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Old 09-03-2014, 14:23   #1
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Bilge pump loop variation

Like many before me, I am trying to figure out how to run a discharge hose and avoid water siphoning back into the boat. Initially I was going to put a Y valve into my manual bilge line and be done with it.

The through hull exit for the manual bilge pump is 2" above the waterline. the hose rises out of 27" deep sump. The hose rises another 3 feet to the bottom of the galley counters, and straight over to the port thoughull. The PO said she never had water siphon back into the boat in 20 years of cruising . But I suppose that was because the Henderson Bilge pump has a flapper valve in it?

So now I am planning to put in a separate through hull. The highest place I can put the new through hull is only 11" above the water line. There isn't a place in the boat to run a vented loop up to the underside of the deck, so I am wondering if I could install a vented loop on the centerline of the boat that is at waterline level and run a hose from the hole in the loop up to deck to elevate the siphon break well above the angle of greatest heal. There is a picture of this on page 205 in Nigel Calder's Cruising Handbook. The photo is of an engine cooling system that shows a raw water line which runs fro the heat exchanger into the exhaust.

I cannot run the hose out of the transom because there is not a clear path to run a hose aft. The engin and sump are not under the cockpit sole, rather the engine is just behind the mast in the exact center of the boat. The bilge is shallow and it is not practical to run exposed hoses over the floorboards.

Your comments will be appreciated.

Some boat statistics:
LOA 36.5
Beam 10.75
Freeboard - 42"
Galley counter height is 10" above manual bilge pump which is the heighest point I am able ot install a through hull.
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Old 09-03-2014, 17:33   #2
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Re: Bilge pump loop variation

I don't think raising the top end on the siphon break valve makes any difference. If I understand you correctly the top of the loop will not be much higher than the waterline, which would make it vulnerable to flooding.

I am not clear if this new discharge line is for the manual pump or an electric pump. If manual, why couldn't you have a seacock on the thruhull, and open it only when you need to pump?

On my boat there is little (vertical) room for a vented loop, so a check valve was used. I wish there was a better solution, but fortunately it is a cat so doesn't heel to speak of.
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Old 09-03-2014, 17:49   #3
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Re: Bilge pump loop variation

I have a well about 5 inches square and 16 inches deep cast into the concrete keel filling. It also would pump till it stopped then the remaining water in the hose would fall back in and the pump would kick in again. It cycled continuously.
Someone put me onto the water witch switches which have a delayed kick in and a delayed shut off (ie, runs for about 8 secs after the bilge is empty)
Fixed my problem, may be suitable for you? (Edit: If you go electric pump)

Cheers
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Old 09-03-2014, 18:47   #4
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Re: Bilge pump loop variation

Thanks for your response. My concerns are all related to the new electric bilge pump. the manual pump has a through hull 2" above the water line. I am considering either using th existing through hull with a vented loop in the shared hose. The vented loop would have a hose attached to the duck bill on the vented loop that goes up to deck level near the center line. Presently there is not a vented loop in the manual bilge pump hose.
If the electric bilge pump had its own dedicated hose, the new through hull would only be 11" above the waterline.
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Old 10-03-2014, 01:22   #5
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Re: Bilge pump loop variation

Quote:
Originally Posted by Onemoreproject View Post
.... There isn't a place in the boat to run a vented loop up to the underside of the deck, so I am wondering if I could install a vented loop on the centerline of the boat that is at waterline level and run a hose from the hole in the loop up to deck to elevate the siphon break well above the angle of greatest heal. There is a picture of this on page 205 in Nigel Calder's Cruising Handbook. The photo is of an engine cooling system that shows a raw water line which runs fro the heat exchanger into the exhaust....
Your comments will be appreciated.

....
Hmm .. I think there's a bit of confusion in at least one subsequent post about what Nigel was suggesting.

