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Sea Kayaks Techniques Bulletin Board
Priming and impellers
Posted By: Brianne Corbett In Response To: Re: High Volume Pumps (Robert Woodard)
Date: Tuesday, 26 November 2002, at 8:04 p.m.
: Brianne,
: Any chance you've run across a self priming pump? A bilge pump needs to sit
: in the water, and my problem with my Rule pump is that when it gets down
: to the last 1 1/2 - 2 1/2 inches or so, it starts sucking air because the
: water can't rush in around the pump fast enough. Once the pump chamber
: fills with air I have to shut the pump off for the water to get back in
: where it will pick it up again.: I figure with a self priming pump I could build a strum box (pick up) and it
: wouldn't matter if the pump started sucking air. I think it would go a
: loooong way to getting those last few inches of water out of the boat.Woody...
I am not a pump expert but from what I know... a self priming pump is not a dry run pump. It isn't going to help you when you run out of water. Water doesn't like to go uphill and it has no tensile strength so you need to power it up there somehow. Suction is one way... the "pull" method... but when ya get down towards the bottom of the boat the suction breaks and the "pull" stops. But bilge pumps are built differently that other pumps.
On the Attwood and probably on others, the impeller is flush with the floor or maybe a quarter of an inch above. The water is not pulled into the impeller through a suction tube. The water surrounds the impeller at all times. Water is thrust out or "pushed" by the impeller. So it stills pumps even without a prime.
I think this is what we want... not a pump like a pool pump where you have to "pull" the water thru the returns. Here a prime is necessary because you can't get the water to the impeller without it. The bilge pump's water is already at the impeller so it still has "push" power.
That's why I don't use these cheesey little pumps... they have no impeller power. A bilge pump has to thrust the water out... not suck it out... it wants power to push the water up and out through the bilge hose.
Your note speaks of the last few inches of water... believe me if I have a few inches of water... my Attwood will be sending an 1 1/8" solid stream of water shooting a foot or more out the side of my hull just behind the cockpit. Properly installed, you should have no more than a quarter or half inch or water in the boat. That last quarter or half inch of water is really no big deal. It does not present a safety concern. You can sponge it up when you get to a safe harbor.
Just some of my thoughts on it.
Safe Paddling!
Brianne
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