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Some years ago Practical Sailor reviewed all the magic-no-moving-parts bilge pump switches and found there was one common problem with them all: They still fail.
Either there's slime or oil or crud in the bilge, which tricks capacitance and conductance gizmos, or there's some other subtle problem. You could probably make it work in any number of ways, but they found a cheap float mechanism to be about as robust and reliable as anything else in the long run. A toilet float assembly, magnet and reed switch, all nicely sealed and potted, and you'd have something quite good enough. Emphasis on "nicely sealed and potted".
If you do go with electronics? Remember the engine starter will routinely throw 200VDC spikes into anything that is always connected--like the bilge pump switch--and you'll need to spike protect it as well as keep it dry.
Last edited by hellosailor; 01-21-2010 at 09:28 PM.