Re: Pumps do not do what you think
A couple of observations:
1) As MarineSail indicated most centrifugal pumps don't deal with back pressure very well. So the length and nature exit pipe makes a big difference with such set-ups. Smooth hose, as large a diameter hose as practicable, large radius bends, NO valves, and as short as possible is the way to go. Also, if the exit is on the transom, and well above the waterline, you shouldn't need an siphon break (it's just another tight radius turn anyway). A one-inch corrugated hose with lots of twists and turns, and a check valve, and maybe a couple of elbows here and there, is going to pose a serious challenge for any small centrifugal pump relative to a smooth, large diameter hose without unnecessary fittings.
2) Your video is something of a "worse-case" test as far as the pump is concerned. As the bilge fills the pump has less "head" against which it has to work. In other words, the last five gallons should take the longest. If the bilge is filled to the sole it doesn't matter if the hole is at the bottom of the bilge (as far as the rate of water ingress is concerned) -- the water will come into the boat as if the hole were right at the surface of the rising water. The difference can be as much as a meter, or more, on some boats (it's close to a meter on my little 27-foot boat); AND the pump won't have as much head pressure to work against.
You are right to point out that the "rated capacity" of a bilge pump is rather deceiving. It's as if car manufactures advertised the milage that their cars get just cruising on the freeway at 45 mph, without factoring in real freeway speeds, or the gas it takes to accelerate to speed, or hills, et cetera. However, you can improve the capacity of a bilge pump, sometimes pretty dramatically, with a little large diameter hose and some common sense. Also, realize that while your pump(s) might not be able to completely keep up with a hole, or busted thru-hull, or whatever, they could well buy you enough time to get a handle on the problem, or get to shore, or for the CG to get to you.
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Last edited by SlowButSteady; 06-22-2013 at 05:25 PM.