Air locking a bilge pump? How to prevent it?

[Mark42]

Reading the install instructions for an Attwood Sahara automatic bilge pump, it says to be sure that the hose has no dips where water can collect because it can air lock the pump.

I'm having a hard time routine the bilge hose so there is no drop. The problem is right off the pump, the hose must go under the gas tank and that is where it will sag to the hull causing a low spot.

How can I prevent air locking the pump? What causes an air lock?

Here is a link to the install doc: http://www.attwoodmarine.com/products/Install_Instructions/69370-C Sahara Pumps.pdf

 

[Beefer]

Re: Air locking a bilge pump? How to prevent it?

You may not have a problem going under your tank. An airlock is like the P-trap under your sink. Water completely fills a section of the tube, blocking air.

If your tube is 1" in diameter, and the dip is 1/2", there will still be enough air to prevent the lock. If the dip is gradual, and spread over a distance, (width of the tank), it may still be ok. It'll just depend how much water remains in the tube after the pump stops.

I would try it, and see how it goes.

 

[Mark42]

Re: Air locking a bilge pump? How to prevent it?

I was wondering if the hose needed a small hole, about 1/16" , drilled near the pump outlet so air could escape when the pump kicks on?

This weekend I'll be able to get back in the boat and check out my routing options.

Thanks!

 

[joed]

Re: Air locking a bilge pump? How to prevent it?

The issue is to let the air out as the bilge fills up. The hose must be open to the air at the end so that as water rises it will fill the pump. If the air can't get out the pump will have a air bubble in it.
Think of it like pushing and empty bottle under water with the opening down. No water will go into the bottle.
The small hole will work if it doesn't get clogged. It will also spray water as the pump is running so be aware of where it is pointing when you drill it.

 

[Moody Blue]

Re: Air locking a bilge pump? How to prevent it?

The link doesn't work for me.

Good question. I will be installing a pump later in the spring and was going to LEAVE a trap in the line to minimize the amount of water in the line running back through the pump into the bilge. Better rethink that approach I guess.

I have a 1 1/8" bilge line that must run about 3 1/2ft in total. Once the pump shuts off, all that water in the line will run back into the bilge, will it not?

 

[Mark42]

Re: Air locking a bilge pump? How to prevent it?

OK, I fixed the link.

I was in the boat this afternoon, and there is a way to route the hose so its all uphill to the thru-hull. Don't forget to add the high "trap" where the hose rises from the pump to a point higher than the thru-hull, then curves back down to the thru-hull. This prevents water from entering the bilge through the hose.