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Tuesday, November 16, 2010
Forward Bilge Pump Testing
Bilge pumps come in all sorts of different sizes rated by pumping capacity often described in GPH (Gallons Per Hour). Question is, how big is big enough? There is no real answer to that in a boat this size. It is like trying to answer the question "How big of a hole do you plan to put in your boat?" Well, the SMALLEST of course!
So, in our case, a pump was chosen by physical size. The largest that would fit in the space planned for the pump. As our primary electrical system will be 24 volts DC, our choices are limited (suppliers tend stock mostly 12 volt pumps, 24 volt versions available by special order).
Our choice: The Rule 16A, 24v, 3700 GPH Bilge Pump. It has a 1 1/2" hose discharge which matches the thru-hull. For the float switch, we are using the Aqualarm 20378 Bilge Pump Switch and Alarm, which provides automatic and manual operation from a remote panel, plus an alarm should the pump run (in automatic mode) for more than two minutes.
As a test, I emptied one of the fresh water tanks into the bilge. The water tanks have been full since the summer as a test for leaks (there are none). With winter coming and the occasional freezing temperatures, I thought it a good idea to empty the tanks.
The initial setup includes 1 1/2" hose, up through a vented loop, to the overboard discharge thru-hull. At the pump outlet, a "non-return valve" (also known as a check valve or one-way valve) is installed inline with the hose. This helps keeps the remaining water in the hose from rushing back into the bilge, and tripping the bilge switch, restarting the pump, again, in an endless cycle. (The reality is, even this valve will leak, so the bilge will back-fill anyway, cycling the pump, just more slowly and less often. I am still working on how to best deal with this.)
The results: The 3700 GPH pump easily kept up with the draining of one of the tanks. In fact it outperformed the tank. Which revealed a problem: these centrifugal pumps, when they start sucking air, begin to lose their ability to pump against the head pressure in the discharge line. As a result, the water level in the bilge kept rising until it was well above the base of the pump (intake strainer). Then the pump would eventually "prime" and empty the bilge, then do it all over again, causing a "spitting" behavior. All this because the pump is "too powerful"!
This is not necessarily bad. As in the event of a real hole in your boat, the water ingress could very well be more than a tank draining. To help this situation, I will install a smaller capacity pump (maybe 500 GPH) along side the big one and adjust the float switches such that the small pump will be the "first line of defense". If it can't keep up, then the big one (with the float switch a couple inches higher) will "kick in".