[C320-list] Bilge Pumps and Check Valves

[John Frost]

ElectricMarine.com has this sound advice:

"They become upset when their bilge pump stops and the water in the hose to the outside rushes back to the bilge, raising the water level.
People tend to hate water in their bilges.

I hate check valves and here are four reasons why.  The pump may have some air in it when it shuts off and the check valve closes.  The pump can become air locked the next time you want it to run.  "Air lock" means that there is air trapped in the pump below bilge water level keeping the impeller from getting traction on the water.  Centrifugal pumps are not self-priming, meaning they must have water at the impeller to start pumping.  An air lock can last long enough to sink your boat (presumably any way), and it does not develop every time.  One solution is to drill a small hole in the hose below the check valve.  This will eliminate the airlock until the hole gets plugged up with lint.  In the mean time, it will also squirt a thin stream of water across your bilge which can be unnerving the first time you see it.  The new solid-state augmented float switches extend the pump run time after they actually trip off.  This gets the pump trying to suck air at the end of it's cycle and increases the possibility of airlock with a check valve in the system.

The second problem with check valves is that you can trap so much water weight above the valve with a high head that the pump can't open the check valve from a standing start.  Centrifugal pumps do not like to be stalled out.  With the inevitable air cushion below the check valve, the pump sees a gradually increasing back pressure culminating in complete blockage,  The check valve can stick shut a little too.  The pump may not develop enough static pressure to open the check valve.   The inertia of the water moving through the discharge pipe aids the pumping action, if you think about it.  An operating pump is a dynamic system with water moving through it.  If you stop the water from flowing, you have changed the system conditions enough that water may not always start flowing again.  Bilge pumps are puny little things and do not always tolerate unusual conditions.

The third problem with check valves is they leak.  The leakage increases as they age.  This means that the check valve you installed to stop your little oscillating pump system now just oscillates with a longer period as the water drains back to the bilge at a slower rate.  If the pump only runs every 15 minutes this may be tolerable.

The fourth problem with check valves is that they inhibit a good thing.  All that water rushing back down the discharge pipe backflushes the debris screen(s) at the intake to the pump.  Mostly these screens do not get clogged and this is why.    

John
2009 C320MKII
Hull  #1118
Lake Guntersville, AL

-----Original Message-----
From: C320-list <c320-list-bounces at lists.catalina320.com> On Behalf Of Warren Updike
Sent: Wednesday, September 2, 2020 4:00 PM
To: c320-list at lists.catalina320.com
Subject: Re: [C320-list] Bilge Pumps and Check Valves

I've heard that a check valve in the bilge pump line is not recommended. The concern is that if it failed, it could interfere with the pump function. That said, I did use a check valve for a long time until I started having problems with the pump which I replaced. In the process I removed the flapper in the check valve and have not replaced it. 
As for your question, I suggest you raise the float switch temporarily or get a similar replacement for your original pump. 

Warren & Pattie Updike
C320 1994 #62 “Warr de Mar”
Middle River, MD (Chesapeake Bay)



-----Original Message-----
From: Charles Martin <chiprmartin at gmail.com> 
Sent: Wednesday, September 2, 2020 3:54 PM
To: C320-List at catalina320.com
Subject: [C320-list] Bilge Pumps and Check Valves

Dear Shipmates:

My bilge pump recently failed, and I have done a quick and dirty fix while I am waiting for a new pump from Catalina Direct (cheaper but less powerful than the closest model from West Marine or Defender).

My question is:  the small rule pump I have installed as a temp fix doesn't sit as low as the intake hose did for the factory installed pump; and as a result, after it pumps out any water in the bilge and stops, a lot of water comes back into the bilge from the hose leading up to the above-water through-hull.  When that happens, the float switch rises, the pump turns on again, and tries once more to drop the level of the water.  This happens over and over--an infinite do loop.  I could just raise the float switch, but I'm wondering if a check-valve in the line might be a more elegant solution?  Did the diaphram pump installed originally have a built-in check valve?  Any thoughts?

Best,
Chip Martin
*Bonaventure* -- Sail #767
(410) 231-0199
chiprmartin at gmail.com