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Re: Bilges - On vs. Automatic; Capacity?
Easy question first: If you've got a centrifugal pump (like a Rule), then yes, it is quite typical for the bilge to never get completely emptied. What happens is that all the water in the discharge hose from the pump up to your anti-siphon loop will drain back into the boat when the pump is turned or cycles off. Some people install check-valves in the hose run to keep this backflow from happening -- please don't do it! There have been many threads discussion this here on SN; do a search and read on.
Getting a little more complicated, now...
A common set up for multiple pumps is for one high capacity pump on a float switch, paired with a small capacity pump that has either a manual or dual ("manual" & "auto") switch. The reason is that the smaller pump will typically have a much smaller hose bore (like 3/4") as opposed to a high capacity pump (which will normally have a 1-1/8" to 1-1/2" hose.) The smaller hose will hold less water in the run up to the anti-siphon loop, and thus you'll have less water flowing back into the bilge when the pump stops.
If you go elect to go this route, make sure you mount the float switch for the larger pump higher in your bilge than the backflow water level from the first pump.
As far as switches go, I actually prefer to have dual function switches; if your float switch fails, you can always bypass it at the selector switch by simply flipping it from auto to manual.
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'85 BYB Vagabond 39