Centrifugal Pumps ( Aerator) and Airlocks 101

Flow-Rite Australia's chief, Lincoln Johnson, can answer all of your livewell questions here - timer switches, pumps, aeration and recirculation. Pimping your livewell? This is the place to start.

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Centrifugal Pumps ( Aerator) and Airlocks 101

Post by FlowRiteAus » Wed May 25, 2011 8:54 am

Centrifugal Pumps ( Aerator) and Airlocks 101

Most of us have experienced an airlock in a livewell system at one time or another. They are easy to identify; you hear the pump run, but no water is being pumped. If you have ever backed up a boat in order for the pump to start pumping water, you have most likely experienced an airlock. With a little understanding of how centrifugal pumps work and exactly what an airlock is, there is no reason to experience this problem again.

Most of us created an airlock when we were kids playing with a drinking straw in a glass of water. If you held your thumb over one end of the straw and then dipped the other end into the water, what happened? Water would not enter the straw unless you removed your thumb. The trapped air in the straw “locked” the water out. This is an airlock.

So now that you’re an expert at creating an airlock, let’s discuss some basic facts about centrifugal pumps. Unlike positive displacement pumps commonly used for wash-down systems, centrifugal pumps are not self-priming and must be mounted below water level to work. Centrifugal pumps do not suck water. They push water. As water is pushed out, new water rushes in and fills the pump impeller housing since water seeks it own level. This rush of water is easily confused with suction, but it is just a matter of displacement and water seeking its own level.

Going back to the drinking straw analogy, imagine having a pump located in the middle of the straw. Put your thumb on top of the straw and dip it in the water. No matter how long the pump runs, water will never reach the pump due to the airlock created by your thumb. This is exactly what happens in a boat. Instead of a thumb causing the airlock, trapped water in the hose (due to improper hose routing) does the same thing.

If water is not allowed to drain from the hose when the system is turned off, it will create an airlock. Once the hose leaves the pump outlet, it can run horizontally, up, or up then down. The one thing the hose can NOT do is to run down then back up. This dip in the hose will collect water and block the hose just like your thumb did with the straw. This is the number one cause of all airlocks in livewell plumbing today.


Another cause of an airlock is a large hump in the hose on the inlet side when the pump is mounted remotely (rather than directly through the transom).

If your plumbing is correct and pump is running there are still some conditions that mimic airlocks. They include mounting the pump too high (above waterline), slippage of the impeller on the pump motor shaft, clogged inlet strainer, cracked pump housing or outlet and debris in the pump or hoses.

Centrifugal pumps are fantastic for moving a lot of water when filling, circulating and aerating livewells and baitwells. The primary pump brands used for baitwells and livewells include Attwood, Johnson and Rule. They are very inexpensive and will serve you well when installed and plumbed properly.

With the information you have just read, you should now be able to resolve any airlock problems, or better yet avoid creating one in the first place.

Long Live Fish!

Roger Miller and Lincoln Johnson

Flow-Rite Controls


(07) 3299 3007

Steve Morgan
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Re: Centrifugal Pumps ( Aerator) and Airlocks 101

Post by Steve Morgan » Wed May 25, 2011 1:16 pm

Good info, Lincoln.

ph:(07) 3387 0888
fax: (07) 3887 0889

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