Re: Blue water White Death

[Phil Nuytten <72020.572@compuserve.com>]

Hi, Ken:
        There are a number of ways to effect an 'auto-hover' or
'depth-hold' - usually electronic, these days . .it is difficult, if not
impossible, to consistently hold depth at usual decom levels if you are
using a good-sized soft or fairwater ballast system - typically the exhaust
is under the tank and the air is not all 'left behind' but a good part of
it forms a bubble under the sub - the shallower the depth, the more
pronounced the problem. With elec. soft ballast control you dump through
large solenoid valves atop the ballast tanks and add by an LP solenoid
valve from your onboard air.With manual, the same but you physically turn
the valves on and off.This is a clumsy system, IMHO. I much prefer to use
soft ballast only on the surface - use syntactic foam (and lead shot bags
to match payload) to bring the sub almost neutral - just slightly positive,
and use a small hard ballast system to make up for depth compression, 
picking up small objects, etc.  Most of the up and downing is down with the
vertical thrusters. With this type of system, you are neutral throughout
the dive and when you're ready to call it a day, you just make yourself 20
lbs or so light and float quietly to the suface - once on the surface, blow
the fairwater  ballast and rise  as high as your tanks. If you have good (
read 'expensive') thrusters, you can easily auto-hover by a hover-lock
button - this simply cuts an electronic pressure sensor into the thruster
control system  - it locks on the depth (pressure) - if the outside
pressure increases, it speeds up the vertical in an up thrust - and
reverses the procedure for pressure decrease. This calls for a fairly
sophisticated thruster control system - it is the system used by most
high-end work ROV's . . .and is used with a 'normally neutral' system. The
ability to have very fine vertical control is a major reason that some
systems owners accept the cost and complexity of variable pitch
thrusters.With the NEWTSUIT, for example, the operator could easily hold
vertical position within a couple of inches ( with a visual reference) as
this was a requirement for working mid-water. We used a Nuytco designed
'cruciform' thruster for this, with seperate variable pitch propellors
fixed at ninety degrees from each other, powered by a single motor. You
should see the right angle drive with a hollow center to accept the
pitch-changing shafts!! . . .yup, could use both at once. System runs on
460 volts DC to minimize umbilical size and ran at continous 3000  RPM -
almost no current draw at zero pitch - but you could cut in full pitch
instantly! A good operator could turn somersaults with this system -
providing you had somewhere to hook your toes or manips!

Regards
Phil Nuytten