T&T: Bilge pump sizing

[Candy Chapman and Gary Bell]

Sam Agajanian asked:

With all the talk about bilge pumps, general usage AND emergency,  wouldn't  
a 110 Volt basement pump work as a good back up. Cheap, powerful  and able to 
deploy anywhere needed?



Mister Science replies:

Yes, those pumps work really well, I have one and use it regularly.  For 
an onboard emergency however I could not count on finding reliable 
nearby 110 volt power.  Likewise at an extended anchorage I would not 
have the genset or inverter running all the time, and the inverter/house 
bank alone could not survive long running even one of the secondary 
pumps.  I have a 12 volt 3700 gph bilge pump in a bucket.  The bucket is 
one of the colorful square three gallon buckets I bought full of laundry 
detergent.  It is pierced by a thousand or more drilled holes, is 
topless and the bucket bale is bent to an inverted V shape to facilitate 
hanging the bucket from a tether line.   I have a ten foot line attached 
to the bale, and the electric line and the discharge hose are clamped to 
it.  The pump bucket is kept handy to lower into any flooded space, and 
I have also used it as a portable emergency sea chest when I packed up 
my regular cooling water intakes with weeds in a shallow anchorage.  I 
dipped the bucket in the water and patched the discharge hose into the 
affected cooling water intake line.  The electric lead has a cigar plug, 
and all the spaces aboard have cigar plug sockets.   For real emergency 
power I keep a group 31 battery in a portable battery box (otherwise 
used for a trolling motor, for occasional jump starting and of course 
for upper body weight training), and it too is fitted with a nice cigar 
socket. 

My boat is a power catamaran, and of course I have twin primary and 
secondary bilge pumps to accomodate my twin bilges.  For routine bilge 
pumping, I have a small primary pump mounted as low as possible, so as 
to slurp the regular run of condensate, drips and dribbles to as close 
to dry as possible.  I built a three inch high dam to limit those 
routine drips to the aft quarter of the bilge, keeping the forward three 
quarters dry and clean.   In addition to the aforementioned power 
availability issue I mentioned above,  as a practical matter, I don't 
see any of these little pumps in 110 volt, while they are abundantly 
available in 12 volt.  I have a much bigger 12 volt secondary pump with 
its sensor float switch mounted about five inches above the little 
primary pump's, and it is intended to take up the heavy pumping if I 
have some flooding.  The third stage is the bucket/pump mentioned above, 
and fourth is a plain bucket. 

Incidentally, in Katrina, a sistership of my boat had one of its bows 
totally crushed by a hit-and-run casino barge, and after consoling the 
owner, we PDQ owners were tickled to find that completely flooding one 
hull will not sink the boat, as water never rises above the bridgedeck 
leaving the other hull dry.  The designer even told me that the 
intrinsic floatation of the foam cored hull structure alone should float 
a completely swamped normally loaded boat. 

That ought to stir up a lively discussion... ;-}
Gary Bell, aka Mister Science