submarinehunt Wrote:
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> Just to make sure I have understood correctly, in
> general use, the ballast tanks are either full or
> empty.

That's right.

> The fine regulation of depth is by
> adjusting the subs buoyancy, and this is done
> using "regulation tanks".

No, depth is controlled by means of hydroplanes.
From the beginning: on the surface, the sub has positive buoyancy
(the ballast tanks are empty). To submerge, the positive buoyancy
has to be destroyed - the ballast tanks are flooded. When the sub
is submerged, it would be ideally, if it has buoyancy equal to zero.
In such case, it would float in the water. As you said, it is very
difficult to achieve such condition - so the buoyancy is almost equal
zero - but without propulsion (that is acting the force resulting
from propellers and hydroplanes) it has tendency to sink or to surface
(that depends on if the current buoyancy is negative or positive).

So, assuming that boat is submerged with buoyancy almost equal to zero,
it can control its depth by means of hydroplanes (that is it changes
depth dynamically).

As you know, the weight of the boat is changing during the patrol: the
food supplies, the fuel, the torpedoes are used. So each day, the sub
had to be trimmed - it submerged, and then - fine flooding (or blowing)
regulating tanks, the "almost zero" buoyancy condition has been restored.

When changing depth dynamically (with hydroplanes and propulsion), when
you are going deeper, things become complicated. There are some leaks
(through the stuffing boxes for example) which makes boat heavier.
The pressure hull is also compressed by increasing external pressure. That
means, that its volume (buoyancy) is decreasing. So going deeper, you
have to compensate this loss of buoyancy - by pumping out regulating tanks.

> I have read that a sub maintains depth using two
> methods, buoyancy control and using the
> hydroplanes ( dynamic control). Perhaps we should
> discuss dynamic control later in another post. But
> I have read somewhere, that precise control using
> buoyancy alone is very difficult. I.e. say as
> commander, i wanted the U boat to be stationary at
> periscope depth, would this be something easily
> done while stationary....

Keeping the boat at the same depth, when it is stationary is very
difficult - it requires the boat to be perfectly trimmed (adjusting
its buoyancy to zero). At the periscope depth there was a trick:
if you have boat with almost zero buoyancy, you can rise up your
periscope. If the boat is too heavy - it starts sinking, so the total
volume of the boat (with scope) become larger and the buoyancy is
increasing - the boat stops sinking. If the boat is too light - it
starts surfacing: the total volume (with scope over the surface) become
smaller and thus buoyancy is decreasing - the boat stops surfacing.

> or would the submarine always be in motion, in order to use the
> hydroplanes ?

Generally the boat has to be in motion to keep depth. The greater
difference between current buoyancy and zero buoyancy - the greater
speed is required to keep current depth.

--
Regards
Maciek


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