One-way trap on bilge pump?

[Bigjim]

I was running my bilge pump to clean out the old gunky water from the bilges. I noticed that when the pump stopped water flowed back into the bilge. Does anyone use a one-way trap on your bilge hoses to prevent water from coming back through the pump and into the bilge?

[Cheechako]

I have used one. I wouldn't again though. Almost sank my boat once due to a check valve. Mucky things from the bilge clog check valves. They restrict flow and may cause a siphon. Some people use a small diaphragm pump to get out the last bit of water.

[denverd0n]

To elaborate on Cheechako's answer...

You should NEVER have a check valve on your main, "keep the boat from sinking" pump. NEVER!

Many people, however, arrange a main, large bilge pump for emergencies, and also a smaller pump -- usually a few inches lower than the main pump -- to try to keep the bilge dry. If you have an arrangement like this, then a check valve on the smaller, "keep the bilge dry" pump is not a terrible thing.

But again, NEVER on the main pump.

[Aimlessness]

My two-cents worth...
Put a check valve on the secondary pump (the smaller automatic pump that is ALWAYS on). If there is ANY (even a small amount) back-flow to the pump it will continuously cycle draining your batteries.
However, as stated the switched pump is another issue. The Johnson 4000GPH pump I have wired to a Auto-Off-Manual switch has a check valve / flow control that came with the pump. Works fine. Don't ask what mistake I made to know that it does work fine.

[Cheechako]

Originally Posted by Aimlessness

My two-cents worth...
Put a check valve on the secondary pump (the smaller automatic pump that is ALWAYS on). If there is ANY (even a small amount) back-flow to the pump it will continuously cycle draining your batteries.
However, as stated the switched pump is another issue. The Johnson 4000GPH pump I have wired to a Auto-Off-Manual switch has a check valve / flow control that came with the pump. Works fine. Don't ask what mistake I made to know that it does work fine.

Beware that with a check valve, in certain situations, whether the pump is off or on... a siphon may be created filling the boat with water. I got very lucky once, I had been to sea for hours motoring. That boat "squats" a bit when motoring hard. The boat seemed "sluggish". I went below and lifted a floorboard. The water had filled the boat to just starting to touch the floor support beams! it took a very long time to pump the boat out. I was pretty surprised... at first I thought I wasn't gaining getting water out at all... The boat hadn't had a problem before. I got rid of the check valve after that.

[Aimlessness]

I agree. There are many issues to consider based on circumstances that are sometimes unknown.
I lost my last boat in Mathew it was inundated with water, ripped from its mooring and smashed into a debris pile. I can't say whether the bilge pumps tried to keep up, but I had left them both on auto.
My new boat has the same bilge pump setup plus I added a high water alarm because whenever I go out I have so much anxiety about the boat sinking I obsessively run to the cabin to check the bilge. Now I can relax knowing that the alarm is continuously doing what I was obsessively doing.
I think the whole bilge water management issue can be over done; however, I sleep well at night. I was shocked that the previous owner had the boat for 25 years and NEVER installed an electric bilge pump the boat only had a manual pump when I bought it last fall. The owner told me the boat has never taken on water. Now, I think I've gone the opposite direction with over kill. All-in though I have only 'bout a thousand into the system.

[Terra Nova]

NEVER install a check valve on any bilge pump line. For drying the bilge, properly install a diaphragm pump.

[Aimlessness]

To the OP. This forum will provide many expert opinions the decision is yours to make, it is your boat. Here's a link I used to make my decision: ALL ABOUT BILGE PUMPS - Boats, Yachts Maintenance and Troubleshooting
You'll note that the author specifically refers to the diaphragm pumps as inherently unreliable.