Propeller and Rudder

Propellers, Rudders and Bow Thrusters

Burak Acar

Ship Drive Train and Power

Ship Drive Train System

Engine ReductionGear

Bearing Seals

ScrewStrut

BHP SHP DHP

THP

EHP

Brake Horsepower (BHP)

- Power output at the shaft coming out of the engine

before

the reduction gears

Engine

ReductionGear Bearing Seals

ScrewStrut

SHP

DHP

THP

BHP

EHP

Shaft Horsepower (SHP)

- Power output at the shaft coming out of the reduction gears

Engine


ScrewStrut

BHP

SHPDHP

THP

EHP

Engine


ScrewStrut

BHP

SHPDHP

THP

EHP

Delivered Horsepower (DHP)

- Power delivered to the propeller

- DHP=SHP – losses in shafting, shaft bearings and seals

Engine


ScrewStrut

BHP

SHP

DHP

THP

EHP

Thrust Horsepower (THP)

- Power created by the screw/propeller

- THP=DHP – Propeller losses

- THP is the end result of all HP losses along the drive train

PROPELLER

Types of Propellers

• Fotokopiler

The Screw Propeller

7.9 Screw Propeller

DiameterHubBlade TipBlade Root

Pitch DistancePitch AngleFixed Pitch

Variable PitchControllable Pitch(Constant Speed)

Basic Nomenclature:• Hub The hub of a propeller is the solid center disk that mates with the propeller shaft and to

which the blades are attached. Ideally the hub should be as small in diameter as possible to obtain maximum thrust, however there is a tradeoff between size and strength. Too small a hub ultimately will not be strong enough.

• Blades Twisted fins or foils that protrude from the propeller hub. The shape of the blades and the speed at which they are driven dictates the torque a given propeller can deliver.

• Diameter The diameter (or radius) is a crucial geometric parameter in determining the• amount of power that a propeller can absorb and deliver, and thus dictating the amount of• thrust available for propulsion. With the exception of high speed (35 Knots+) vehicles• the diameter is proportional to propeller efficiency (ie. Higher diameter equates to higher• efficiency). In high speed vessels, however, larger diameter equates to high drag. For• typical vessels a small increase in diameter translates into a dramatic increase in thrust• and torque load on the engine shaft, thus the larger the diameter the slower the propeller• will turn, limited by structural loading and engine rating.

Basic Nomenclature:• Revolutions per Minute (RPMs) RPM is the number of full turns or

rotations of a propeller in one minute. RPM is often designated by the variable N. High values of RPM are typically not efficient except on high speed vessels. For vessels operating under 35Knots speed, it is usual practice to reduce RPM, and increase diameter, to obtain higher torque from a reasonably sized power plant. Achieving low RPM from a typical engine usually requires a reduction gearbox.

• Pitch The pitch of a propeller is defined similarly to that of a wood or machine screw. It indicates the distance the propeller would “drive forward” for each full rotation. If a propeller moves forward 10inches for every complete turn it has a 10inch nominal pitch. In reality since the propeller is attached to a shaft it will not actually move forward, but instead propel the ship forward. The distance the ship is propelled forward in one propeller rotation is actually less than the pitch. The difference between the nominal pitch and the actual distance traveled by the vessel in one rotation is called slip.

Screw Propeller

• Variable Pitch (the standard prop):

- The pitch varies at the radial distance from the hub.

- Improves the propeller efficiency.

- Blade may be designed to be adjusted to a different

pitch setting when propeller is stopped.

• Controllable Pitch :

- The position of the blades relative to the hub can be

changed while the propeller is rotating.

- This will improve the control and ship handling.

- Expensive and difficult to design and build

Right and Left Hand Props

Right HandLeft Hand

Pressure Face

Suction Face

Leading Edge

Trailing Edge

Propeller Walk

• Due to a difference in the pressure at the top and bottom of the prop (due to boundary layer), the lower part of the prop works harder.

