I'm using a pump that is sold in hardware stores. It is supposed to be attached to an electric drill. I have adjusted (improved) the seals and attached the driveline of an old electric screwdriver (Johnson engine with double planetary gears).
The pump has quite a large flow (33l/min) but by using a geared down engine the flow is reduced. This way I can fill my 2 liter ballast tank in an acceptable time.
The whole unit is reversable and usable to fill and empty the tank.
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