Siphon break valves are rather unreliable in salt-water plumbing, because salt has a tendency to precipitate out of solution and crystallise when it comes in contact with air, so the little valve element can easily be immobilised or blocked off. At this point, the fitting is functionally identical to a plain elbow, and nothing is gained.

What Calder is showing on that diagram is an ALTERNATIVE to a siphon break valve, particularly but not exclusively for salt water lines, namely a T connection (in practice, often an unequal T) where the 'branch' of the T is connected to a hose which is open at the far end, sufficiently far above the waterline that the pressurised water flowing in the main pipe cannot mount sufficiently far up to flow out the opening. (for every 1 kPa, you need 100mm, or for you leftists/masochists, for every 1psi, you need an additional 2.3 feet above the waterline)

This has always been the preferred option; the siphon breaker is a less reliable compromise usually used (largely for the convenience of the builder, or to save cost*) on leisure craft, and one which you would never find on a military or a rescue vessel.

*perhaps a bit like brass seacocks ...

Some siphon break valves lend themselves to being converted to the preferable "vented loop" described above, but the valve element should always be removed if doing this, or nothing whatsoever is gained.
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Old 10-03-2014, 10:51   #6
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Re: Bilge pump loop variation - Question

Thank-you for your extremely informative response! I can make that work if there is enough head room in the boat.
If I am understanding you correctly, I will need 2.3 feet of hose for every pound per square inch.
Are you speaking of PSI for water flowing back into the boat? How many PSI would you estimate that to be? I am not sure I have enough room to do that as there is only 32 inches from where the T in the hose is to the top of the cabin ceiling. Although I could get another 8" out of it by extending the hose into the dorade box vent.
Does the hose have to be straight or could there be some bends or elbows in it to prevent pinching?


By the way, do you have any recommendations for reliable bilge pumps in the 1200 GPH size? For a swich I am considering the Water Witch or Ultra made for deep narrow bilges.

Thanks again!
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Old 10-03-2014, 16:16   #7
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Re: Bilge pump loop variation

Thanks for the nice recognition, Onemoreproject; you're more than welcome.

In the case Nigel Calder's diagram covers, the flow in that part of the circuit is under pressure when the engine (and hence salt water pump) is running.

I forgot to return to the situation you raised:

<<I am wondering if I could install a vented loop on the centerline of the boat that is at waterline level and run a hose from the hole in the loop up to deck to elevate the siphon break well above the angle of greatest heal.>>

In your case you, too, have a "dynamic" pressure situation, because the bilge pump, forcing flow through a length of piping, will be fighting the resistance to flow in that pipe.

And that resistance to flow will also force the water up your vent hose.

The easiest way to measure the equivalent "static" pressure is simply to use clear hose for the vent, and empty a bucket of water into your bilge sump.

The distance the water climbs up the hose tells you the "static head", and it will always be above the waterline (provided either that you do this test with the boat in the water, or the outlet fitting through the hull is above the waterline)

You need to add a margin for heeling/pitching if the open end of this hose is not central, plus a margin for 'heave', plus a margin for the hose or outlet blocking up somewhat with marine growth etc. (which will increase the 'back pressure')

And, if it's easy to do, an added margin in case at some future time you upgrade the bilge pump to a higher throughput, and forget to reestablish the necessary height of the vent for the loop.
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Old 10-03-2014, 16:21   #8
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Re: Bilge pump loop variation

As for recommendations: while I recognise they are well suited for some types of boat, and many philosophies of boating, I don't personally have any great interest in or enthusiasm for electric bilge pumps, so I can't be of any help there, I'm afraid.
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Old 11-03-2014, 04:27   #9
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Re: Bilge pump loop variation

Quote:
Originally Posted by Onemoreproject View Post
...I am considering either using th existing through hull with a vented loop in the shared hose...
NO! shared bilge pump hoses!

The trouble you are having with back-flow is due to selecting the wrong type of pump. Instead of a submersible, use a diaphragm pump. Mount the pump well above the waterline.
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