Presentation on theme: "Chapter 3 / The Propeller. Ch3. Propeller -Ahead movement -Astern movement -Transverse thrust."— Presentation transcript:
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
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
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
Ch3. Astern / Transverse thrust Conclusion: pronounced turn to Sb when engine is going astern Similar effect with headway, sternway of vessel stopped
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
Ch3. Interaction between propeller and rudder Engine ahead: Propeller flow strikes rudder and increases the rudder effect. Action of propeller flow on rudder more pronounced when vessel is stopped or 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.
Ch3. Twin 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.
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
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