Two electric bilge pumps using a common outlet - feasible

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tillergirl

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I have a shallow bilge so a large pump is not possible. Having just experienced hard pumping greater capacity seems a good idea. Adding a second electric pump elsewhere in the bilge is feasible with a separate power circuit and a non return valve (as currently in-line with the existing electric pump), would the common outlet handle the capacity. The outlet is 1": pump 1 would be 950 US gallons per hour, pump 2 1350 gallons per hour, total panic capacity on the common outlet 2300 US gallons.

Does anyone know the capacity of a 1" outlet. I'm guessing that would restrict capacity?
 
The capacity of a 1in outlet depends on the pressure behind it.
The back pressure from two pumps sharing will reduce the flow from either pump.
As will the non return valve.
You will probably shift more water if you just exhaust the second pump straight over the gunwhale.
You could make up a portable pump with a rigid tube and a hose. I have a small one where the rigid tube is 20mm plastic overflow pipe. For a larger pump you could use 32 or 40mm. Mine is great for deck wash and emptying tenders, and it's ready to use in case of crisis.
It's also easy to lift out of the bilge to clear the strainer. Which will clog!
It's not hard to mock something up and measure the actual flow rate pumping from a big bucket.
My so called 600gph does about half that when lifting 1m and going through a total of 2.5m of hoses. At 12.8V that is. A bit more at 14.4V.

I don't trust non return valves, in my experience they leak back after seeing bilge water for any length of time.
 
NRVs also can stick closed and leave your pump whirring away uselessly.

(I now have two pumps fitted. Could not work out why pump no.1 was spinning but not pumping, only discovered there was a NRV when I came to plumb in the replacement pump!)
 
Hi Tillergirl, to calculate the output I need the horizontal distance to the outlet and the vertical height from the pump to the outlet. Phil
 
tillergirl said:
I have a shallow bilge so a large pump is not possible. Having just experienced hard pumping greater capacity seems a good idea. Adding a second electric pump elsewhere in the bilge is feasible with a separate power circuit and a non return valve (as currently in-line with the existing electric pump), would the common outlet handle the capacity. The outlet is 1": pump 1 would be 950 US gallons per hour, pump 2 1350 gallons per hour, total panic capacity on the common outlet 2300 US gallons.

Does anyone know the capacity of a 1" outlet. I'm guessing that would restrict capacity?
Click to expand...

I would estimate an appropriate size from the discharge connections on the two pumps.

For each measure the outlet diameter and calculate the cross sectional area

Add the two areas together .

Work out what size outlet fitting you will need from the sum of the two areas.

I suspect you will arrive at a figure between 1¼" and 1½". That may not sound much more than 1" but remember 1½" diameter is slightly more than twice the area of 1"diameter.
 
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Phil - I am estimating the vertical height on each pump will be around 1.2m, horizontal, on the basis that the two pumps would be equidistant to the common outlet at about 2m.

I note several comments about NRV failure. Dare I put the mockers on mine by saying that it has survived for 26 years - eek! But what is the alternative? Separate outlet's and another hole in the side I suspect but this may be necessary.

I am not wild about the suggestion re the portable set-up. If single handed and a crisis occurred, setting up a portable set-up would lose critical time.
 
A good starting point is to assume a velocity of 1m/sec in the discharge pipe. Use this to calculate the flow rate that can be accommodated. I can't be bothered to do the calcs this time of night. There are several factors to consider.
1. The quoted pump rates will be a lot less in practice
2. Two non return valves will be necessary but will reduce the output.
It's likely that the most efficient solution will be to have separate outlets for each pump but you won't get anywhere near 2300gpm.
 
ghostlymoron said:
A good starting point is to assume a velocity of 1m/sec in the discharge pipe. Use this to calculate the flow rate that can be accommodated. I can't be bothered to do the calcs this time of night. There are several factors to consider.
1. The quoted pump rates will be a lot less in practice
2. Two non return valves will be necessary but will reduce the output.
It's likely that the most efficient solution will be to have separate outlets for each pump but you won't get anywhere near 2300gpm.
Click to expand...

I did the calc earlier based on 4ft/sec and came up with needing a 2" diameter outlet ... needs someone to check the arithmetic though!
 
I think, as I anticipated from the outset, it has to be two outlets. Reading some Whale data sheets the 950 is quoted at 850 GPH @ 1m head, the 1300 at 1100 GPH. Obviously the actual output will be significantly less because of the practical route from pump to outlet. Worth planning that best rather than just dropping a new pump somewhere here or there. 3.5 metres is stated as the max output. Also both Whale pumps recommend outlets for each at 1"/1 1/8". My wish for a quick easy solution is dashed. Not a problem though. Ghostly and Vic and Phil - ta thanks.

STOL71 - four and a half minutes actually dry to floor boards at 9am last Monday.
 
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