Siting an anti-siphon loop in the bilge pump outlet

[jwilson]

I agree about syphoning back the whole or at least most ofthe hose content, syphons are like that!

Are you running a sauna? I don't believe I'd want to be on a yacht where condensation is enough to pump out.
Yes you can get a surprising amount of bilge water in an AWB, from wet people, sails, spray etc.
Racing is usually messier...
But it takes a lot to even fill the hoses of a bilge pump that would be big enough to take seriously.
A manual pump or electric diaphragm pump that will pump air is better.
But lifting the middle of the cabin sole and wiping up with a j cloth is normally what's needed when putting the boat away.

No, not a sauna, but 4-5 people do create moisture. The bilge pump isn't actually used that much. I start the season with totally dried bilges, but water does creep in. I'm talking about extracting a pint or maybe 4 a year, unless I decide to wash down under the floorboards, which I do do occasionally, when the bilge pump does get used. And when you do use it you have to sponge or Pela out the last few pints, or if left on it will cycle endlessly, thanks to the long long pipe run.

The water that does get in is not from a drip-free stern gland, or keelbolts, or skin fittings, or deck leaks. The boat is a Jeanneau with old-fashioned non-double-skin construction. People with egg-box double skin hulls a la most modern Beneteaus etc often THINK they have totally dry 'just-dust-them' bilges because the glossy smooth little compartments under the floorboards stay dry. There's often some water between the skins, just not immediately visible.

 

[lw395]

No, not a sauna, but 4-5 people do create moisture. The bilge pump isn't actually used that much. I start the season with totally dried bilges, but water does creep in. I'm talking about extracting a pint or maybe 4 a year, unless I decide to wash down under the floorboards, which I do do occasionally, when the bilge pump does get used. And when you do use it you have to sponge or Pela out the last few pints, or if left on it will cycle endlessly, thanks to the long long pipe run.

The water that does get in is not from a drip-free stern gland, or keelbolts, or skin fittings, or deck leaks. The boat is a Jeanneau with old-fashioned non-double-skin construction. People with egg-box double skin hulls a la most modern Beneteaus etc often THINK they have totally dry 'just-dust-them' bilges because the glossy smooth little compartments under the floorboards stay dry. There's often some water between the skins, just not immediately visible.

4 pints won't even fill the bilge pump plumbing on many boats.
With a syphon break and even 3/4inch hose, you are still going to have more water running back than I'd leave in the bilge, so what's the point?
A syphon break is however a very good idea if the bilge pump outlet might be underwater, for instance on an open boat that runs the pump while racing.
Any double skinned or 'egg box' boat with water between the mouldings has a serious problem.

 

[RichardS]

The relative height between pump and switch makes no difference - the variables which determine whether it cycles or not are volume of water in the pipe, the debit rate of the pump, and the timing hysteresis of the switch. The pump has to carry on running for long enough after the switch turns off to clear a volume greater than that stored in the outlet pipe - otherwise the water in the pump runs back and switches the pump back on.

If you make the switch higher as you suggest, all that happens is that you store more stagnant water in your bilge.

Pete is spot on with the reason why the higher switch will stop the cycling.

And you are right that the higher switch will store more water in the bilge ...... but this is the inevitable price of preventing the cycling.

The only 100% effective solutions are to fit a pump which will pump air as well as water or stop the leaks.

Richard

 

[girlofwight]

Slightly off topic, but worth mentioning, we had a check valve fitted on one our pumps and even after six months it held water against it, so much so the bilge pump couldn't actually prime past it.

Empty valve pump ran, leave it and return, and it wouldn't run. Only found this by accident as the pumps were tested once in commissioning and this one - we have four - hadn't been used since. Solution, of course, move check valve, but it's worth noting they can cause unexpected problems.

 

[Ric]

Pete is spot on with the reason why the higher switch will stop the cycling.

Richard

Depends on the shape of the bilge. If the bilge gets wider as the switch is moved higher, then yes, there will be a point where the volume of water between switch on and switch off is greater than the volume in the pipe, and the cycling will stop.

However, my bilge pumps are fitted into a sump with vertical sides, so moving the switch higher makes no difference at all to cycling. Higher switch just means there is more stagnant water in the sump.

 

[RichardS]

Depends on the shape of the bilge. If the bilge gets wider as the switch is moved higher, then yes, there will be a point where the volume of water between switch on and switch off is greater than the volume in the pipe, and the cycling will stop.

However, my bilge pumps are fitted into a sump with vertical sides, so moving the switch higher makes no difference at all to cycling. Higher switch just means there is more stagnant water in the sump.

It doesn't make any difference. As you move the switch further above the pump the volume of water between the switch trigger point and the pump minimum suction level gets larger and larger. At some point that volume is more than the volume of residual water in the outlet hose ..... and at that point cycling stops.

The shape of the bilge could be concave or vertical sided, with or without a sump.

Richard

 

[Ric]

It doesn't make any difference. As you move the switch further above the pump the volume of water between the switch trigger point and the pump minimum suction level gets larger and larger. At some point that volume is more than the volume of residual water in the outlet hose ..... and at that point cycling stops.

The shape of the bilge could be concave or vertical sided, with or without a sump.

Richard

How is the pump turned off in your system? With most mechanical float switches there is mechanical hysteresis so that the pump switches off at a lower level than it is switched on. The volume pumped out between switch on and switch off has to be larger than the amount in the pipe to prevent cycling, so the shape of the bilge does make a difference.

You seem to be discussing some sort of smart pump that turns itself when it no longer has any water to pump.

 

[RichardS]

How is the pump turned off in your system? With most mechanical float switches there is mechanical hysteresis so that the pump switches off at a lower level than it is switched on. The volume pumped out between switch on and switch off has to be larger than the amount in the pipe to prevent cycling, so the shape of the bilge does make a difference.

You seem to be discussing some sort of smart pump that turns itself when it no longer has any water to pump.

You're right. It's not something I've thought about before but with a normal float switch it's more complicated than I imagined.

Richard