Re: [PSUBS-MAILIST] Hydraulic Drive Unit

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

Walter Starck wrote:

>Gary Boucher wrote: " 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 do too but with over 40 years experience in designing, building,
>operating and maintaining a wide variety of underwater and marine
>equipment I have gone through the process.

I have no idea what your credentials are.  Forty years of "experience" is 
pretty vague, but lets say that you have experience in actual design and 
construction of manned submersible propulsion systems.  The issue that I 
raised stands.  You are making a blanket condemnation of hydraulic 
propulsion and this is an uninformed declaration.

>Alec Smythe Wrote: "In an earlier post Gary pointed out that in
>hindsight, he recommends compensated trolling motors for their
>simplicity."

Lets set the record straight here.  I do recommend pressure compensated 
trolling motors for most applications where people are designing 
PSUBS.  There is a simple reason for this.  It is the easiest approach for 
the PSUBer who has limited knowledge of other approaches.  A thru-hull 
shaft is probably the very last thing I would recommend for these 
people.  I don't recommend hydraulics for every application.  I don't 
recommend hydraulic propulsion for most subs.

You probably have no idea what the design philosophy of my sub is.  You 
have no idea what the intended purpose of my design was, or is.  You are 
placing yourself in the position of an expert and basically saying that all 
hydraulic propulsion is a bad idea.  I strongly disagree.

If hydraulic propulsion is such a bad idea, why don't you take this 
campaign to the manufactures of thrusters that are driven by hydraulic 
fluid.  They are on the market.  They must sell because they still make them.

>This seems to agree completely with what I have said although Gary now
>seems to disagree.

No, no change in my opinion.  The main reason that I would rethink my 
design if I had it to do again is weight.  I am marginal on my weight and 
would for that reason like to have some extra buoyancy provided by motor pods.

>Sean Stevenson wrote: "For the homebuilder, overcoming the efficiency
>issue is the only real hurdle for emplying a hydraulic system."
>This like saying overcoming gravity is the only real hurdle to building
>a flying saucer.  High friction losses are inherent in hydraulics.  For
>brief or intermittent operation or anywhere power is not limited this
>loss may not be important.  In small submersibles however, available
>power is a limiting factor and taking a 30% or more efficiency hit on
>usage is an important consideration.

I will quickly admit that efficiency can be an issue.  Whether this is a 
deciding issue or not cannot be judged by anyone without first fully 
understanding what the design emphasis is.  Engineers learn very quickly 
that nobody can build the perfect car, airplane, boat, submarine, 
etc.  Compare a Jaguar  to a Lincoln.  Each is an excellent car in its own 
right.  Each has a totally different functional design, a totally different 
purpose.  If fuel efficiency is your issue, buy a Taurus.

Each design is an optimization of purpose based on a very extensive set of 
tradeoffs.  The engineer's main purpose is to make judgements as to what is 
important and what is not, what is going to promote the design philosophy 
and what is not.  Good engineers are going to weigh the merits and balance 
the pros and cons.

I hear a lot of talk about propulsion on PSUBS.  I hear some really 
outlandish proposals.  Most are not feasible, but I seldom discard these 
ideas, in that I put many of them on the shelf for later consideration.

>Carsten Standfuss obviously understands the issues.

Carsten apparently is a good engineer and builder.  But, just because he 
elected to not use hydraulics for his controls does not make, in itself, 
hydraulic controls a poor decision.


Gary Boucher