Transcript Ch3.Propellor

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 discharge
From propeller pressure of water more regular on left side of
Rudder
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
Rudder Amidships
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
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
Result
Gentle 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
Rudder Amidships
Ch3. Astern / Transverse thrust
Ship follows
Conclusion:
the rudder:
Ship will tend into the wind:
•pronounced turn to Sb when engine is going astern
Shipeffect
will tend
port very
easily of vessel stopped
•Similar
withto
headway,
sternway
Ship does not tend to starboard easily
Rudder Amidships
Ch3. Astern / Transverse thrust
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
opellers
/ Rudders
Ch3. Interaction
between propeller and rudder
means of controlling the stern
Engine ahead:
Thrust
Propeller flow strikes
rudder
and increases the rudder
effect.
Action of propeller flow on
Side Force
rudder more pronounced
when vessel is stopped or
Rudder Force with sternway.
Ch3. Interaction between propeller and rudder
Engine astern and Rudder amidships: the vessel is
Swinging 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 effect
of propeller astern is stopped.
Ch3. Rudder counter effect to control propeller effect
1. Rudder to Sb
2. Engine astern
3. Put rudder amidships and gradually to Sb
4. 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 speed
2. Hard to port
3. Half or full ahead
4. Rate of turn increases
5. Short bursts on the engine
to avoid increase of speed
6. 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 starts
with the first turn to Sb. Otherwise will the propeller effect oppose
the rudder effect
Ch3. Half turn in heavy wind condition
Pos 1 : Engine half/full astern – the stern comes into the wind
Pos 2 : Rudder hard to port and engine half/full ahead
Pos 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 helical
discharge.
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.
Shiphandling: Twin Screw Ships
Ch3. Twin
Propellers
Split propellers – Torque effect
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 turning
In the upper half of the circle of
rotation when viewed from astern
If Sb propeller is put astern it will
be rotating in the opposite direction
Ch3. Twin propellers / Transverse thrust
Outward turning fixed pitch
propellers
(Sb ahead & Ps astern):
Helical discharge of Ps propeller
deflected up and onto Sb quarter
of the ship.
Transverse thrust is assisting the
torque 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 and
increase the effects of rudders and
torque.
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 engine
astern / 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 / rudders
Hard 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 Azimuth
Unit.