• This leads to a slight turning moment.

• Right hand props cause turns to port when moving ahead.

Prop Walk Solutions• Twin Screws

• Counter rotating propellers (one shaft)

• Tunnels/shrouds (nozzle)

Shrouded (nozzle) prop

Skewed Screw Propeller

Highly Skewed Propeller

DDG51

- Reduce interaction between propeller and rudder wake.- Reduce vibration and noise

Advantages

Disadvantages

- Expensive- Less efficient operating in reverse

Propeller Theory

Propeller Theory

• Speed of Advance

Q

PWake Region

SV WV

0waterV

Swater VV

• The ship drags the surrounding water so that the wake to

follow the ship with a wake speed (Vw) is generated in the stern. • The flow speed at the propeller is,

WSA VVV Speed of Advance

Propeller Cavitation

• Cavitation : Definition

- The formation and subsequent collapse of vapor bubbles

on propeller blades where pressure has fallen below the

vapor pressure of water.

- Bernoulli’s Equation can be used to predict pressure.

- Cavitation occurs on propellers (or rudders) that are

heavily loaded, or are experiencing a high thrust loading

coefficient.

1 atm=101kpa =14.7psi

Blade Tip Cavitation

Sheet Cavitation

Navy Model Propeller 5236

Flow velocities at the tip are fastest so that pressure drop occurs at the tip first.

Large and stable region of cavitation covering the suction face of propeller.

Consequences of Cavitation

1) Low propeller efficiency (Thrust reduction)

2) Propeller erosion (mechanical erosion as bubbles

collapse, up to 180 ton/in² pressure)

3) Vibration due to uneven loading

4) Cavitation noise due to impulsion by the bubble

collapse



• Preventing Cavitation

- Remove fouling, nicks and scratch.

- Increase or decrease the engine RPM smoothly to avoid

an abrupt change in thrust.

rapid change of rpm high propeller thrust but small

change in VA larger CT cavitation &

low propeller efficiency

- Keep appropriate pitch setting for controllable pitch

propeller

- For submarines, diving to deeper depths will delay or

prevent cavitation as hydrostatic pressure increases.


• Ventilation

- If a propeller or rudder operates too close to the water surface, surface air or exhaust gases are drawn into the propeller blade due to the localized low pressure around propeller. The prop “digs a hole” in the water.

- The load on the propeller is reduced by the mixing of air or exhaust gases into the water causing effects similar to those for cavitation.

-Ventilation often occurs in ships in a very light condition(small draft), in rough seas, or during hard turns.

Other forms of propulsion

A one horsepower cable-drawn ferry!

RUDDER

Ship rudder

• rudder is the most important ship control surface

• a fin-like projection under the counter and below the waterline – placed as far aft as practical

• mounted onto a circular shaft referred to as the stock – penetrates the hull through bearings

Ship rudder – basic definitions

Ship rudder – forces on a foil

Area and shape of rudders

• no fixed rule for determination of the size

• in practice, rudder area, expressed as a fraction of the product of the length and draft or centerline plane area, often selected by comparison with a ship with similar maneuverability requirements

• (Rudder Area)Cargo Ships = 0.017 * LOA * T

Area and shape of rudders - types

Area and shape of rudders - types

• rudder consists of two parts: the blade (flat part) against which the water pressure acts and the stock (shaft) which transmits motion of the steering gear to the blade

• there are 3 types of rudders:

balanced: a portion of the blade area is disposed symmetrically through the rudder height and fwd of stock

unbalanced: blade is entirely aft of stock

semi-balanced: area fwd of stock does not extend to the full height of the blade aft of the stock – upper portion may be considered unbalanced and the lower portion, balanced

Area and shape of rudders

Modern double-plate,semi-balanced rudder in a single screw ship

BOW THRUSTERS

Rotatable Thrusters and Propellers

Actuator Models

Actuator Models

Education

Propeller and Rudder