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Begin forwarded message:

> From: Jared Sherman 
> Peter-
>   don't function well in the water nearer the surface that contains more
> foam
> than deeper water with less air bubbles.>
>
>  Not right at all. Cavitation is, specifically, that phenomena where
> water literally BOILS in the relative low pressure zone at the
> trailing edge of the screw. Since the water pressure is lower closer
> to the surface, it is easier for air to boil out in the water closer
> to the surface.
>
>  Take a big enough prop, put enough horsepower behind it, and the
> right shape to the edge. and you can make it cavitate at any depth.
> Reduce the horsepower, reduce the prop speed, and you reduce the
> pressure differential and the boiling stops.
>
>  Another one explained in the engineering texts.

Well, lets treat with suspicion the word 'cavitation' and start again.
Water closer to the surface tends to contain more air bubbles than
water lower down. As the surface becomes more agitated - big seas - it
becomes a real advantage having the prop as low as possible, as it
can't function effectively in foam.  Some boats, catamarans especially,
have outboards that can be lowered into the water when needed. Its not
a bad idea; has the virtues of simplicity, no drag when not needed,
less weight (sorry, mass) ease of servicing, etc, but does also have
this important disadvantage, that the prop which typically sits just
below the surface tends to 'cavitate' rather too easily.

In the  Volunteer Bushfire Brigade I belong to we use that same term -
cavitation - for when air finds its way into the water pumps, the
effect seems to be the same. The remedy is to re-prime the pump.

   

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