Here are some pictures of a work-in-progress prototype for a simple axial flow water circulation pump:

After my test of the “pump payanak”, I kept thinking about the fact that it is pumping the water up 1.5m, making the water do a 90 degree turn and then dropping the water back into the pond. I don’t actually require a change in elevation, since I am only using it as a circulation pump.

It is worth noting that technically, the pump does need to overcome the friction of the water moving through the pipes which I calculated to be equivalent in pressure to about 30cm of water height, given my pipe length, flow rate, and pipe diameter.

But that is still 5x less pressure required than pumping the water up 1.5m just to drop it back down. So in theory, an equivalent pump that was perfectly horizontal might be 5x more efficient at producing the same amount of water circulation.

This leads me to consider the possibility that my ideal circulation pump might be perfectly horizontal.

Additionally, I read that rapid impellers can kill off some of the good microflora/microfauna that you want in a healthy pond. (Killing off the good guys just allows the bad guys to take over… or so I have read.) And since everything I have read suggests that bigger, slower propellers/impellers are more efficient anyway, this doesn’t seem like it has any downside. (There may even be some safety benefits to a slower impeller, also.)

So my ideal pump might have a much slower impeller speed, also.

Since we live on a canal, I bought a second hand Thai long tail boat a while back and have been impressed with both the simplicity and elegance of the design. I’ve also been impressed with the abundance of inexpensive equipment and parts.

So I picked up a 12.5″ diameter long tail boat propeller (220 THB/$6.5), bought a long tail boat shaft and had it cut down to about 70cm (1100 THB/$32.5), and we welded on some scrap metal left over from the excavator project to mount it all into a piece of the 12″ PVC pipe. It’s worth noting that (of course) we had to grind down the 12.5″ propeller a little bit to fit inside of the 12″ pipe.

This is a really big propeller with a really high pitch, so it ought to be able to push a lot of water at relatively low spin speeds.

Useless trivia: the lady at the boat shop always tries to talk me out of such large propellers since on a long tail boat this would be for a ridiculously large engine/boat. The colloquial phrase in Thai to say something like “this propeller is really big with a large pitch” is กินน้ำเยอะ(“gin naam yeu”) which translates as “it eats a lot of water”. Maybe it is just me, but I always found this Thai version a bit lyrical.

Compare these prices to the 20000THB/$600 we spent on the “pump payanak” (not including the motor), and this option is looking quite interesting for the 3 big circulation pumps and 1 small one that I will end up needing for my pond design. If I can get it to work.

Next we have to test this prototype setup. In order to do that, we will need to weld a sprocket and chain onto the end of the shaft and then mount a motor with sprocket to drive the other side of the chain. The two sprockets should have a very high gear ratio in order to achieve a relatively low speed. We happen to have an old 36V 800W DC motor laying around that might work well for this since DC motors are easy to adjust the speed. Starting out, I just need to test the propeller balance since the grinding of the propeller might not be perfectly even. A significant amount of vibration would propably tear the PVC apart. Water tests come later.

To be continued…