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Hydrodynamics > Propulsion > Flashcards

Propulsion Flashcards

1
Q

Give Va in terms of Vs and the wake fraction

A

Va=Vs(1-w)

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2
Q

On a BP delta diagram there is a code such as: B4-70. What does it mean?

A

B is the screw series. 4 would be the number of blades. 70 would be 0.7 BAR

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3
Q

How do you find Pd from Pb?

A

Pd=Pbxshaft efficiency

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4
Q

How do you get the behind hull diameter from the open water diameter?

A

Do is about 0.95Do

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5
Q

Give an equation for BAR

A

Ad/Ao

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6
Q

Define a right handed propeller

A

Pushes stern to starboard so bow to port due to its clockwise rotation (when viewed from behind)

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7
Q

Define hub

A

The shaft Nose. End of shaft

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8
Q

Define the front and back of the propeller

A

Back is the back of the propeller blade when viewed from aft. Front is the front of the propeller blade when viewed from aft.

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9
Q

Define the fillet area

A

The blade root. Where the blade attaches to the hub

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10
Q

Define the diameter of the propeller. How does diameter effect efficiency?

A

The diameter of the circle that is made by blade top when the propeller rotates. A larger diameter will give a better efficiency.

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11
Q

Define mean face pitch

A

Face pitch is a tangent drawn down the pressure side. Since this varies along the blade you get the mean value.

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12
Q

Define skew and explain why skew is used.

A

Skew is the shift between the blade reference line and the directrix. Skew is used to ease the shock loading and pressure pulses due to propeller entering wake shadow.

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13
Q

Define projected blade area.

A

Area of the blade when looking straight on.

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14
Q

Give some assumptions of the axial momentum theory.

A

Ideal fluid, axial flow, thrust is uniformly distributed over disc, propulsion is an actuator disc.

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15
Q

Give som conclusions of the axial momentum theory.

A

Upper limit to propulsion efficiency, larger diameter gives better efficiency, to accelerate the flow the propulsion has to spend energy in the axial direction

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16
Q

Breifly define a propulsor

A

The particular component of a propulsion system which produces thrust to overcome resistance of craft or overcome resistance and support the craft.

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17
Q

Give all the efficiencies that are on the propulsion system diagram.

A

Hull efficiency ηH, Propulsive eff ηD, behind hull eff ηB, shaft eff ηS, relative rotation efficiency ηR, propeller eff ηo.

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18
Q

Give the different powers and forces on the propulsion system diagram.

A

V forward speed, T thrust, R resistance, Pd delivered power, Pt thrust power, Pb brake power, Pdopen open water delivered power, Pe effective power.

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19
Q

What is nominal wake?

A

Wake in the propeller plane without the action of the propeller

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20
Q

Give the three wake components.

A

Potential wake, viscous wake, wavemaking wake

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21
Q

What is effective wake?

A

Wake in the propeller plane with the action of the propeller

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22
Q

What does wake fraction depend on and what happens as hull roughness increases?

A

Wake fraction depends on length and fullness of the ship and increases with hull roughness.

23
Q

Give the resistance from the thrust of a ship.

A

R=T(1-t)

t is the thrust deduction factor

24
Q

What is the purpose of open water tests?

A

Establish torque, thrust and efficiency of the propeller.

25
Q

What is the purpose of self propulsion tests?

A

Power prediction and to investigate propeller hull interaction. Collect data.

26
Q

What is the purpose of cavitation tests?

A

Study the nature and extent of the cavitation on the propeller

27
Q

What are the main propeller tests that are performed?

A

Open water, self propulsion, cavitation, sea trials (measured mile)

28
Q

How are standard series and models propeller tests used?

A

Used to find optimum parameters of the propeller for given characteristics. Find Kt, Kq, J, and efficiency and use these to find optimum P/D and diameter using BP-δ diagrams.

29
Q

How are Kt, Kq , J diagrams used to find optimum values?

A

Plot 10Kq against J for a set of tests. Where this crosses the maximum efficiency line read off J and use this to find D opt and interpolate to get P/D.

30
Q

How are BP-δ diagrams used to find optimum characteristics?

