speed vs cavitation

Discussion in 'Boat Design' started by rfleet1066, Jul 11, 2012.

  1. rfleet1066
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    rfleet1066 rfleet1066

    I am working on prop specs for a work boat. I can choose shaft speed via selecting a gear ratio and select prop size. What effect does prop speed have on cavitation and the loss of thrust therefrom?

    Ryland
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Well, first a brief over view of cavitation.

    A prop is made from a foil section, usually called a blade. As water passes over it, there is suction, or lift, on one side, the upper surface and then a positive pressure on the lower surface. We know from Bernoulli that as velocity increases, pressure decreases. So, as the speed of the water over the blade increases the pressure reduces.

    There comes a point where the pressure on the blade reduces by a certain amount, as the rpm increases, that the absolute pressure is so low water vapourises. In other words, at increasing rpm the water no longer remains in contact with the blade, and an air cavity is formed and spreads. Which leads to a reduction in thrust and other effects.

    If you're lucky the prop your selecting will have whats called a Cavitation Bucket diagram, and is dependent upon the characteristics of the blades geometry.

    cavitation bucket.jpg
     
  3. daiquiri
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    daiquiri Engineering and Design

    There is a typo here, AH. It is a water-vapor cavity, or a gas cavity - not an air cavity. In fact, just prior to that you have correctly said that the water vaporizes due to low fluid pressure.

    Cheers!
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    OOpppss!!..well spotted.
    I shouldn't reply early in the morning..my brain doesn't wake up until the afternoon :eek:
     
  5. mydauphin
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    mydauphin Senior Member

    Guys, I think your both wrong. Isn't technically a vacuum and the water is vaporizing to fill it.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Hmm…we’re probably going to get into semantics here.

    A liquid is a body that has no form, but a fixed volume. The molecules are close enough to attract one another and not “fly apart”, they are in equilibrium.

    A gas, has a temperature in which is can no longer liquefy. Below this temperature it is called a vapour, and above this temperature it is called a gas. Vapours can be liquefied by compression. Air for example, has its critical temperature at -140c, otherwise it would be rather hard to scuba dive breathing in liquid air instead of simple compressed air! The critical pressure of air is 39 atm’s (3.95 MPa)

    So back to this.

    Water therefore has a pressure at which it vaporises; this being a mixture of water (fluid) and fluid + air. This being at 219atm’s (22.19MPa or 3218psi), or its critical temp of 374c.

    So below this pressure (temp), water remains as water, above this pressure limit the water molecules are no longer in thermodynamic equilibrium and can ‘escape’ as such and become vapour. Since the critical vapour pressure is 374c, the water is not boiling! The water does not go to a vacuum, it becomes a vapour, a mixture of water and/or (air + water). The critical pressure of air is even lower at 39amt’s or -140c. So it must change state from a liquid to a vapour first.

    Thus for cavitation to occur the local static pressure must fall below the water vapour pressure, that also being dictated by the temperature of the water.
     
  7. rfleet1066
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    rfleet1066 rfleet1066

    problem to solve

    The problem to solve is this.......... I'm operating in shallow water at times. The prop will not be far below the surface. The choice is a large slower prop or a smaller diameter faster prop. I am concerned about cavitation because of proximity to the surface.

    Ryland
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Yes this does influence the cavitation.

    You could then, if space allowed and the hull shape too, add a simple cavitation plate, like you see on outboard motors. Not much else you can do, unless going for a SPP.
     
  9. daiquiri
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    daiquiri Engineering and Design

    Yeah John, to hell with these forums - go to sleep some, my friend... ;) :p

    I am affraid the anti-aeration plate wouldn't help against the cavitation, because it doesn't influence the hydrostatic pressure (which depends only on vertical distance of the prop blade from the water surface).
    But the plate do serve against the aeration, the sucking of the atmospheric air into the suction side of the blade, when the prop (or the rudder) is operating close to the surface.

    The way to reduce the cavitation consists, essentially, in the control of the prop loading. So you will either reduce the power in shallow waters, or will adopt as big prop diameter as practically possible, or will distribute the required power over several propellers, or will reduce the gear ratio (prop speed), or will adopt any combination of these.

    Cheers
     

  10. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

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