The propeller is fitted on the outboard end of the propeller shaft
It is also called as tail-end shaft or tail shaft
It passes through the Stern Tube
The outboard end of the Shaft is tapered
Propeller Boss fits on the tapered part of the Shaft
Propeller Nut is screwed on the end to lock the Propeller
Oil Lubricated Stern Tube
Stern Tube is made of Cast Iron
Propeller Shaft passes through the Stern Tube
Stern Tube carries the weight of the Propeller Shaft and the Propeller
Stern Tube are lined with white metal & lubricated with Oil
Both ends of the Stern tube has lip seals made of nitrile rubber.
Oil in the stern tube is maintained at a certain pressure between the lip seals
Chrome liners are provided on out board and in board ends of the Propeller Shaft
The elastic lip of the seal grips on the rubbing surface provided by chrome liners
Seals prevent the entry of seawater and also prevents the loss of lub oil
Large tanker/ships with large draught changes are fitted with two oil header tanks – one for ballast condition and another for fully loaded condition
Basic Marine Engineering Book
Page No. 151 Fig 8.2 (a) and Page No. 152 Fig 8.2 (b)
Types of Propellers
- Solid Propeller (Fixed-Pitch Propeller)
- Controllable-Pitch Propeller
- Contra-Rotating Propeller
- Vertical-Axis Propeller
Solid Propeller (Fixed Pitch Propeller)
Blades are attached to the Boss/Hub
Blades are Helicoidal form (Skewed) [blade has a twisted appearance when viewed from tip to centre]. When Propeller is rotated – it SCREWS or THRUSTS its way through the water by giving momentum to the column of water passing through it. Pitch is the axial distance that a Propeller will move in one revolution. Fixed Pitch = for each revolution the axial distance moved is same
Basic Marine Engineering Book Page No. 153 Fig 8.3
Controllable Pitch Propeller (cpp)
Difference between CPP ( controllable ) and fixed pitch propeller.
| Sl. No | Solid / Fixed Pitch Propeller | Controllable Pitch Propeller |
| 1 | Speed of Ship is controlled by varying the speed of Engine | Speed of the ship is controlled by varying the pitch of the propeller |
| 2 | Engine with varying speed is required | Engine with constant speed is enough |
| 3 | Hence Shaft driven auxiliary cannot be fitted | Hence Shaft driven auxiliary can be fitted |
| 4 | Astern movement of the ship is by reversing the engine {engine should be of reversing type} | Astern movement of the ship is by varying the pitch of the propeller {unidirectional engine is enough] |
| 5 | Full power of the engine is not available during astern running | Full power of the engine will be available during astern running also |
| 6 | Hence Stopping time and distance is more | Hence Stopping time and distance is less |
| 7 | Manoeuvring in confined water is difficult | Manoeuvring in confined water is easy |
| 8 | Propeller efficiency is more | Propeller efficiency is less since large diameter boss/hub is required |
| 9 | Cost is low | Cost is high |
| 10 | Cost of repair and maintenance is less | Cost of repair and maintenance is more |
Maximum Continuous Rating (MCR) – It is the maximum power output (kW/BHP) that an engine can produce while running continuously at safe limits and conditions
Propulsion Efficiency
Propeller Efficiency: ηp = Pt Vi
2π N T
Propulsive Efficiency: ηt = Rt V
2π N T
Pt = Thrust (Propeller axial thrust force)
Vi = Inflow Velocity (Mean inflow velocity)
N = Rotation rate (Rotational speed of the propeller) in rev/sec
T = Torque (Propeller shaft torque)
Rt= Total Ship resistance
V = Ship Speed (Ship velocity)
SFOC (or) SFC: Specific Fuel Oil Consumption = Mass of fuel consumed per hour g/kWh
Power developed in KW
Consumption of Fuel Oil per Unit Energy at the Output Shaft of the Engine for given time [g/kWh (or) g/BHPh]
(Or)
Mass of Fuel Oil Consumed per average Shaft Power developed by the Engine for a given time
SFOC = Co D 106
h Pe
106 is multiplied to convert the fuel oil unit in tonnes to gram
Co = Fuel Oil Consumed Over the period in m3
D = Temp. Corrected Density in kg / m3
h = Measuring period in hours
Pe = Brake Horse Power bhp or kW
Take flow meter reading for specific time interval [usually 1 hr period, h = 1 hr) Diff between initial readings and final reading (after 1 hr) = Co in m3
Density is given in Bunker Delivery Note in kg /m3
Calculate Density at the Temp near the flow meter.
Temp Corrected Density D = (Density of Fuel Oil @ 150C) * [1-{(T-15) * 0.00064}] kg/m3
Calculate Shaft Power (BHP/kW) at the given interval Pe
SFOC is used to determine the Efficiency of the Engine
Power-to-weight ratio of engine = Power generated by the engine
Mass of the engine
Example:
Engine’s Power = 250 kW
Engine’s Mass = 380 kg
Power-to-Weight Ratio = 0.65 kW/kg
Power to Weight Ratio is used for comparing the Performance of one Engine with the performance another engine.
