Building the Frederic Mistral - Water pump Mk II

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Water pump MkII is a diaphragm pump, otherwise known as 'A modified fish-tank air pump'.  Here's how I did it; you can improve it no end:

The air pump has a diaphragm flexing over a plastic body.  The body contains two 'flap' valves of thin plastic, arranged so one controls the suction and the other is a non-return valve on the delivery.  The delivery is via a moulded-in pipe to suit 4mm soft tubing.  The air gets into the pump via a small hole (more like a bit missing) on/in the body

What we are going to do is mount the diaphragm part onto a geared motor (mine happens to be 150 RPM at 12V) which has an eccentric bearing on its shaft.  Connecting the eccentric and diaphragm will be a simple rod so the diaphragm wobulates in-and-out of its housing just like it used to do.

To mount the diaphragm we will replace the original box-like casing with a stiff backplate with a connection for the suction pipe.  We will bung-up the original air-inlet hole so anything going in has to go through our backplate.

the guts of an air-pump

All we require of the original air pump.  Keep the rest for a rainy day.
One of the flap-valves is retained by the small, black blob (top of the white body in this view) - it's a 'D'-shape of rubber pushed into place.  The other valve is hidden from view on the other side of the white body.

the guts of an air-pump

There's an M3 screw through the diaphragm.  Keep it and its washers and nut; there's no point removing them, just to replace them later.  In this group you'll also notice the rubber seal that used to go between the white 'body' and the original outside case.  Modify it, later, with a hole sort of inline with the suction pipe.  (That's the hole off to the right in this picture.)

My version of the connecting rod (eccentric strap would be a more correct name, if only it looked better) is sawn from 5mm brass sheet.  It is bored 10mm diameter to suit a 5mm ballrace's OD.  The other end is drilled and tapped M3 by about 6mm deep.

The wee hole for air-intake is blocked with resin putty gunge.  My first thought was to machine the plastic body down to level with the bottom of this hole but looking at the 'D' plug, I chickened out.

Don't let the five holes fool you, only three are used!

The backplate is another chunk of 5mm brass, drilled to clear the centre body-retaining screw, and tapped M3 for two outer holes for mounting the thing to the motor brackets.  There are actually two sets-of-two M3 holes because I changed my mind about which way the outlet should face... Oh! I won't bore you with the details.

Despite all my natural inclination, I actually measured-and-marked-out for the suction nipple.  And then drilled a whopping great 6mm hole around about that spot.

The nipple is a bit of brass turned to 6mm one end and about 5mm the other, to suit airline tubing, and bored through 3mm.  It is simply soft-soldered into the backplate.

In case you thought this is a precision job, here is the eccentric being turned.  I don't have a four-jaw chuck for the lathe so a bit-of-wood provides the offset that I would otherwise have spent ages 'getting right'.

This end will be 5mm dia. to suit the ballrace I have.  The other end is drilled 6mm to suit the motor shaft.

And an M3 grubscrew keeps the thing on the motor shaft.

The shaft needed shortening by 6- or 7mm, and it all ended up like this.

The two brackets are standard hardware-store strips, hammered in the vice till they were about 90o and nearly the same length.  They fix to the motor gearbox with short M3 screws.

Next, fasten the backplate in place.  Again, short M3 screws.  Notice I have put the backplate inside the brackets to reduce strain on the screws - the brackets take the push when water is being ejected from the pump.

The ballrace/eccentric strap is pushed onto the eccentric..

and retained by - you guessed it - a short M3 screw.

The diaphragm is screwed (fairly tightly) onto the free end of the eccentric strap ..

And pushed back onto the pump body.

Now swing it all round to fix to the backplate.  Don't forget to line-up the extra hole in the 'gasket' with the suction hole, and tighten the centre retaining screw quite severely.  I didn't, and the pump gently filled the boat as water jetted out somewhere between the body, the gasket and the backplate.

Test the blighter against your boat's battery, at/in the bath, sink, indoor fountain.  It is self-priming so you will see water gradually sucking up through the suction tube.

On 6 Volts (my boat runs on accumulators) it pumps steadily, though not fast.  I shoved a syringe into the output pipe and watched the plunger being pushed back up the syringe.  It actually pumped 60mL in 30 seconds.


Of course this would make a superb bilge pump - don't bother with the suction nipple, don't bung-up the original air hole in the body.  Mount it so the backplate sits down in the bilge, take the delivery overboard somewhere to suit.

Leave the begger running - it draws only a few milliAmps when sucking air and will pump water when water is available.

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