Re: [Paddlewise] bilge pumps - batteries vs feet?

From: "Keith Wrage" <keith.wrage_at_charter.net>

> Some folks use a battery-powered bilge pump - what do they use for a power 
> source?  

Folks seem to choose different solutions.  Some use NiCd, others lead acid.
You'll need at least 1 Ahr capacity by my reckoning, assuming you'll do 
repeated pumping during rolling and rescue practice.  It doesn't seem
to cost a lot more to get 2Ahr if you choose lead acid batteries.  You'll
only get about half the capacity with lead acid unless you want to shorten
the life of the battery - sucking out less than 20% of the capacity results 
in long life.  NiCd can be fully discharged every time (well, down to about 
1V - after that they suffer IIRC).

My preference is for AGM lead acid - these are completely sealed and are
not considered hazmat for transport.  Liquid cell and sealed gel cell are
considered hazardous, BTW.  I use a 5Ahr 12V that I got used from a 
surplus store for C$5 (a 2kg brick - but I can use it to recharge AA
camera/VHF/GPS batteries while on a trip).  New, it would be about C$40 
locally and I figure the used one will last at least $5 worth of battery
life.  The reliability of lead acid is high and self discharge rate is low.

Some people use NiCd D cells (though many NiCd D cells are C cells in a big
case).  You'll need a dozen ,s since 12x1.2V = 14.4V, which is about what 
a 12V battery really puts out freshly charged. As well, when at minimum
charge, it will still output 12V.

Similarly, you could use AA size NiMH (at 1.6 Ahr, 1.2V; most 2000 mAh aren't) 
- a dozen would be around C$60 new.  These would be small and light, but would 
have twice the self discharge rate of NiCd.  Not suitable for a several month 
long trip without a solar charger.  You'll lose about 30% of the capacity in a 
month, so I think the self discharge problems are somewhat overstated if you 
consider a one-two week trip as typical.

There are many submersible pumps on the market designed for marine 
applications.  I found some reviews online that show that the ratings
are often overblown.  They either can't pump as much water as they
claim or use more power or both.  Atwood and Whale are two that 
impressed me - Atwood is one of the few that is conservative in
rating their pumps.  As well, there is a lot of variation in how
much power the pumps actually require.  My Atwood 625 draws 0.9A 
in the kayak (measured while unflooding the cockpit) - less that its 
rating and less than the rating of the smaller Atwood 450!  In fact, it 
has one of the lowest power demands of all the small pumps.  If you want 
to minimize your power requirements (ie want the smallest, lightest battery), 
it's worthwhile investigating this stuff.

I now leave my hand pump at home and keep the minimum legal bailer as a 
backup.

Mike

PS - the foot pump in the recent SK article looks good - better than many
foot pumps I've looked at in the past.  The soft spring and the duckbill
valves look like a good choice.






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