In my search for a suitable bilge pump, I followed the advice of U33 and 6705russell and embarked on a sortie to Halfords (5th July please note!)
There were probably only 3 or 4 models to choose from but previous customers had thoroughly mixed them up on 6 or 7 pegs so, after much rummaging , one was selected - on the basis of appearing to be the smallest (HWP16). Mrs Corposant, who happened to be present, sanctioned the outlay of £17.99. On arriving home, it ran well on 6v BUT was not self-priming! Much kicking of self over not concentrating on orientation of ports when making the selection! Not wishing to waste £18, the problem was handed over to the guy in the drawing office - who came up with the following:
The inlet nozzle was cut off and an acrylic scoop made to fit on the impeller.
This showed not the slightest hint of working. Thought "Oh bother!" (or something very similar). The chap in the drawing office got a right flea in his ear, I can tell you. His response was:
I.e. not trying to draw water in laterally but shove it upwards. It seemed simple enough - to make a single turn Archimedean screw by threading a disc onto a spiral of brass wire. An initial one, using a disc made from acrylic sheet, showed signs of working, so another was made using brass and soldered into place.
The disc is 5mm diameter.
After soldering
Drilled and tapped for inlet aperture disc.
This worked but only sometimes! Thought "Oh bother!" (or something very similar). Decided to try again, going up from 5mm to 10mm in diameter.
How on earth do you make a 10mm disc from sheet with only a 1.3mm hole in the middle? A 12 BA screw just sheared off as soon as the tool touched (by only 1 thou.) the edge of the roughly formed disc. (Same result on second try.) Thought "Oh bother!" (or something very similar). Ended up mounting it using a 12 BA tap.
5mm and 10mm versions.
Mounted on impeller.
In all this, I have spent infinitely more time making jigs than making impellers!
The 10mm impeller still didn't work reproducibly but seemed to indicate the knub of the problem: centrifugal force overtakes the upward thrust. So... glued a collar round the impeller. Performance much improved but now realised lack of reproducibility down to air locks forming in the outlet tubing. Cutting slots in the collar seems to allow sufficient water in to force them out.
Showing the slots.
Mounted on pump.
The three attempts.
All this has taken almost two months! (In between trying to resurrect the lawn and lay the base for Mrs Corposant's arbour.)
The main purpose of this post is to let you learn from my mistakes!
Mike
