Bilge Pump Discharge - New Thru Hull ??? - SailNet Community
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post #1 of 5 Old 04-30-2003 Thread Starter
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Bilge Pump Discharge - New Thru Hull ???

Hi Everyone,
Looking for some advise on installing an electric bilge pump in my sailboat. Do I have to install a new thru hull fitting or is it possible to connect the new bilge pump discharge line (3/4") into a "Y" connection that would be installed in the existing (1 1/2") manual bilge pump discharge line ... aft of the manual pump itself ???
Any advise would greatly be appreciated.

Thank you,
Terry Clarke
CAPTIVA, Port Whitby
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post #2 of 5 Old 05-09-2003
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Bilge Pump Discharge - New Thru Hull ???


I am not heavy in to plumbing, but I do know when you ''Y'' two pumps together like that you need to install check values (anti-siphon, duck bill, etc.) in both lines to keep one pump from pumping into the other. Personally, I think separate discharge lines would be best, but that costs lots of money.

(Actually your bildge line should already have a check value.)

Good luck, Ron
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post #3 of 5 Old 05-10-2003
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Bilge Pump Discharge - New Thru Hull ???

Yes, you can install two pumps to discharge through a single through-hull; but you should not use an existing (properly sized) thru-hull to also accomodate an extra pump. It may not, therefor, be practical.
1. NEVER install a check valve in a Bilge Pump discharge line. While the valve reduces the amount of water that returns to the bilge, after the pump shuts down; they are VERY prone to failure. It''s difficult for a typical bilge pump to open a fouled check valve (and they DO clogg up) A new & clean 1.5" check valve represents the equivalent hydraulic friction loss of about 14 Feet of SMOOTH-BORE Pipe..
2. You CAN install a "Y", providing a single thru-hull discharge for two pumps.
The "Y" will consist of an assembly that has the free area of the thru-hull equal to not less than the sum of the areas of the pump discharge ports (actually, this is only a close approximation).
The pump hoses will each rise to an anti-siphon loop (risers), then drop to the "Y" fitting, then discharge. This prevents the cross-pummping that Ron mentions.
This riser loop should consist of gentle bends, so as to limit the amount of waterflow turbulance introduced into the discharge line(s). Turbulant waterflow increases friction head, reducing pump output.
3. So, for your example (a 0.75" and a 1.5" pump discharge) will have a "Y" assembly consisting of:
a 0.75" internal diameter upper leg, another upper leg at 1.5" (= 2.2 Sw. In total), and a 1.67" internal Diameter(>2.2 Sq. In.) lower discharge leg. Because 1.67" I.D. not available, you''d have to increase discharge thru-hull leg to 1.75 or 2" diamter.

Perhaps a second 3/4" thru-hull might be easier, after all.

Hope this helps.


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post #4 of 5 Old 05-10-2003
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Bilge Pump Discharge - New Thru Hull ???

You know, it is beyond me why anyone would install two outflows in a "Y" fashion on a single discharge thru hull. The idea is failure redundancy, necessitating two thru-hulls. Done it, it''s easy. Go with low amperage auto pump with internal switch at the low point of the bilge and put a higher capacity pump on a manual switch up 8" above that. If those won''t move the water necessary, you''re screwed anyway.
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post #5 of 5 Old 05-10-2003
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Bilge Pump Discharge - New Thru Hull ???

If you''ve got a little daysailer then yea, you can put two bilge pumps on the same outlet and use a check valve too. But if you plan to sail out further then you can swim, you need to installed the second pump as a completely separate entity.
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