Re: [PSUBS-MAILIST] Hydraulic Drive Unit

["Gary R. Boucher" <engineer@sport.rr.com>]

Mr. Starck,

    I am not an advocate of this method for all subs.  Much to the contrary 
as you would find by reading my notes to SubQuandary trying to dissuade him 
from the use of hydraulics.  There are approaches that are more efficient 
and simpler for many applications.  But, in engineering you will find that 
throwing a blanket condemnation on a certain practice closes many doors to 
you.  Also, many of the "Successfully Addressed" points that you bring up 
are perhaps successful, but at what cost?  Lets look at some of your simple 
alternatives.

Directional Control:

Rudders are nowhere as efficient as vectored thrust in turning a large boat 
such as mine.  Dive planes and rudders to a perhaps lesser extent require 
forward velocity of the sub to be effective.  Nozzles are a most expensive 
alternative not to mention many complexities of design.  Remember Mr. 
Starck that these are not builders with high levels of expertise or 
resources.  Steerable motors to me represent the only viable alternative as 
I have mentioned in my posts.  Most on PSUBS will be limited to modified 
trolling motors.

Carbon/Ceramic Shaft Seals:

Running one or two shafts from inside the pressure hull to outside 
propellers is no small engineering task, as you would see if you attempted 
a design from the ground up.  I know it sounds simple, but try engineering 
a sub such as I designed and constructed using power coupled to the 
propeller system with thru-hull shafts.  It can certainly be done, but you 
might be surprised at the extra measures you must take to make this work.

Reversible Motor Switches:

Certainly this can be done, but as an electrical engineer, I will tell you 
that there are issues which will limit the type of motor you can purchase 
for this application.  For example, any motor that is not a permanent 
magnet motor will have to have field and armature currents reversed 
separately.  This may require a major alteration of the motor itself.  Even 
with PM motors, reversing will require some level of effort.

Weight Distribution:

How can you say that weight distribution is not an issue?  Weight 
distribution is just as much an issue as any of the other issues I am 
addressing here and is certainly a component of the package of sub 
design.  It's all part of the big picture.

Ballast Control:

My sub does not have dive planes at all.  Few subs fall in this 
category.  I obtain 100% of my up-down bubble with weight control and that 
requires hydraulics given my boat geometry.  Electric motors would add an 
order of magnitude in complexity.  But, I am sure you have a simple fix for 
this too.  There is absolutely no safety issues related to using the 
hydraulic system for both applications.  A smaller hydraulic system would 
be required anyway to move the weight that I use for pitch control.

Pressure Housings:

Sure they are reliable, but they must be designed.  They must be 
constructed.  And, they must be checked often.  They must also be capable 
of dissipating the amount of heat generated while holding the motor at a 
reasonable temperature.  Compensation is not simple in most cases.

Leak Detectors:

Sure, lets put in some inspection ports in our external motor 
housing.  Lets add some leak detectors too.  We can hook the detectors into 
the instrument system so the pilot can see if the motor housing is leaking.

     Now, I am going to make a few statements here and leave this issue for 
the time being.

(1) I am not advocating using Hydraulic Propulsion for every or even many 
designs.  It worked for me in solving several major problems in the 
engineering process.

(2) Coming from the standpoint of most PSUBS people, what you think is 
simple is going to create major issues for them.  Believe me, I have been 
through the design process of many systems, both mechanical and electrical.

(3) Engineering anything such as a PSUB is going to require much thought 
before a final decision is made in regard to anything in the design.  To 
chart out an absolute path illuminating possibilities prior to studying the 
other factors that go into a design is...  Well... Unacceptable.

(4) I get somewhat irritated when people that have not gone through this 
process sit back like armchair quarterbacks and make broad reaching 
technical statements.  I can assure you that if you have not gone through 
this experience, you have much to learn about the sub engineering process.

(5) Hydraulic Drive is used on a number of subs, and subs that are not 
under the amateur heading.  I am not sure of how many there are out there, 
but do some research and fine out why they used it.

