The last post in the linked thread sums up both the problem and the reason why raising the switch won't work in a bilge with nearly vertical sides.
" OK, When I read about shimming the float switch up I thought it was a great idea. However the more I thought about it, shimming the float switch up should not make a difference, Sure a higher switch will let the water level in the bilge rise a little higher before the switch closes but when the switch opens the water will also be higher. You have the same difference between the switch closing and opening regardless of the height of the switch. What we are really concerned about here is the deadband OR the difference in water height from switch close to switch open. This is independant of switch height unless there is a significant difference in the shape or volume of the bilge at the two different heights."
Since I (and others with H340's, plus plenty of other boats) have a problem with short-cycling due to backflow, I can only see a few possible solutions. One is a smaller diameter discharge hose, so less water flow back.
Another is a check valve, which I have, but it causes the pump to run for a long time sometimes before catching a prime. Centrifugal pumps often generate very little head pressure until some water is flowing, and the check valve holds that long column of water in place, so it has trouble creating enough pressure to get it started. Once started, it pumps fast - it's the startup that's a problem.
A float switch with a longer excursion would be the perfect solution in my narrow but tall bilge. Then I could remove the check valve and just let the hose backflow without turning the pump back on. I can't find much by way of specs for these things. My standard Rule doesn't have enough.
I was thinking about modifying one by adding what amounts to a miniature toilet valve type float, with a short arm with float on the end, but I'm reluctant to make a heavy modification to such a switch for fear of a failure (I suppose I could put another switch higher up, wired in parallel, as a backup in case of a failure to start caused by the modified one.
Has anyone ever done something like this? Or does anyone know of a float switch with a larger distance between on and off than the standard Rule switches?
" OK, When I read about shimming the float switch up I thought it was a great idea. However the more I thought about it, shimming the float switch up should not make a difference, Sure a higher switch will let the water level in the bilge rise a little higher before the switch closes but when the switch opens the water will also be higher. You have the same difference between the switch closing and opening regardless of the height of the switch. What we are really concerned about here is the deadband OR the difference in water height from switch close to switch open. This is independant of switch height unless there is a significant difference in the shape or volume of the bilge at the two different heights."
Since I (and others with H340's, plus plenty of other boats) have a problem with short-cycling due to backflow, I can only see a few possible solutions. One is a smaller diameter discharge hose, so less water flow back.
Another is a check valve, which I have, but it causes the pump to run for a long time sometimes before catching a prime. Centrifugal pumps often generate very little head pressure until some water is flowing, and the check valve holds that long column of water in place, so it has trouble creating enough pressure to get it started. Once started, it pumps fast - it's the startup that's a problem.
A float switch with a longer excursion would be the perfect solution in my narrow but tall bilge. Then I could remove the check valve and just let the hose backflow without turning the pump back on. I can't find much by way of specs for these things. My standard Rule doesn't have enough.
I was thinking about modifying one by adding what amounts to a miniature toilet valve type float, with a short arm with float on the end, but I'm reluctant to make a heavy modification to such a switch for fear of a failure (I suppose I could put another switch higher up, wired in parallel, as a backup in case of a failure to start caused by the modified one.
Has anyone ever done something like this? Or does anyone know of a float switch with a larger distance between on and off than the standard Rule switches?
