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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #11  
Old 01-30-2007
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wildcard: diaphram pumps can be run dry since they make a vacuum that suck the water upward. Theoretically, you could have a pump above water line that draws all of the water out of bilge until its bone (well more like moist) dry. Centrifugal pumps are generally "wet sump" meaning that they require the water to be at the level of the impeller or higher. I've only used centrifugal pumps, not because they are better, but because they are much cheaper and slightly more energy efficient. I've been thinking about a check valve on my own system. It won't eliminate the little remaining water in the hose from coming back in, but it will slow it enough to not cause the pump to spin backwards (and yes, this is very bad for pump motors, especially those that are of the "mag drive" type)
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Old 01-30-2007
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The other advantage of diaphragm pumps is that they can also pass small obstructions, which impeller and centrifugal pumps have issues with.
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Old 01-30-2007
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  #14  
Old 01-30-2007
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I like the two pump suggestion. But I would also suggest the two float switch method. I got a wet surprise, of the bilge variety, when my one float switch decided to stick, and it was a new float switch too, as the old one had decided it preferred a heads down approach.

Second tip: wet-and-dry vacuum cleaners are great for getting the last irritatingly smelly drops out (plus any "articles carefully designed to block bilge pumps").
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Old 01-30-2007
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Folks, I have looked at a nice site which included an article on bilge pumps http://www.yachtsurvey.com/maitenance.htm. Many of the tips you have shared here I see there too.

Happy Sailing.
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Old 01-30-2007
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The problem with having a suction line on a pump mounted above it is that "lift" is now entered into the equation. A centrifugal pump has a theoretical lift capability of 25 feet. Any amount of suction lift reduces the capability of the pump. BTW, an elbo located in that suction line adds the equivalent of 5' of lift. Now head is a different matter. The centrifugal pump can "push" water much more effectively than it can pull or lift it. Most are, in fact, designed to have a certain amount of head pressure against discharge. A sometimes easy way to pick up prime on a centrifugal that may be air-bound is to throttle the discharge down to almost closed until pumping is acheived.

The chief advantage to centrifugal pumps is volume pumped. A secondary is simple, trouble free, rotational motion design. A centrifugal designed for pumping water with debri should have open, versus closed, impellers. The slight loss of pump capacity will be offset by the reduction in impeller fouling. Sewage ejection pumps are frequently of this design.

The problem with diaphram, and all "positive displacement" pumps, is capacity. For a given size, and given size prime mover, we cannot get the volume out of a positive displacement pump that we can out of a centrifugal pump. With this trade-off in mind, it may make good sense to mount the positive displacement pump at the lowest point in the bilge where it can perform a "stripping" function, while mounting the high capacity centrifugal higher where it will be able to do it's job without as much risk of fouling impellers. Coincidently, that is roughly how you pipe up an oil tanker for discharge. The other problem with positive displacement pumps, be they diaphram, piston, or gear, is maintenance. These pumps by the nature of their design, wear more. Most of them involve reciprocating components and that always means friction and wear.

Ironically, I just saw the first "Wilden" pump I've seen in a number of years-my auto mechanic has one for pumping oil. Wilden is the world class leader in air-powered diaphram pumps. NO merchant ship is without one, for a variety of uses. They come in all sizes-my mechanics was about 10" square. The size seen on merchant ships are usually the size of a portable generator. The pumps are made of aluminum and thus light and portable. It is child's play to rebuild one and parts are available world-wide. You do need air to power one though, which brings me to my point. In the post mentioned earlier, the surveyor mentioned having a bilge pump running off the engine. I did not have time to read his idea thoroughly, but it occured to me that it might be easier to mount a compressor on the engine-similar to a Peterbilt. With a supply of air you could run a Wilden pump anywhere-in your bilge, in the bilge of the boat along side, in your dingy, hell you could lower it over the side and use it for wash-down if you wanted to get crazy. Wilden does make attachments, via cam-lock fittings, to fit a suction line on at the strainer box, in essence making a water shop-vac out of it. Larger boats could use this to good effect. Compressor draw on the engine would be the limiting factor I suspect. Giulietta could also then use a pneumatic wrench instead of his electric winch buddy!

The best line check valves I know of are made by Maas-Midwest and available in the sizes required. They generally come threaded female, each end-but male x female are probably available. They are an all bronze spring check (no plastic) and are of superior construction and reliability. I recommend an oversized check valve for bilge application due to the possibility of small stuff/line debri. If discharge is 1", I'd go 1-1/4" on the line check. I advocate mounting the check valve as close to the pump as possible. If the hose above the check is already full of water it will aid in "loading" the impeller via head pressure when the pump kicks in. It will also be much more effective at stopping water running back through the pump into the bilge. Mounted at the through-hull the water in the line will slowly seep back through the pump and into the bilge-although it will not allow water from the through hull to pass. The valves are more effective against head pressure than thay are against vacuum on the other side.

That's probably enough to provoke comment-and I haven't even gotten to the employment of ejector pumps(probably not what you may think they are), but I think they'd be of limited use on a boat. Although the USCG still requires their carriage on merchant ships.
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Old 02-07-2007
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Personally I use a dual pump set up like what has been described above, except for boats over 50' where I might consider going with 3. The only thing I would add is that FOR EMERGENCY PURPOSES ONLY it can be very usefull to be able to use the engine intake as a bilge pump. I have a selector switch on my boat for this purpose, and while I have never used it, the one time i did something like this on a raceboat that was damaged, it really helped out.

Greg
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  #18  
Old 02-07-2007
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Rubingr-

Given how little water the engine pumps, I would hesitate doing that. Also, is it worth endangering the engine in an emergency, by possibly getting bilge crud in the cooling system. That would make a bad situation worse.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

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  #19  
Old 02-08-2007
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SD- I have seen a lot of articles in the yachting press recommending the use of the raw water pump in emergencies (via a Y-valve between the inlet seacock and the strainer). They claim the pump actually has a very large capacity - compared to the average bilgepump. I was thinking of adding the facility together with a strum box. Comparing the impeller sizes on my boat, the raw water pump certainly has more volume, its on the end of the diesel pump drive shaft, so it runs at engine rpm. Snag is, the route the raw water takes is tortuous, so the pump sees a lot of back pressure. I guess I should measure it; to see how long does each pump take to empty a bucket of water.
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Old 02-08-2007
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Im not going to get into the pump debate already going but I would like to suggest that instead of connecting the bilge pump hose to the emergency hand pump hose, you may look at teeing it into the hose from your cockpit scoopers. I dont see any sense in putting ANOTHER hole (granted its has a seacock) below the waterline of your hull if not needed.
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