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post #6 of Old 04-26-2009
Maine Sail
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Originally Posted by soulfinger View Post
Hmmm...check valve, eh? I guess that's what the brass thing is about a foot up the line from the pump. It doesn't look like any of the pictures of check valves I've found online, but I suppose they come in a bunch of shapes. Can I "un-stick" it, or should I just replace the check valve?
Get rid of it all together! Check valves on centrifugal bilge pumps are a safety hazard IMHO.

I would also advise re-routing the bilges hose so they each have their won discharge line..

As far as bilge pumps go I think we need to be very careful taking any centrifugal pump at face value as many do because items like a check valve can reduce the pumps capacity dramatically. Having two large pumps a wise idea with centrifugals!

Flow rates, as rated by bilge pump makers, can be quite misleading. They should serve only as a rough guide of a pumps "best case" capacity compared to others of similar design.

Bilge pump capacity is usually rated as “open flow” or what's called “open bucket” rate. This means the figures account for no, nada, zero vertical lift and also no discharge hose friction, radius bends, check valves or discharge outlet restriction.

Actual flow rates, under real operating conditions, can be drastically lower. Water must be lifted up and out of the bilge and then be pushed through lengths of hose to the discharge point. This resistance is called head. Head pressure is basically the weight of the water and the frictional resistance of the hose, bends, check valve and outlet.

Most centrifugal pumps, like the ones made by Rule or Johnson Pumps, have large internal tolerances to allow the passage of bilge crap. Sadly because of this design their flow rate decreases dramatically with increases in head pressure. Unlike a vane pump, which would be less affected by head loss, the pumps rotor or impeller does not come in direct contact with the pumps walls. Impeller/vane pumps and diaphragm pumps are less affected by head pressure than a centrifugal pump is and they may actually be more effective, and move more water, despite a lower "labeled" GPH rating. Some of the vane/impeller pumps can handle 60+ feet of head pressure with ease. Because of the large tolerances in centrifugal pumps it can lead to, and create, cavitation if the head pressure is extreme enough like in the case of a check valve.

I'm sure many of us have seen the bilge pump kick on and then not actually suck any water but instead just create noise and bubbles in the bilge. This can usually be eliminated by removing those ridiculous head boosting check valves and the crappy corrugated bilge hose and replacing it with smooth walled hose..

The output of many centrifugal bilge pumps can diminish by as much as half with only a few feet of head and can stop entirely at between 13 and 20 feet, depending on the size of the bilge pump. Remember head is not just the peak height/lift of the hose it is the friction, height, bends and fitting restrictions all added together.

Another serious consideration is voltage. What is your pump rated at? Is it 14.2 volts or 12 volts. This will and can make a difference as the pumps motor increases and decreases output based on voltage.

There have been a few tests like the ones conducted by West Marine. They rated pumps based on voltage and head pressure. Most manufacturers ratings (open bucket / no head ratings) were off between 10-50%. the output on average was reduce by about
5% for every foot of head pressure. With voltage the drops from 13.6 volts to 12.2 volts were another 15-30% off on top of the head pressure loss. So your 800 GPH centrifugal pump may not even deliver 1/3 of that rating in a real word situation and these numbers are still without a check valve..

I guess what I'm getting at is this; unless you have a "monster truck" grade bilge system, do not count on anywhere near the face value rated capacity from your bilge pump and please, what ever you do, remove that check valve..

Some other things to consider:

- Use smooth walled discharge hose as it has significantly less head resistance than does corrugated.

- If the discharge outlet is close to the waterline you must use a high loop in the discharge hose to prevent siphoning. It is advised to add a siphon break at the highest point to ensure it won't back siphon. Even with a high loop a bilge pump can still siphon without a siphon break. I have been on more than one boat that self siphoned when under sail and power.

- Do not succumb to the temptation of using a "check valve" with a centrifugal pump. If you have 5' of height, in a 1" hose, the pump will most likely cavitate before it can throw open that check valve with that standing water behind it..

-Maine Sail / CS-36T

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