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Old 30-01-2014, 19:29   #1
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Push or pull water

I am in the process of building a shower using an in-line propane heater that will hang from my stern rail and the person having the shower would be on the swim platform.

Here's my dilemma, is it better if the pump was attached to the same frame as the heater and then draw water about 6' straight up, or should it be on the swim platform and push the water up.

I've done internet searching and I don't get an absolute answer, so I thought I could find the answer here.

Thanks in advance...

Oh yea, my boat is a Carver Aft cabin.
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Old 30-01-2014, 20:23   #2
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Re: Push or pull water

All pumps like to push water rather than pull water, but some pumps are able to pull water better than others. Since I see the word "shower", I would "assume" that you might select a diaphragm pump, which can pull water from heights generally 3 feet or less. However, if there is a place on the swim platform, that's the best place to be, as long as the electric motor doesn't go swimming.
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Old 30-01-2014, 21:27   #3
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Re: Push or pull water

Yep, Generally push works better. A diaphragm pump is a positive displacement pump, so its better at dealing with NPSH (net positive suction head) But for a six foot lift, its better to have the pump below the water line with flooded suction. That is inside pulling off a water through hull. It should work ok on the swim platform though.
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Old 30-01-2014, 22:27   #4
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Re: Push or pull water

Pumpcity and Sailor Chick have given you the technically correct engineer's answer.
That Sailor Chick may be a cute blond, but she's scary smart. The less distance the pump needs to lift the water the better.

That said, if it's more convenient to mount the pump closer to the six foot height, you'll probably be ok depending on the pump. I just looked up the Jabsco Par Max 1.9 which is a pretty typical single outlet pump and it will self prime to almost 8 feet. My advise is to look up the specs of the pump you plan to use and mount it where it is convenient as long as it's some what below the maximum self prime height.

Of course 1.9 GPM is a pretty wimpy shower in my book. With six feet of suction it wouldn't even put out the rated 1.9 gpm. You might want to get something like the Jabsco 32600 which will put out 3.5 gpm and self prime to ten feet.

Link: Jabsco 32600-0092 3.5 gpm 40psi Water Pump
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Old 30-01-2014, 22:59   #5
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Re: Push or pull water

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Pumpcity and Sailor Chick have given you the technically correct engineer's answer.
That Sailor Chick may be a cute blond, but she's scary smart. The less distance the pump needs to lift the water the better.

That said, if it's more convenient to mount the pump closer to the six foot height, you'll probably be ok depending on the pump. I just looked up the Jabsco Par Max 1.9 which is a pretty typical single outlet pump and it will self prime to almost 8 feet. My advise is to look up the specs of the pump you plan to use and mount it where it is convenient as long as it's some what below the maximum self prime height.

Of course 1.9 GPM is a pretty wimpy shower in my book. With six feet of suction it wouldn't even put out the rated 1.9 gpm. You might want to get something like the Jabsco 32600 which will put out 3.5 gpm and self prime to ten feet.

Link: Jabsco 32600-0092 3.5 gpm 40psi Water Pump
Ah grasshopper....

Its here that I point out that you have to factor in the friction loss of the suction hose and the pressure drop across the strainer or hose inlet into the suction lift. 10 feet is not vertical distance, its hydraulic distance, which factors in suction head, suction hose pressure drop and inlet losses.

1/2" hose has a pressure drop at 3 gpm of about .19 feet per foot (Hose has a really high friction loss compared to hard wall pipe, BTW) So for say 8 feet of hose is 1.5 feet, probably more like 2 feet of head. Then there is either a inlet orifice loss, with a coefficent of about .62 or a inlet strainer with a loss of roughly depending on model of .8 to 1.5.

In other words, your going to be real close to that 10 foot max suction head which tends to make the pump work lots harder for less flow. I try not to go to the edge of capacity in a pump.

With that said, if you go with 3/4" suction hose it would probably work ok.
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Old 30-01-2014, 23:16   #6
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Re: Push or pull water

Sailor Chick, I love it when you talk engineer to me.
I don't understand the math but I get that there are losses through friction in the hose. I also agree that lower is better but you can still get a pretty decent shower with the right pump and a six foot lift.

I wondered about the larger diameter hose. How does the weight of the column of water in the hose affect the height at which the pump can self prime?
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Old 30-01-2014, 23:35   #7
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Re: Push or pull water

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I wondered about the larger diameter hose. How does the weight of the column of water in the hose affect the height at which the pump can self prime?
Oh very good question. The short answer is generally not that much up to a point. Going one or two sizes is usually, no problem. If you got crazy and used say a 6" diameter pipe it might get a little wonky depending on the pump. With a diaphragm pump, it just might take a really long time to prime. With a centrifugal pump, its probably not going to be a happy camper.

Really your dealing with a column of water and air pressure at 14.7 PSI. So at zero gauge pressure there is still 14.7 psia pushing down on the water. Pulling suction on a hose reduces the internal pressure lower then the outside atmospheric pressure. So the air pressure pushing down on the water pushes some of the water up to balance the inside and outside forces.

Every pump can pull a little NPSH, some more then others. Positive displacement pumps generally are very good at pulling suction. Though there is a maximum lift it can do with the available horsepower.

Ok less then short answer is the 3/4" hose would work fine. Actually is very nice design to have the suction pipe be larger then the inlet of the pump. But for dinky pumps its not too much of a problem to use the same size, unless your running close to design parameters.
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Old 31-01-2014, 09:22   #8
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Re: Push or pull water

I'm using a Shurflo Model - 4028-100-E54

The place we got the heater, Camping World in the US, recommended this pump and said it would lift 10' easy....but I'm not so sure.
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Old 31-01-2014, 09:29   #9
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Re: Push or pull water

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Originally Posted by sailorchic34 View Post
Yep, Generally push works better. A diaphragm pump is a positive displacement pump, so its better at dealing with NPSH (net positive suction head) But for a six foot lift, its better to have the pump below the water line with flooded suction. That is inside pulling off a water through hull. It should work ok on the swim platform though.
Appears I am not the only Mecheng here.
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Old 31-01-2014, 09:30   #10
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Re: Push or pull water

Pulling water can over heat a pump faster then pushing if the line gets plugged up (cavitation).
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Old 31-01-2014, 11:23   #11
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Re: Push or pull water

That pump is rated at 2.3 gpm. That's pretty marginal if you're lifting the water more than a couple of feet. Of course if you're using one of those damn water saver shower heads, you're not going to get much of a shower anyway.
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Old 31-01-2014, 11:29   #12
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Re: Push or pull water

Love water saving shower heads. The key with those is to either remove the orifice plate, located at the inlet to the shower head, or if its not removable, drill the hole larger. Then the pressure problems go away...

So much nicer that way...
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Old 31-01-2014, 11:30   #13
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Re: Push or pull water

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Appears I am not the only Mecheng here.
Actually there are a few bouncing about here.
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Old 31-01-2014, 11:33   #14
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Re: Push or pull water

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Love water saving shower heads. The key with those is to either remove the orifice plate, located at the inlet to the shower head, or if its not removable, drill the hole larger. Then the pressure problems go away...

So much nicer that way...
Sorta defeats the purpose entirely, but first thing I do when I buy a shower head is exploratory surgery
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