Roger Long wrote:

"Stephen Trapani wrote:


Or, of course, the capacity of the batteries.


And, in this case, it would have had to be pretty impressive capacity.
After the fiasco with the oil sensing switches, I'm all for the
simplicity promoted above. What's driving this now is the fact that
the pumping capacity I want (enought to have a fighting chance of
surviving a burst stuffing box hose), combined with the hose lengths
forced by the design of the boat, means that any float switch on the
market will go into an endless cycle of pumping out the hose.

****ing big centrifugal pump (or even two) with big hose with its float
switch mounted above that for a little, positive displacement diaphram
pump with a small hose?

The little pump doesn't affect the reliability of the big one but should
be enough to cope with the runback from the big hose.

Alternatively, back to the relay idea:
Take big pump and upper float switch (NO), wire in series, pump -ve to
batt -ve, other side of switch to +ve supply. Wire the relay (NO)
contacts accross the upper float switch. Wire a lower float switch (NO)
in series with the relay coil and feed it from the pump motor +ve.
Return the other side of the relay coil to the pump -ve (which is also
the battery -ve) Wire RC snubbers accross both the relay coil and the
motor as near as practical to them and a reasonably tolerable buzzer
accross the motor. Feed from an appropriate fuse directly off the
battery with a high dB output pizeo beeper accross the fuse. Put the
sounders and the relay (in a socket) somewhere where you can get at them
if you have to. e.g. back at the panel. You need four wires down to
the bilge, (Upper switch +ve, common junction of upper switch -ve &
lower switch +ve & motor +ve, lower switch -ve [to relay coil], motor -ve)

Now lets consider the possible failure modes:

1. Upper switch fails open: No alarm, bilge can flood silently till boat
sinks.

2. Upper switch fails closed: Continuous buzzer, bilge is emptied until
eventual flat battery or pump burnout

3. Motor OC: Continuous buzzer if enough water in bilge until the boat
sinks

4. Motor SC (or jammed so fuse blows): Continuous extremely loud
unignorable high pitched tone untill fault cleared and fuse replaced,
beeper smashed or disconnected :-) or boat sinks

All the above are the same as for a simple pump setup with one switch
and no relay. The only difference is you have two audiable alarms.

5. Relay fails open or lower switch fails open: Circuit reduced to
simple switch and pump, you hear a double (or more) buzz due to runback
restarting the pump, boat does not sink.

6. Relay fails closed or lower switch fails closed: Continuous buzzer,
bilge is emptied until eventual flat battery or pump burnout or untill
you pull the relay out of its socket which puts it back to the simple
single switch case.
**** This is the only increased risk ****

The snubbers have a resistor + a capacitor in the same package.
Resistors almost always fail by going open circuit which would make the
snubber ineffective but not cause an immediate circuit failure or the
capacitor might short, in which case the resistor would limit the
current until it went open and the circuit would keep working. If you
are fairly (justifiably?) paranoid, you might use two snubbers in
parallel in place of a single one in each circuit position.

Finally for backup security fit the second pump which only needs a
simple switch a little higher in the bilge than the first pump's upper
switch with pizeo sounder accross its fuse and another accross the pump
(+ another RC snubber) off your other battery. This catches any fault
with the first pump that haven't sounded its alarm.

(NO stands for Normally Open contacts)

N.B. *DONT* PUT THE LOWER SWITCH FOR THE FIRST PUMP IN SERIES WITH ITS
MOTOR (as per your JPEG) OR YOU DOUBLE THE CHANCE OF A FAILED SWITCH
STOPPING THE PUMP WORKING WITH NO ALARM.

Hope that helps, Roger.
Have I missed anything Larry?
--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
[at]=@, [dash]=- & [dot]=. *Warning* HTML & 32K emails -- NUL:
'Stingo' Albacore #1554 - 15' Early 60's, Uffa Fox designed,
All varnished hot moulded wooden racing dinghy.