<..>
> > Oh, and on the 'cavitation vs ventilation' issue, take a look at:
> >
> >
> http://www.powerboat->
training.co.uk/articles/articles-propellers-venti
> > lation,%20cavitation.htm
> >
> > No idea if it's right or not, but it sounds authoritative to me.
>
> Looks just about reasonable explanation to me.
> The effect (on the boat) will be the same - after all one is
> mixing water
> with gas : air=ventilation; steam=cavitation (yes steam -
> water boiling =
> steam, even at low pressures as experienced by cavitation).
That's the nub of the matter. Whether you call it cavitation or
ventilation and however it's caused the end result is much the same
(except that vapourisation of the water flowing over the prop can cause
erosion of the blades which is very unlikely when the problem is due to
air being drawn in from the atmosphere)
This has been very enlightening and educational and I can see some
justification for differentiating between the two different processes
that produce what is commonly known as "cavitation". Never the less, I
suspect that "ventilation" will continue to be called "cavitation" on
the cut for the forseeable future!!!
In any case, I am yet to be convinced that it is incorrect to use the
term cavitation to describe the *result* of ventilation. Whether the
"holes" are caused by air being drawn in or by vapourisation of the
water flowing over the blades the end result is pockets of low pressure
vapour (be it steam or air) in the prop wash which cause turbulence,
noise and vibration. That, by my lights (and according to the dictionary
definition), is cavitation.
The *cause* may be ventilation (which does seem to be a term in common
usage although I have some difficulty with it myself as it implies a
deliberate and desirable provision of air) or vapourisation and
distinguishing between the two is obviously important since the remedies
are totally different.
Vapourisation can, I would think, be ignored in inland waterways craft
since it occurs on high performance propellers turning at high
revolutions. Props on narrowboats and inland cruisers are not generally
high performance and they don't go round that fast!
Ventilation (still don't like the word but we'll stick with it) may
result from poor hull design eg; a swim shape that causes a standing
wave in just the wrong place (as someone described earlier ISTR) or an
uxter plate that traps a pocket of air under the counter or simply bad
trim. However, whatever the cause, getting the stern deeper in the water
is, assuming the boat doesn't sink first, an infallible cure for the
problem.
As Sean mentioned President used to cavitate due to ventilation (there,
that's everybody either kept happy or terminally peed off!!!) and the
proplem was cured during one of the trips I was involved in by moving a
couple of tons of bagged coal from the front of the hold to the back of
the hold. I've run into the same problem on other working narrowboats
and the cure has always been the same - shift some of the load to the
back of the hold and get the blunt end deep enough in the water to make
the problem go away
It's noticeable that most modern narrowboats built for leisure use run
with the counter just barely touching the water at best if not actually
out of the water. A loaded working motor narrowboat would have the
counter *in* the water by several inches (and a loaded FMC steamer at
full chat will damn near pull the counter *under* the water!)
This also explains why cavitation due to ventilation often goes away at
higher revs / speeds. At low speed, the blade isn't sucking air in, as
the speed rises it starts to ventilate and cavitation occurs. Then as
speed further increases the stern of the boat will tend, by differing
amounts depending on the hull design, prop shape etc., to "sit down" in
the water increasing the draft at the stern by enough to stop the
ventilation occurring.
This thread has been a pleasant interlude and a welcome flash back to
the good old days when such weighty matters were furiously debated to
the delight and erudition of one and all. Can we start again in about
three weeks please 'cos now I've got to go and get on with getting ready
for the great GIG and National Festival excursion which commences in a
rather worrying 2 days and a bit!
ATB
Bru
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