine stopped /bowthruster to Sb:

• Pivot point close (1/3L) to the stern

• vessel turns on her heels: bow fast to Sb.

• Very effective with sternway

Pivot point position

Bowthruster stopped / Sb engineastern / Ps engine ahead :

• Pivot point close (1/3L) to bow

•Bow turns slowly to Sb

• Stern turns fast to port

Pivot point position

Bowthruster stopped / Sb engine Ahead / Ps engine astern / ruddersHard to Sb:

•Pivot point very close (1/4L) to bow

•Sterns goes to port

•Rate of turn increased due to rudder position

Ch3. Pivot point position

Bowthruster to Sb/ Sb engine astern/Ps engine ahead / rudders amidships:

• pivot point close to center of gravity and behind

• bow turns faster then stern due to the position of the pivot point

Ch3. Position of pivot point

Bowthruster on / Ps engine ahead /Sb engine astern / rudders hard Sb:

•Pivot point at center of gravity

•Ship turns around her center of gravity

•Equal Rate of turns at bow and stern

Ch3. Voith Schneider propulsion

Ch3. Voith Schneider propulsion

Ch3. Voith Schneider propulsion

Multi directional propulsion unit /rotating vertical blades

Ch3. Voith Schneider propulsion

The use of two thrust units placed side by side facilitating spectacular manoeuvrability of the vessel

Ch3. Kort Nozzle

Ch3. Azipod propulsion

Rotating AzimuthUnit.