Propellers

Propellers

Provides the most important source of force on a ship.Provides the most important source of force on a ship. (Usually) makes ship go forward.(Usually) makes ship go forward. Most ships have 2 propellers.Most ships have 2 propellers. Aircraft carriers / Patrol Craft have 4.Aircraft carriers / Patrol Craft have 4. Frigates have 1.Frigates have 1.

Propeller parts

A. Blade Tip The maximum reach of the blade from the center of the propeller hub. It separates the leading edge from the trailing edge. B. Leading Edge That part of the blade nearest the boat, which first cuts through the water. It extends from the hub to the tip. C. Trailing Edge That part of the blade farthest from the boat. The edge from which the water leaves the blade. It extends from the tip to the hub (near the diffuser ring on through-hub exhaust propellers) D. Cup A small curve or lip on the trailing edge of the blade, permitting the propeller to hold water better and normally E. Blade Face That side of the blade facing away from the boat, known as the positive pressure side of the blade. F. Blade Back The side of the blade facing the boat, known as the negative pressure (or suction) side of the blade. G. Blade Root The point in which the blade attaches to the hub. H. Inner Hub This contains the shock absorbing hub (described below). The forward end of the inner hub is the metal surface which generally transmits the propeller thrust through the forward thrust hub to the propeller shaft and in turn, eventually to the boat.

I. Outer HubFor through-hub exhaust propellers. The exterior surface is in direct contact with the water. The blades are attached to the exterior surface. Its inner surface is in contact with the exhaust passage and with the ribs which attach the outer hub to the inner hub

J. RibsFor through-hub exhaust propellers. The connections between the inner and outer hub. There are usually three ribs, occasionally two, four, or five. The ribs are usually either parallel to the propeller shaft ("straight"), or parallel to the blades ("helical").

K. Flo-Torq™ Shock-Absorbing Rubber HubRubber molded to an inner splined hub to protect the propeller drive system from impact damage and to flex when shifting the engine, to relieve the normal shift shock that occurs between the gear and clutch mechanism.

L. Diffuser RingAids in reducing exhaust back pressure and in preventing exhaust gas from feeding back into propeller blades.

M. Exhaust PassageFor through-hub exhaust propellers. The hollow area between the inner hub and the outer hub through which engine exhaust gases are discharged into the water. In some sterndrive installations using a through-transom exhaust system, this passage carries air.

• Diameter is the distance across the circle made by the blade tips as the propeller rotates•Diameter is determined primarily by the RPM at which the propeller will be turning and the amount of power that will be delivered to the propeller through the shafts and gears.• The degree to which the propeller may operate in a partially surfaced condition, as well as the intended forward velocity, will also play a role in determining the most desirable diameter.

Pitch (*)•Pitch is the distance that a propeller would move in one revolution if it were moving through a soft solid, like a screw in wood measured on the face of the blade

PropellersForces resulting from the use of the propellers:

-Forward (or reverse) thrust -Side Force

Side Force

Causes stern to move sideways in the direction of propeller rotation.

Propeller Thrust

•A result of the propeller spinning on its shaft.•Caused by a pressure differential between the opposite sides of the propeller blade.

Rotation ofpropeller blade

Water Flow

Low Pressure

High Pressure

PropellerBlade

Resulting Thrust

Fixed Pitch Propellers (*)

•Found on diesel and steam ships especially on seagoing ship.•Cannot change pitch of propeller•Thrust (speed) controlled by changing speed of the shaft•To go backwards, must stop shaft and spin the shaft in the opposite direction.

Controlling Propeller Thrust

Depends on type of propellers•Fixed Pitch Propellers•Controllable Pitch Propellers

Controllable Pitch Propellers (*)

• Found on all gas turbine ships and diesel ship especially warship.• 0 - 12 kts:

I. shaft rotates at 55 RPMII. thrust (speed) controlled by changing the pitch of the propeller blade

• >12 ktsI. thrust controlled by changing the speed (RPM) of the shaft.

• The shaft always spins in same direction whether going forward or backward.

• Can change the position of the blades relative to the hub hence the pitch of the propeller.

• More flexible because you do not have to change shaft direction to change ship direction

The "Push/Pull" Concept

• right-hand rotation propeller• blade is rotating from top to bottom and is moving from left to

right.

• At the time, water must rush in behind the blade to fill the space left by the downward moving blade.

• This results in a pressure differential between the two sides of the blade: a positive pressure, or pushing effect, on the underside and a negative pressure, or pulling effect, on the top side. ‘This action’, of course, occurs on all the blades around the fall circle of rotation as the engine rotates the propeller.

How Propellers Work