Cause of cavitation

[chad tha duck comander]

i was wondering what is causing my boat to cavitate

 

[walleyehed]

Re: Prop advice.

Re: Prop advice.

OK, were here now!!! Magic ain't it???
Give some details as to your boat/engine combo and the prop you have now...what's it doing?....I know, you say "Cavitation"....be a bit more specific please.

 

[ebry710]

Re: Cause of cavitation

An old tug boat captain, Capt John Cristich (SF Bay, 40 years, started WWII) once told me that a prop does not push water, it screws water. He use to call tug boats single screw or twin screws. A prop cuts the surface tension of the water in a cork screw fashion. If you turn the prop faster then the water will allow it will cavitate or spin in a hot steam pocket. The engine has no more resistance from the water and the boat goes nowhere.

On a tug boat it does three things:
1) cause power loss
2) over revs the engines
3) pit the expensive props

Your boat is not a tug, but the principles are the same and its a good story from a great man of the past.
The only way to stop cavitation is to throttle down and start again....usually slower.

 

[walleyehed]

Re: Cause of cavitation

That's VENTILATION...Not cavitation....

 

[ddennis]

Re: Cause of cavitation

http://en.wikipedia.org/wiki/Cavitation for a photo

Ventilation and Cavitation
Ventilation occurs when surface air or exhaust gases are drawn into the propeller blades. The load on the propeller is reduced by the mixing of air or exhaust into the water steam causing over revving.

Anti-ventilation Plate: Large plate cast into gearcase housing directly above propeller. Helps reduce surface air from being pulled into blades.

Cavitation: The aeration (bubbling) and boiling effect of water caused by creation of a low pressure area. Generally caused by a solid shape (propeller blade) passing through the water, in such a position and speed, that a low pressure area is formed due to the inability to move through the water in nonresistant manner. An example is, a propeller blade that has a rough edge would not cut efficiently through the water, thus creating a low pressure area. If the pressure drops below the vapor pressure, a cavitation bubble will form in that region. These bubbles will collapse when they reach the higher pressure region of the blade. This causes a rapid change in pressure and can result in physical erosion. You may notice burns (erosion) at some area on the face of the blade.

Common rules of thumb:

Ventilation can lead to excessive slippage.
Cavitation can lead to ventilation and/or slipping.

 

[steelespike]

Re: Cause of cavitation

Cavitation erosion is usually most evident on the back of the blade most props have some evidence of cavitation.