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Go Back   SailNet Community > On Board > Gear & Maintenance > Pumps do not do what you think
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Thread: Pumps do not do what you think Reply to Thread
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Topic Review (Newest First)
06-23-2013 03:13 PM
robwilk37
Re: Pumps do not do what you think

my boat, 40' 9tons relatively shallow bilge, has a pisser (800gph rule) for the minor drips and drabs and occasional wash down of the cabin sole. set about 6" above that is a 2400gph electronic (with a siren alarm thatll wake the dead) mostly in case the pisser fails and a deck hatch is left open during monsoon season. then theres the beast - 8000gph rule - i picked up at a marine swapmeet brand new for $75. its not on auto, you have to throw that switch intentionally. ive got a huge manual whale i can work vertically from the cockpit and a y-valve on the head pump so it can be pressed into service if need be. they all discharge to the COCKPIT and not a throughhull , where ive got more than enough drainage out the transom above the waterline. if your cockpit drains arent up to that kind of flow then thats where you put a scared crew with a bucket. and youve got a visual warning that your pumping when you are at the helm.

next, im essentially double-hulled from the bow to midships about 6" above the waterline as all the v-bearth, settee, and stowage lockers have secure latches and gasketed lids. this isnt as hard to do as one might think, and itll at least dramatically slow or even stop and compartmentalize flooding from a collision until a repair or patch can be well thought out.
lastly, carry several sizes and shapes of 1/2" ply pre-drilled around the perimeters and some truss head self drilling sheet metal screws, and a cordless drill. think it through and when youve calmed down some splooch on some sealant and just drive the screws into the hull.

and of coarse tapered plugs at all the seacocks.
06-22-2013 03:50 AM
SlowButSteady
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.
06-21-2013 11:12 PM
olson34
Re: Pumps do not do what you think

One of the best stock setups I have seen was the big Edson manual pump mounted at the foot of the companionway on a Santa Cruz 52. Slot in the sole section gave it away...
The broker told us that owners often referred to it as their "Oh My God" pump!
Gallon a stroke!
Handle was about 4' long.

You'd still have to have someone finding the leak and doing damage control, as others have pointed out.
06-21-2013 06:13 PM
hellosailor
Re: Pumps do not do what you think

A pump is not there to "unhole" the boat. It is there to help empty the water out, while you are plugging the hole. There's a reason why serious dewatering pumps are driven by gasoline engines and resemble medium size gensets.
06-21-2013 05:49 PM
fryewe
Re: Pumps do not do what you think

This thread confirms step 1 of damage control during flooding into the ship: STOP THE LEAK!

You have to reduce the leakage to less than your dewatering capacity before all your reserve bouyancy is lost.

Nothing else can save your ship from sinking.
06-21-2013 09:29 AM
BubbleheadMd
Re: Pumps do not do what you think

I was wondering about the corrugated hoses. Thanks for the info, Maine.
This will help me in my upgrade plans.
06-21-2013 07:54 AM
Maine Sail
Re: Pumps do not do what you think

This is not at all unusual for a centrifugal style bilge pump. I always figure on 1/3 or less of face value rating as installed.

Some things most are not aware of:

#1 These pumps are rated at the open bucket rate. This means a ZERO head rating.. This is a VERY DISHONEST way of selling a bilge pump to unsuspecting buyers..... I personally find this type of marketing disgusting at best. Almost as bad as companies selling "marine" tankless LPG fired water heaters that meet NONE of the safety standards. Caveat Emptor.......

#2 These pumps deal horribly with added head pressure and can pretty easily lose 50-75% or more of their rating as installed.

#3 These pumps are rated at charging voltages. This is NOT completely honest as most sailboats would not have the motor running when the bilge pumps was in use. All one needs to do is look at the amperage consumption to see how much the voltage can affect these pumps. A Rule 2000 will draw 12A at 13.6V and just 8.4A at 12.0V....... The current a pump draws is directly related to GPM/GPH volume

#4 Corrugated hose adds a LOT of head restriction.

#5 Check valves should not be used on the outlet of the pump because the pumps often don't have the oomph to overcome the standing water in the hose or the check valve sticks.

#6 Every elbow or turn adds more head restrictions.

#7 The height from bilge to siphon break creates even more head.

#8 Undersized wire can add yet more unwanted voltage drop to the pump causing even more reductions in output.

#9 Discharging below water, such as teeing into a sink drain, can also slow the pumps capacity...
06-21-2013 01:55 AM
jrd22
Re: Pumps do not do what you think

I've always figured that 12V electric pumps are only going to buy me a little time to find/stop the leak unless it's just a small one. Any serious hole is going to sink the boat if it can't be plugged or slowed down.
Most boaters seem to be overly impressed with the "GPH" rating of their pumps without doing the math to find out what that equates to in the much more important "GPM", to say nothing of the friction losses and head.
06-20-2013 03:08 PM
vtsailguy
Re: Pumps do not do what you think

The pumps are pretty new and the hoses are to spec. You should try the test yourself, you might be surprised!
06-20-2013 01:14 PM
Tim R.
Re: Pumps do not do what you think

Quote:
Originally Posted by vtsailguy View Post
So I did some real world tests and calculations on my pumps.

Four pumps will net me about 160GPH

A one inch hole lets in 2,400GPH

Nowhere near enough for even a relatively small hole. Basically all pumps are useful for is open windows and children's dribble?

I made a video, the math is at the end, can this be right?

That Sinking Feeling | Sailing With Kids
Out of curiosity, how long are your hose runs? Are there any back flow prevention valves on these hoses? Is the hose diameter sufficient throughout the run of the hose? How old are your pumps? How high do your pumps have to pump? Have you verified that the vanes of the pumps are clean?
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