On Mon, 14 Aug 2006 13:30:48 +0100, "Bruce Peckett"
<[EMAIL PROTECTED]> wrote:
><..>
>
>> Sorry guys but everything described upto now is Ventilation
>> not Cavitation!
>
>Justify!
To be honest Bruce I feel that you have justified my statement by your
reply!
>
>cav.i.ta.tion (kv-tshn)
>n.
>1. The sudden formation and collapse of low-pressure bubbles in liquids
>by means of mechanical forces, such as those resulting from rotation of
>a marine propeller.
>2. The pitting of a solid surface.
>3. Medicine The formation of cavities in a body tissue or an organ,
>especially those formed in the lung as a result of tuberculosis
>
>Read 1, QED!
We are not talking about cavitation but air getting to the prop. The
concept of getting the stern deeper in the water will not correct
cavitation as defined above.
>
>*Strictly* speaking, when the blades of the prop are running partially
>in air that isn't itself cavitation although it will invariably
>*produce* cavitation. The noise and vibration that occur when this
>happens are, however, the result of cavitation due to air being drawn
>into the prop wash
>
>In marine and inland craft applications, the distinction is a moot point
>(although it isn't in pumps etc.) and when the prop is running partially
>in air and producing a disturbed wash, noise and vibration it is said to
>be cavitating.
>
>I have never heard of cavitation being called "ventilation" nor have I
>ever heard of it being caused by the weedhatch although it's not
>impossible I guess.
>
>Whatever name you want to call it by (and since it's been known as
>cavitation since screw props were first fitted to marine craft I'll
>stick with that!) the solution is to stop the prop drawing air in and
>almost invariably that means getting the stern of the boat deeper in the
>water by adding ballast at the stern or possibly by reducing weight
>towards the bows
>
>Bru
Dave Gerr's book "Propellor Handbook" discusses cavitation as per your
definition but then goes on to discuss Ventilation
I quote
"Ventilation is often confused with cavitation though actually it's
quite different. Whereas cavitation comprises actual areas of partial
vacuum, ventilation is caused by the propellor's sucking air down from
the waters surface. This is not usually as severe a problem as
cavitation, but it can lead to vibration and loss of thrust."
He then discusses getting the prop deeper into the water and the use
of raked blades on the prop.
I personally feel that the majority of the narrow boat industry is
using the wrong name for the problem. The link found by Robin Lewis
again describes the differences correctly.
Hopefully not starting another discussion its a bit like the use of
"centrifugal force" when what is described is "centripetal force".
Richard
Nb "Pound Eater" Parkend G+S
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