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Go Back   SailNet Community > On Board > Gear & Maintenance > Bilge Pumps
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Thread: Bilge Pumps Reply to Thread
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Topic Review (Newest First)
02-08-2007 12:20 PM
Razman23
Quote:
Originally Posted by T34C
Raz- You have to be really carefull with this aproach as it is possible for water from cockpit to backflow down into the bilge.
Ah crap! Youre right.....................nevermind.
02-08-2007 12:05 PM
Idiens SD - I see the engine pump as additional capability. Clearly, choose your pumping capacity and there is always a little bigger hole below the water line that will overcome it. In such circumatances the pumps are just winning time while one struggles to patch or bung the hole.
Yes, crud can stop any pump, but that applies to the electric ones too, so strum boxes are needed with a mesh that only allows acceptable sized dirt through. Since the raw water pump already has a strainer, adding a strum box to the "emergency inlet" plus a Y-valve is a fairly competitive way of increasing the time before the life raft starts looking attractive.
02-08-2007 11:02 AM
T34C
Quote:
Originally Posted by Razman23
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.
Raz- You have to be really carefull with this aproach as it is possible for water from cockpit to backflow down into the bilge.
02-08-2007 10:36 AM
sailingdog I've seen more boats have problems due to water cooling issues, and don't really see the need to risk it on a boat that is already having some issues. If the engine continues to run, you can run as many large electric bilge pumps as you need...but as soon as the engine fails... you're down to your batteries and whatever manual bilge pumps you have...

It is really a personal choice, but I think that having a small bilge pump for the day-to-day stuff, and one or two large bilge pumps for emergency use, that have a higher float switch than the day-to-day pump does makes far more sense. As long as you have diesel and can keep the motor running, you'll have electricity to keep the pumps going. If you use the raw-water cooling as part of the pumping solution and it gets blocked with crud... then you're down a pump, but also have lost most of your electrical generation capacity, and possibly the ability to move your boat toward safety...since the diesel is also powering the prop...

A small hole in the bottom of a boat, say 3' below the waterline because of a grounding, even if it is only an 2" in diameter will let in 136 GPM... Given that most bilge pumps are rated at GPH, not GPM, the largest 12V bilge pump I've been able to spec out rates at 3000 GPH, or 50GPM, which would require three of them to stay ahead of a small 2" hole. That is an optimistic estimate as well, since the pumps are rated at 0' of head, and in this case, we'd have at least 3' of head, to get the water out of the bilge and up past the waterline. I don't believe a raw water cooling system runs at 50 GPM, given the small diameter of most of the tubing and passages in it.
02-08-2007 08:40 AM
Razman23 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.
02-08-2007 04:42 AM
Idiens 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.
02-07-2007 10:45 PM
sailingdog 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.
02-07-2007 07:52 PM
Rubingr 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
01-30-2007 07:58 PM
sailaway21 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.
01-30-2007 02:50 PM
rheaton 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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