One way valve Versus anti-siphon device on bilge pump pipework

Durcott

Durcott

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Hi Folks,

I have two bilge pumps on my (bilge keel) boat - one manual diaphram type, pumping out to the usual sort of outlet, and an electric one, which pumps out into the boat's outboard well in the stern quarter.

The problem I have, is that when the boat is floated off her river mooring, sometimes she takes on a severe list. I reckon the outlet from the electric pump gets submerged, and then water siphons in until the hull unsticks from the mud. The water does pump out again, but the damp and mud (or worse) is taking its toll on the cabin. I reckon at worst I get 40 or 50 litres in the cabin, which sloshes around due to the acute angle until she rights herself.

So - apart from the screamingly obvious - move the ***** boat - I'd like to consider putting in a valve or an anti siphon device.

I could merge the output from the electric pump into the pipe feeding the manual bilge pump, taking advantage of that pump's internal non-return valve, but then the electric pump would have to work 'through' the diaphram,

OR I could fit a non-return valve to the electric pump, and hope that it doesn't get blocked in an open position,

OR I could fashion an anti-siphon device, aka the thing you see on sea toilet plumbing now and again - not that I have a great deal of freeboard to work with.

I suppose I could add a vertical loop to the electric pump's output, but I've not got a clue how much 'head' a typical pump can drive.

I've got no experience with bilge pumps, and all of my options feel like bodging. Has anyone got any advice to offer?

Thanks

Jeff
 
From your description, I don't think an anti-syphon valve would help. You need to install either a loop, raising the discharge pipework above the heeled waterline, or else a manual or non return valve somewhere in the pipework. I'm not a fan of n/r valves in bilge applications as they can easily be jammed by debris.
If using a manual valve, don't forget to open it after launch.
Small centrifugal pumps are very sensitive to pressure head, so a loop might reduce its performance somewhat.
 
Could you perhaps move the electric pump outlet to somewhere that doesn't submerge as she floats off?

Another option would be a seacock on the outlet, but that's not much good if you want the pump to work unattended while the boat's moored.

I would be wary of non-return valves in the electric pump output. Centrifugal pumps don't seem to generate all that much pressure, it looks to my uneducated eye as if they work more by "whooshing" the water up the pipe by momentum, so any constrictions in the pipe (even too many or too tight bends) will affect the flow rate. Also the valve may stick, especially if mud is being washed in.

I have a feeling that once water is flowing over a loop, the height of it doesn't matter. The water falling down one side helps pull the other side up. The head a pump can produce should be in the specs in the manual (hopefully available online) or on the product page in better online shops.

Possibly replace the electric centrifugal (I'm assuming that's what it is as these have nothing to prevent backflow, hence the problem) with an electric diaphragm? Probably won't be cheap to get the same capacity though.

Sorry, no one concrete suggestion, just some thoughts.

Pete
 
I have a one way valve in the electric pump pipe for the same reason -not easy to get a loop above the waterline. works fine. Problem of debirs will always be there but can be minimised by keeping the bilges clean.
 
I found a small Rule pump would only pump into about 5 or 6 ft head.
I used one for rinsing the deck of my yacht.
The flow reduces to zero at that point.
But if the loop is sealed, or has just a small leak to break the siphon, the 'down' leg will reduce the head that the pump sees.
I would move the outlet well above the waterline.
If you only get half the nominal GPH figure, it is usually still plenty.
 
It's a standard rule never to rely on a check valve as the only component keeping water out of a boat. I realise that the boat has not yet sunk because of the current arrangement but it may only be a matter of time. Check valves sold at chandleries are poor things with only a small flap of rubber to make the seal. I have known a couple in which the flap broke off after a short period of use. Far better off raising the outlet or incorporating a loop.
 
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