Gary Boucher





At 11:22 PM 11/19/2002, you wrote:
>The points you mention have all been successfully addressed in numerous
>designs without incurring the additional cost, bulk, complexity and
>inefficiency of hydraulic propulsion.
>
>On your specific points.
>
>1. Directional control is adequately provided in almost all larger subs
>by rudders, nozzles and diving planes. In small subs requiring high
>stationary maneuverability external steerable motors are preferable and
>have proven to be very reliable.
>
>2.  Carbon/ceramic shaft seals are readily available, highly reliable
>and not expensive.
>
>3. Reversible motor switching is again common, reliable and not costly.
>It presents no problems that  make hydraulics preferable.
>
>4. Weight distribution of electric motors is likewise a non-problem in
>this application.
>
>5.  Certainly hydraulics would be an excellent way of  moving ballast
>weights but this can be done quite effectively by a simple manually
>powered system.  In any case having separate systems is generally
>preferable to having everything dependent on a single central system.
>
>6.  Pressure resistant motor housings present no major problems and are
>reliable.  On the other hand pressure equalization is also not difficult
>to reliably achieve.
>
>7. Anything that leaks presents a problem but the means to avoid this
>are well known and not difficult to do.  Leak detectors and inspection
>ports are also easy to provide.
>
>8. Although hydraulic motors are compact the entire system is not and an
>even larger motor is required to compensate for the power loss from
>hydraulic and mechanical friction such a system imposes.
>
>The reason existing subs don't use hydraulic propulsion is not because
>everyone else is stupid or unaware.  Many subs do in fact often use
>hydraulics for other functions.  They don't use them for propulsion
>because they don't offer any important advantage and do have
>disadvantages for this purpose.
>
>You can, if you choose, do it and if  properly done it will work well
>but it solves a non-problem at the cost of  added expense & complexity
>plus reduced performance.
>
>Walter Starck
>Golden Dolphin Video CD Magazine
>The premiere publication of diving and the ocean world.
>www.goldendolphin.com
>
>
>----- Original Message -----
>From: "Gary R. Boucher" <engineer@sport.rr.com>
>To: <personal_submersibles@psubs.org>
>Sent: Wednesday, November 20, 2002 11:21 AM
>Subject: Re: [PSUBS-MAILIST] Hydraulic Drive Unit
>
>
> > Well, that may be so Mr. Starck.  However, lets think about some
>things
> > that you may have missed.
> >
> > (1) Unless the motor is completely outside the hull you will have
>problems
> > with directional thrust.
> >
> > (2) If the motor is inside the hull you have a thru-hull shaft seal to
> > contend with of which, if it ruptured, you have a bad situation.
> >       With hydraulics there must be two major breaches.
> >
> > (3) The motor will have to be reversible (not the case with hydraulics
> > using a 4-way valve).
> >
> > (4) The bulk of the weight of the propulsion system can be more easily
> > centered in the hull.
> >
> > (5) The hydraulic system can serve dual purposes in both propelling
>the sub
> > and furnishing hydraulic actuation to move heavy keel weights as in my
>sub.
> >
> > (6) Pressure compensated motor compartments are a headache as they
>require
> > continuous air, or else a bladder to keep pressure away from the
>seals.
> >
> > (7) Corrosion can be a major problem in motor housings that leak.
>Just
> > knowing that there is a leak requires special inspection.
> >
> > (8) The higher HP systems are easier to implement this way, because
> > hydraulic motors are very compact and light weight per amount of power
> > produced.  The bulk of the system can be located in-board.
> >
> >
> > Gary Boucher
> >
> >
> >
> > At 06:10 PM 11/19/2002, you wrote:
> > >Using an electric motor to drive a hydraulic pump that powers a
> > >hydraulic motor to turn a propellor to  propel a sub is a really
> > >complicated way of doing what could be much better done by using the
> > >motor to turn the propellor directly.  The latter is far cheaper,
>less
> > >bulky, more reliable and more energy efficient.
> > >
> > >Hydraulics are a good way to distribute power especially where high
> > >force is required but they consume a lot of energy in themselves and
> > >offer no advantage in this application.
> > >
> > >Walter Starck
> > >Golden Dolphin Video CD Magazine
> > >The premiere publication of diving and the ocean world.
> > >www.goldendolphin.com
> >
> >
> >