Pitch is the axial distance that a Propeller will move in one revolution
P = 2 π R tan θ metres
P = 2 π R1 tan θ1 + 2 π R2 tan θ2 + ……. 2 π Rn tan θn metres
n
P = Pitch Ratio x Diameter of Propeller in metres
P = Pitch in metres
R = Radius from the centre of shaft to section in metres
θ = Pitch Angle in degrees
tan θ = Natural Tangents under Mathematical Tables in Science Data Book (Clark’s Tables)
Scientific Calculator – Select the Mode – ‘Deg’
Enter the Value of θ (given in degrees)
Press ‘tan’ key
Propeller Speed (Theoretical Speed) (VT) is the distance the propeller will move in unit time if working in an unyielding fluid
VT = P x N x 60 Knots (Nautical Miles/hr) (nm/hr)
1852
VT = Propeller Speed (Theoretical Speed) in Knots (Nautical Miles/hr) (nm/hr)
P = Pitch in metres
N =Revolutions per minutes in rpm
N x 60 = n (Revolutions –No Units)
Theoretical Distance = P x n in Knots (Nautical Miles) (nm/hr)
1852
Apparent Slip (Percentage Slip) is the difference between the Propeller Speed (VT) and the Speed of the Ship (Actual Speed) (V) expressed as a percentage of Propeller Speed (Theoretical Speed) (VT)
Apparent Slip (Percentage Slip) = VT – V x 100 %
VT
VT = Propeller Speed (Theoretical Speed) in Knots (Nautical Miles/hr) (nm/hr)
V = Speed of the Ship (Actual Speed)
V = Actual Distance
Time taken to cover the Distance
Apparent Slip = Theoretical Distance – Actual Distance x 100 %
Theoretical Distance
Wake Speed
When a ship moves, due to the friction between hull and the water, the water around the ship also moves. This moving water is known as Wake.
Speed of moving water (Wake) is known as Wake Speed (Vw)
Vw = w x V Knots (Nautical Miles/hr) (nm/hr)
w = 0.5 Cb – 0.05
w = Wake fraction
Cb = Volume Displaced
L x B x D
L = Length in m B = Beam in m D = Draught or Draft in m
Volume Displaced = Δ Volume = Mass
1.0125 Density
Δ = Displacement in tonnes
Speed of Advance is the Propeller added Speed (Speed added by the Propeller)
Speed of the Ship (Actual Speed) (V) = Wake Speed (Vw) + Speed of Advance (Va)
V = Vw + Va Knots (Nautical Miles/hr) (nm/hr)
Basic Marine Engineering Book Page No. 154 – Figure
Real Slip or True Slip is the difference between the Propeller Speed (Theoretical Speed) (VT) and speed of advance (Va) expressed as a percentage of Propeller Speed (Theoretical Speed) (VT)
Real Slip = VT – Va x 100 %
VT
Power Estimation
Rt = ρSVn
S α (Length)2
Δ α (Length)3
S α Δ2/3
Rt α Δ2/3 V2
P α Rt V
α Δ2/3 V2 V
C = Δ2/3 V3
P
Rt = Total Resistance of a Ship
ρ = Density of Water (kg/m3)
S = Wetted Surface Area (m2)
V = Speed (Knots)
Δ = Displacement (tonnes)
P = Propeller Power / Engine Power (kW)
Index ‘n’ is taken as ‘2’ since ships are slow or medium speed
ρ is considered constant since ships will be in sea water
C is constant known as ‘Admiralty Coefficient’
Power Estimation
(A) Power = Δ2/3 V3
C
(B) Δ12/3 V13 = Δ22/3 V23
P1 P2
If Δ1 = Δ2
P1 = V1 3
P2 V2
P = Power of the Engine in kW
Δ = Displacement in tonnes
V = Speed of Ship (Actual Speed) in Knots (Nautical Miles/hr) (nm/hr)
C = Admiralty Coefficient
Δ2/3 – Scientific Calculator First Take Cube Root & then Square it
Fuel Estimation
- Daily Fuel Consumption α Δ2/3 V3
α V3
Fuel Coefficient = Δ2/3 V3
Daily Fuel Consumption
Daily Fuel Consumption = Fuel Consumption / day in tonnes
= Daily Consumption in tonnes
Δ = Displacement in tonnes
V = Speed of Ship (Actual Speed) in Knots (Nautical Miles/hr) (nm/hr)
Voyage Consumption = Total Fuel Consumption during the Voyage in tonnes
= Total Voyage Consumption in tonnes
Voyage Distance = Distance Covered during the Voyage in Knots (Nautical Miles/hr) (nm/hr)
= Total Distance in Knots (Nautical Miles/hr) (nm/hr)
(2) Δ12/3 V13 = Δ22/3 V23
Daily Fuel Consumption1 Daily Fuel Consumption 2
(3) Daily Fuel Consumption1 = Δ1 2/3 V1 3
Daily Fuel Consumption 2 Δ2 V2
(4) Daily Fuel Consumption = Voyage Consumption
No. of Days on Voyage
(5) No. of Days on Voyage = Voyage Distance
V x 24 hrs
(6) Voyage Consumption1 = Δ1 2/3 V1 2 Voyage Distance 1
Voyage Consumption 2 Δ2 V2 Voyage Distance 2
(7) Daily Fuel Consumption = Voyage Consumption x V x 24 hrs
Voyage Distance
(8) Daily Fuel Consumption1 = V1 3
Daily Fuel Consumption 2 V2
Note:
If Voyage distance not given assume voyage is for 24 hrs (i.e. 1 day)
Voyage Distance for 1 day = V x 24 hrs
Usually 1st Voyage is considered for 24 hrs (or) 1 day