> Justify!
>
> 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.
That's what I meant by 'true' cavitation, which first appeared as a
major problem in Parsons' TURBINIA. A propeller blade works essentially
like an aerofoil, with most of the force being developed as 'lift' on
the forward surface - the more rapid flow over this convex surface
reduces the pressure of the water, and can reduce it to below the
vapourisation pressure of water at ambient temperature, in which case
the water will 'boil' and the collapse of the resulting bubbles of water
vapour will pit the surface of a propeller very rapidly (half an inch in
a few hours) - extensive research was carried out on this in the 1950s
by the Navy as discussed by DK Brown in 'Rebuilding the Royal Navy'
(Chatham Publishing 2003). However, this sort of problem appears on
destroyers with 40,000 hp and I assume Terry's power is rather less?
In a destroyer the propellers are well below water and below the hull,
and cannot easily suck in air from the surface, but on a nb if the
propeller is only a few inches down, this easily happens - when I first
got my boat and she had no plates, there was a whirlpool about 18"
across on the right side - pretty, but not very efficient! 'Ventilation'
is a useful term to distinguish this from 'true' cavitation as the
remedies are quite different - 'true' cavitation won't be solved by
ballasting and needs a large propeller or a very special propeller
design to solve it. As I said before, it gets worse as power/speed
increases so I don't think it's the problem here.
>
> Bru
>
Sean
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