# Bilge Pump as a Top Off Pump??



## Paul the Kayaker (Nov 7, 2004)

Can you use a bilge pump as a top off pump? Just push the plunger down rather than pull up...

Also looking to make my own 4" barrel pump and just looking for more info from anyone that has made one. I have leafield valves on my raft so no need for a check valve on the pump, so I am thinking it would be easy and cheap to make a pump, but confused about the plunger construction mainly. Anyway some chat about how to make it happen with someone would be great.


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## kengore (May 29, 2008)

Here are some detailed instructions on building a DIY bilge pump...
DIY PVC Boat Blige Pump

As for a 'combo' pump that could act both as a bilge and a top off pump. In theory it may seem possible, but I think it would be difficult to pull off. Water and air behave very differently under pressure.

In one function you are trying to make vacuum, in the other trying to make presssure. So the valve system would need to be reversible. The size and shape of the valves that are optimized for air are quite different than the size and shape that would work best for water. . 

Rafts are high volume, low pressure, most boats top off around 3 psi. Water pumps tend to be more low volume, moderate to high pressure. You couldn't move much water at 3 psi.

Most commercial air pumps have a big barrel, 4-5 inches in diameter. The big volume is needed because air compresses a lot before enough pressure is created to open the valves. Water on the other hand doesn't compress, a hand powered bilge with a 4-5 inch diamter would take some BIG muscles to operate.


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## Paul the Kayaker (Nov 7, 2004)

I mean isnt a bilge pump similar to a K-pump at all? Just wondering why a k-pump is 80 bucks and a bilge only 20...


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## MT4Runner (Apr 6, 2012)

kengore said:


> Here are some detailed instructions on building a DIY bilge pump...
> DIY PVC Boat Blige Pump
> 
> As for a 'combo' pump that could act both as a bilge and a top off pump. In theory it may seem possible, but I think it would be difficult to pull off. Water and air behave very differently under pressure.


Yup.

All a bilge pump really does is lift water so it can spill out the outlet.

You are actually compressing air with a top off pump.

Water molecules are relatively large, so your seals don't have to be that precise. Air molecules are tiny, combined with being pressurized, and making seals takes more effort. I'd like to build a big 4-6" pump. "Maybe next winter".


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## Paul the Kayaker (Nov 7, 2004)

That answers it, thanks! Ya and I didnt care about having a pump that did both, I dont need a bilge, just thought I could save some money but it makes sense that they are designed very differently. Still going to make my own 4" pump, I'll post pics when its done.


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## Schutzie (Feb 5, 2013)

Paul the Kayaker said:


> I mean isnt a bilge pump similar to a K-pump at all? Just wondering why a k-pump is 80 bucks and a bilge only 20...


Rule 19
19) If you reveal that what you want to buy is to be used as river gear, the price triples and the availability drops to zero.


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