A

Calculate BP from data, use the diagram by doing to th lien of maximum efficiency and then reading off delta and P/D (interpolate). For behind hull characteristics these can be adjusted with approximations and re-entering the diagram

31
Q

What are the 4 non dimensional parameters used for scaling?

A

Froude number, Reynolds number, cavitation number and advance coefficient.

32
Q

Give the different types of cavitation

A

Bubble, sheet, cloud, vortex (hub and tip), propeller hull vortex

33
Q

What is cavitation and how does it form?

A

Bubbles of gas and or vapour in the water. Formed from low pressures which are a result of high velocity.

34
Q

Give some effects of cavitation.

A

Performance breakdown (at about 20-25% cavitation), noise, vibration, erosion

35
Q

What is a voight Schneider propeller?

A

Vanes on a rotating plate. The vanes rotate on the plate so that they are always perpendicular to the radials from a movable centre point. When centre point is moved from the centre of the plate, thrust is generated perpendicular to the direction in which the centre is moved. Thrust in any direction by moving centre point. No shaft or rudders. Good manoeuvrability but lower efficiency than a conventional propeller

36
Q

What is a ducted propeller?

A

A propeller with a duct to accelerate (or decelerate) the flow. Small gap between the propeller tip and the duct. At higher propeller loadings a duct ca increase efficiency. The duct generates part of the thrust

37
Q

What is a top vortex free propeller?

A

Increased loading at blade tips would increase efficiency but flow around tips prevents this. Tip plates reduce flow around the tips and reduce the strength of tip vortex. Difficult to place plates propeller

38
Q

What is a grim vane wheel?

A

A propeller wheel thing added after first prop. For ships with limited propeller size. Vane free to rotate. Inner part of vane driven by wake of main prop, towards outer part of vane the pitch changes causing the vane to generate thrust at these radii

39
Q

What is a Contra rotating propeller?

A

Two propellers on 1 shaft rotate opposite directions. Produces more power for a given propeller radius and efficiency is increased. The propeller induced heeling moment is compensated (used on torpedoes). However the mechanical installation of coaxial shaft very difficult and complicated. Expensive. Maintenance. More mechanical losses in shaft.

40
Q

What is a schneekluth wake equalising duct?

A

Duct which improves flow so improves propeller efficiency from more axial flow and more equal velocity distribution. Reduction of thrust deduction factor, slight lift generated by aerofoil section. Reduces propeller vibrations. Improved steering

41
Q

Explain flettner rotors.

A

Use the Magnus effect for propulsion. Force acting in a spinning body in a moving airstream which acts perpendicularly to direction of airstream.
Cylinder rotated by electric motor. Have unique points of sail

42
Q

What is magneto hydrodynamic propulsion?

A

A string magnetic field accelerates flow in a magnetic duct. No moving parts but the relatively high resistance of water makes it inefficient.

43
Q

What are propeller boss cap fins?

A

Extra small blades on boss cap. Reduces/eliminates hub vortex. Improved erosion of rudder

44
Q

What are azimuthing pods?

A

Electric motor in a streamlined pod” hung on strut outside hull. Can rotate 360 degrees goo dfor manoeuvrability at low speed. Eliminates rudders, log shafts, stern thrusters, etc.

45
Q

What are surface piercing propellers?

A

For when draught is to mull to accommodate a propellers diameter can operate partially submerged. Very skewed to ease impact on blades. Air suction decreases efficiency

46
Q

What are super cavitating propellers?

A

High rotation rates cause excessive cavitation. Supercavitating propellers have blade sections which generate long sheet cavity at one side of blade means cavitation implies far behind blades. Reduces erosion”

47
Q

Give the equation for Reynolds number.

A

VL/ν

48
Q

Give the equation for froude number

A

Fun=V/root(gD)

49
Q

Give the equation for advance coefficient

A

J=V/nD

50
Q

Give the equation for cavitation number

A

σ=P/ρV^2

51
Q

Give the equation for Pt

A

Pt=TV

52
Q

Give the equation for propulsive efficiency

A

Pe/Pd

53
Q

Give the equation for T/Ap

A

1943.3xPdηo/VaAp

54
Q

How do you calculate Vr?

A

Sqrt(Va^2+(2πN0.7R)^2)

Hydrodynamics (3 decks)

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