I am looking for a light-weight high-flow low-pressure low-cost 
reversible DC pump.  (Is that all, you might ask?)  

Specifically, it has to satisfy the following requirements:

High flow to weight ratio:  Specifically, I'm looking for a pump that
can pump its own weight (assume water) in about 5 seconds or less.
So for example, a pump that weighs 1 pound would have to have a flow
rate of approximately 1.5gpm.  5 seconds is just a guideline - I'm 
looking for the highest ratio that is available at a reasonable cost.

The overall weight of the pump is preferred to be in the .5 to 5 pound 
range, but others will be considered.  The requirements are for it to
be reversible, but a two-pump solution would also be okay (note that
the total weight must satisfy above constraint.)

There are no significant pressure requirements for this pump.  In other
words, it only has to "draw" about 1 foot or so.  More is fine as well,
of course.  

It needs to be DC powered.  6-24 volts or so, I would guess.

Lastly, it needs to be low-cost.  Hopefully under $100.

In terms of an answer, a link to an online catalog showing a 
suitable pump (or range of pumps) is best.  If not available,
a reference to a paper catalog (and a fax of the relevant page)
is also fine.

Thanks.

Hello fnordium,
When I first read your question one thing came to my mind "bilge
pump". Bilge pumps are light weight, high volume, low pressure, and
cheap. Here is a link to specs on the pump I would choose to satisfy
your requirements:
http://www.hopkins-carter.com/catalog/BilgePumps.htm

The Piranha Series Bilge Pumps look good and the 1000 gph pump would
more than do what you want.

1000 gallons/hour / 60 min/hr / 60 min/sec = .27 gal/sec

.27 gal/sec x 8 lb/gal = 2.16 lb/sec x 5 sec = 10.8 lb.

The weight is not shown in the specs, but I called Hopkins-Carter Marine
and was told that it weighs less than 2 1/2 pounds. I suspected it
would since the 1500 gph pump listed further down on the page weighs
only 2 lb 10 oz. You can use 2 pumps as you suggest and still come in
well under $100. From personal experience bilge pumps are extremely
rugged and reliable. I believe this is the ideal solution to your
problem.

Good luck, Redhoss

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Request for Answer Clarification by fnordium-ga on 15 Dec 2003 17:27 PST

Thanks Redhoss -

Bilge pumps definitely satisfy my stated criteria,
and I've ordered a pair to experiment with.

I have a couple of concerns about them, and I would
appreciate it if you are able to address those.  Based
on what they are used for, they appear to require 
pressure at the input to work.  Put another way, the
pump must be below water level.  Is this correct?
If so, what other pumps could I use if I was not
able to have the pump below water level.

Also, they appear to have no input connection per se -
they just draw water in through the bottom?  I need to
use these in a "closed" system - i.e. where there is
liquid in different containers connected by hoses and
the pumps will connect to those hoses.  Do I just need 
to create some kind of plumbing enclosure around the pump?

Thanks.

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Clarification of Answer by redhoss-ga on 15 Dec 2003 18:26 PST

You are exactly right. Bilge pumps must have what I would call
"flooded suction". From the way your question was worded it appeared
that a bilge pump would be ideal. As you say you can fabricate some
sort of enclosure around the suction side of the pump. You will need
to make the suction connection at least as large as the discharge
(larger the better). I hope that you can allow water to flow into the
suction because bilge pumps are not designed for suction head and
probably will at least need to be primed. However, it will be well
worth experimenting to make the bilge pump work because other type
pumps that I know of would be much more expensive and probably weigh
quite a bit more. Please let me know how the pumps work out.