Propeller

The propeller consists of a boss with several blades of helicoidal form attached to it. When rotated it 'screws' or thrusts its way through the water by giving momentum to the column of water passing through it. The thrust is transmitted along the shafting to the thrust block and finally to the ship's structure. A solid fixed-pitch propeller is shown in Figure 11.5. Although usually described as fixed, the pitch does vary with increasing radius from the boss. The pitch at any point is fixed, however, and for calculation purposes a mean or average value is used. A propeller which turns clockwise when viewed from aft is considered right-handed and most single-screw ships have right-handed propellers.

A twin-screw ship will usually have a right-handed starboard propeller and a left-handed port propeller.

DiameterHubBlade TipBlade Root

Pitch DistancePitch AngleFixed Pitch

Variable PitchControllable Pitch(Constant Speed)

• 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

Left Hand Right Hand

Engine ReductionGear

Bearing Seals

ScrewStrut

BHP SHP DHP

THP

Shaft

Ship Drive Train System

Propeller

The engine shaft power is transmitted to the propeller with only minor transmission losses. The operation of the propeller results in a forward thrust on the thrust block and the propulsion of the ship at some particular speed. The propeller efficiency is a measure of effectiveness of the power conversion by the propeller

Propeller

The power conversion achieved by the propeller is a result of its rotating action and the geometry of the blades. The principal geometrical feature is the pitch. This is the distance that a blade would move forward in one revolution if it did not slip with respect to the water. The pitch will vary at various points along the blade out to its tip but an average value is used in calculations. The slip of the propeller is measured as a ratio or percentage as follows:

Propeller

The theoretical speed is a product of pitch and the number of revolutions turned in a unit time.

P=Pitch of propeller m

N= RPM

Theoretical speed= (PxNx60)/1852 KNOT

The actual speed is the ship speed. It is possible to have a negative value of slip if, for example, a strong current or wind were assisting the ship's forward motion

Propeller

Brake Horse Power (BHP)

- Power output at the shaft coming out of the engine before

the reduction gears

Shaft Horse Power (SHP)

- Power output after the reduction gears (at shaft)

- SHP=BHP - losses in reduction gear

Horse Power in Drive Train

Ship Drive Train and Power

Ship Drive Train and Power

Delivered Horse Power (DHP)

- Power delivered to the propeller

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

Thrust Horse Power (THP)

- Power created by the screw/propeller (ie prop thrust)

- THP=DHP – Propeller losses

E/G R/GBHP SHP Shaft

BearingProp.

DHP THP EHPHull

Effective Horse Power (EHP)

• EHP : The power required to move the ship hull at a given

speed in the absence of propeller action (related to resistance)

(EHP is not related with Power Train System)

• EHP can be determined from the towing tank experiments at

the various speeds of the model ship.

• EHP of the model ship is converted into EHP of the full scale

ship by the Froude’s Law.

EHP is approximately equal to THP (usually slightly less)

Measured EHPVTowing Tank Towing carriage