Sea Ray 290 Sundancer Manuale del Proprietario Scaricare il pdf (Pagina 63)

4.5

SECTION 4 • BILGE & UNDERWATER GEAR

290 Sundancer

3. UNDERWATER GEAR

A. PROPELLER GENERAL INFORMATION

Propellers should be free of nicks, excessive

pitting and any distortions that alter them from their

original design. Badly damaged propellers should

be replaced, but those that are chipped, bent or

merely knocked out of shape can be reconditioned

by your marine dealer.

When doing extensive cruising, it is advisable to

carry an extra propeller aboard.

BASIC PROPELLER CHARACTERISTICS

Propellers have two basic characteristics:

• Diameter

• Pitch.

Diameter is that distance measured across the

pro pel ler hub line from the outer edge of the 360°

that is made by the propeller’s blade during a

single rotation. Pitch is that distance in inches that

8. Install rear thrust hub, propeller and nut in same

manner.

9. Install propeller shaft anode.

PROPELLER PARTS

(FIG. 4.5.1)

REAR

PROPELLER

NUT

SHAFT

ANODE

PROPELLER

SHAFT

REAR

THRUST HUB

REAR

PROPELLER

FRONT

PROPELLER

NUT

FRONT

PROPELLER

FRONT

THRUST HUB

a pro pel ler will travel if rotated one revolution without

any slippage.

For example, a propeller with a 12-inch pitch, when

ro tat ed 360° would, theoretically, advance 12 inch es

through the water. Actually, no propeller applied to

any boat is 100% efﬁ cient. No 12-inch pitch blade

will, in a single ro ta tion, advance a boat 12 inches.

This variance is re ferred to as slippage.

PITCH

DI AM E TER

PROPELLER PITCH AND DIAMETER

(FIG. 4.5.2)

VENTILATION, ITS CAUSES AND CORRECTIONS

While often called “cavitation,” ventilation is really

a dif fer ent effect. At times when a boat enters or

leaves a sharp turn, the propeller seems to slip and

lose thrust and the engine may over-speed. This

problem is normally caused by air or aerated water

entering the propeller. (A dam aged propeller can

also cause ven ti la tion.) Ventilation can usually be

corrected by one or more of the following:

1. Replace the damaged or incorrect propeller

with the recommended one.

2. With stern drives, set the outdrive at a lesser

trim angle (trim the unit downward).

CAVITATION, ITS CAUSES AND CORRECTIONS

Cavitation is a phenomenon that occurs in all

pro pel ler-driven craft under certain conditions. The

sur face of pro pel ler blades are not perfectly ﬂ at,

and as water is drawn through the blades to be

discharged aft into the pro pel ler’s slip stream, the

water ﬂ owing over the curved surface of the blade

encounters areas of greater and less pressure.

In those areas of reduced pressure, air bubbles are

formed. When they move out of the low pressure

area these bubbles collapse. If they collapse while in

contact with an object, such as part of the propeller

blade or trim plane, the bubbles create such highly

OP TI MUM PER FOR MANCE

CAVITATION

PROPELLER CAVITATION

(FIG. 4.5.3)

1 2 ... 58 59 60 61 62 63 64 65 66 67 68 ... 149 150

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Sea Ray 290 Sundancer Manuale del Proprietario Pagina 63

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