Just an idle question. I was considering whether the depth of a propeller on a full size vessel had any bearing on its efficiency or reduction in cavitation? The deeper it is set the more pressure - so is the water denser - if so is that an assistance?
On full scale ships the main causes of cavitation are the propeller load and the inhomogeneity of the hull's wake field in the propeller plane while the propeller is working.
Examples of highly loaded propellers are those of e.g. tugs or fishing vessels with a large added resistance when they operate in towing.
Also highly loaded propellers are those of ships of great displacement and of high block coefficients (i.e. fullness of the hull, where 1 would be that of a cuboid with the edges of the main dimensions V / L X B X T) such as VLCCs or ULCCs (CB often >0.85, with high curvature gradients in the aft ship).
Or high-speed propellers where beyond certain speeds, since cavitation cannot be avoided anymore, so-called supercavitating propellers are often employed.
As was already quoted from a Wikipedia article, cavitation occurs where the pressure drops beneath the vapour pressure of the water so that the water literally begins to boil in small bubbles until these bubbles reach an area of higher pressure again when they suddenly implode.
Because these bubbles or sheets of cavitation not only disturb the fluid flow around the propeller blades they also cause an increase of resistance due to the virtual widening of the propeller sections' thickness and flow separation which both results in a notable drop of propeller efficiency.
When these sudden implosions of the bubbles occur close to the propeller's (or the hull's or other appendages' surfaces such as rudders) not rarely erosion at the affected surfaces happens which can end up in the whole destruction of the propeller.
Also nasty noise and vibrations can be caused by cavitation.
Though the inception of cavitation is often hard to predict one can easily imagine that on the propeller's suction side (aka the propeller's back because we look at the propeller from stern to bow when defining whether it is rotating clockwise or anti-clockwise),
where the blade sections have convex camber, and the pressure drops significantly because the fluid gets accelerated and lift (or thrust) is produced (Bernoulli's Law)
that there are for sure areas susceptible to cavitation.
But propeller face side cavitation is also quite common.
In opening I mentioned that another common cause is the inhomogeneity of the flow around the propeller blades.
A poorly designed hull shape (especially this of the aft ship) or a hull-propeller-arrangement being coerced into operating in off-design states can also lead to heavy cavitation.
Sometimes the susceptibility to cavitation can be mitigated by using ducted propellers such as in tugs and trawlers.
Because the effect of the propeller's submergence on cavitation was addressed, yes it does have a small effect at least for the propeller blades in the 6 o´clock position because the higher hydrostatic pressure with greater depth may kind of retard cavitation there.
But the effect compared to the mentioned main causes propeller load and propeller wake field are I would think more negligible.
But indirectly the "depth" of the propeller has a great effect indeed.
In general, the bigger the propeller's diameter, the bigger its efficiency. And with diameter inevitably comes submergence.
As also mentioned in another post, water is considered an incompressible fluid and thus its density should only matter as far as its accompanying temperature is concerned.
Of course, the colder the water the later does it begin to boil (at constant pressure).
Now if the answer is yes then can we apply that to models or is the possible difference in screw depth just not significant enough.
Yes, as you presumed, I too would consider the effect in model scale insignificant.