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Author Topic: Cavitation?....Not!  (Read 550 times)

GG

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Cavitation?....Not!
« on: January 26, 2020, 03:02:01 pm »

As we start into a new Year, although with the awful UK weather I've only managed to so far sail twice and fly once, it seems worthwhile to see if I can prevent a few people from crying out "Cavitation" as soon as they they hear a propeller making a "slurping" sound.


Model boat propellers work by accelerating water rearwards and, courtesy of Newton's Third Law of Motion, this creates a force on the propeller which can be used to move the boat forwards.  This results in a pressure difference between the front and rear surfaces of the propeller blades, the rear having higher pressure to force the water rearwards, the front surface having a reduced pressure to in effect suck the water rearwards, rather like an aircraft wing.  By the way I'm considering propellers working in their normal fashion. i.e. mounted at the rear and driving the model forward so "front" and "rear" refer to the bow and stern facing surfaces respectively.


Now water, like most liquids, has a "vapour pressure".  If the conditions are such that the pressure above the surface of the water is below the vapour pressure then it will readily turn into a vapour, i,e, start to do what most people would call boiling.


Water under what we call normal atmospheric conditions, does not boil until you heat it up to around 100 degrees C.  At which point its vapour pressure exceeds the atmospheric pressure and provided you keep putting energy into it, water will boil.  Yes, it can evaporate at lower temperatures but this needs a water molecule having the right speed (in a fluid particles have speed ranging from zero to very high, few at these extremes and most clustered around an average value) and be heading out of the liquid.  So, evaporation is a much slower process than boiling.


Heating isn't the only way to make water boil, if the pressure above it is reduced then it will boil at a lower temperature.  I recall a school science demonstration where the air pressure in a flask partly filled with water was reduced until, much to the amazement of student holding the flask, it began to vigorously boil even though it was at room temperature!  The dreaded H&S may have now stopped this striking demonstration but it was an unforgettable illustration of why astronauts need spacesuits and you cannot brew up a good cup of tea at the top of Mount Everest!(where water boils at about 69 degrees C).


Now if the pressure on the forward face of the propeller were to drop at any place to less than the vapour pressure of the water it is operating in then a bubble or even a sheet of vapour could form, this being the cavity in cavitation.  Actually forming the cavity isn't usually the problem but when it collapses a powerful impact occurs on the blade. 


Now for cavitation to occur on a model propeller we can ignore the water pressure above it since, compared with the full size, our propellers are barely submerged.  This still leaves Atmospheric Pressure which is a nominal 10 to the power 5 Pascals or alternatively 15 psi.  I have to confess that I can always visualize a 15 pounds weight pressing down on a one inch square more easily than 100000 Newtons of force spread across one square metre. The water may be warmed up a little by the action of the rotating propeller, but I doubt it will drastically raise its vapour pressure.


Some of the very highly powered model boats could suffer from cavitation ,if so, they probably ought to be using "Super-cavitating" props which have a different blade section.  But, for the rest of us, it's hard to see how our propellers could create such a pressure reduction on the front surface to justify the cry of  "cavitation" and it must be something else.


"Aeration" is the something else, that is the propeller sucking air down into its front face.  Given half a chance a propeller would prefer to do this since air is less dense than water, with lower viscosity and so requires less effort to move.  You can often see this effect when a model with a submerged propeller and initially at rest, has full ahead power applied.  The water surface at the hull sides just ahead of the position of the propeller forms a depression.  An even more dramatic effect can be made if the model is moving astern before full power is applied.


Avoiding aeration is usually achieved by being a little more gentle with the throttle control.  Taking a few seconds to advance the throttle ought to avoid a noisy start.  If, however the propeller is close to the surface, maybe even with the tips exposed at the top of their rotation, then aeration might be difficult to avoid.  I have occasionally encountered this problem and usually cured (or at least made it more acceptable) by increasing the draught, using a propeller with a smaller diameter or just becoming ultra gentle with the throttle control.


Some model types are worse for aeration than others. Destroyers with slim hulls and propellers close to the surface immediately come to mind.  Models based on single screw merchant vessels can be a real pain.  But, I can usually learn to live with the slurping noise but not the cry of "Cavitation"!


Glynn Guest


       
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Geoff

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Re: Cavitation?....Not!
« Reply #1 on: January 27, 2020, 01:07:27 pm »

Well explained!!


Geoff
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