Pumping shower water overboard

[Britannia]

But if the pump is here, the thru-hull is there, and the distance is X feet... the pump still has to move the water X feet, yes?

-Chris

Yes - but with a hose that's twice the diameter it only has to move the water at a quarter of the speed - so the same power pump will work as for the narrower hose where the water has to move faster. The only thing that matters is the hydrostatic pressure at the pump (ignoring friction). That's determined by how far below the waterline it is.

Richard

 

[BruceK]

Mark, did this system ever work, or is this a new development?
A seacock on the outlet is new information, must be a PITA opening and closing it every shower.
I`m still thinking upgrade the pump, simple easy cheap. Pumps in "all in one box sumps" are usually small low capacity to fit, go as strong as you can, mine is a fierce 1500GPH pump which leaves nothing but some greasiness on surfaces.

 

[Capt.Bill11]

No.

Here's the easiest way to think of it. Let's suppose the pump needs to pump one pint of water. Let's suppose that the narrow hose holds one pint of water in 4 feet. Then the pump needs to move the column of water up 4 feet. Now suppose that a larger hose is used - with twice the diameter and hence four times the cross-sectional area. Now the pump only needs to move the wider column of water up 1 foot to pump the same pint of water. So it moved 4 times as much water up 1/4 of the distance. So the same work was done by the pump in both cases - and it is not affected by the diameter of the hose (ignoring friction).

I hope that helps

Richard

Some of this is ringing a bell from dive class years and years ago.

But I was talking about not a pump pumping X amount of water up an empty hose but having to pump up against a hose already filled with water before it starts to pump. And up above sea level.

But maybe it makes no difference that one size hose would hold more water by volume and mass than the other and I'm just having a slow day.

Although I believe there is a difference between salt and fresh water due to their differences in density.

 

[markpierce]

Mark, did this system ever work, or is this a new development?
A seacock on the outlet is new information, must be a PITA opening and closing it every shower. ...

Only taken a shower in the boat twice in four years; always "flooding" a couple of gallons (wet, soap, and rinse) in the bilge. The gray-water seacock is no more pain than any other of the boat's eight seacocks (five below waterline). In fact, it is one of the easiest to turn.

 

[HopCar]

Mark,
A set up with a shower sump below the waterline, that is equipped with a small bilge pump, like a Rule 500, that discharges through a hose that doesn't rise more than four or five feet before it turns downward to the below the waterline thru-hull should work fine.
There should be a seacock on the thru-hull and there should be a vent at the top of the loop.

 

[markpierce]

That's the message I got from the boat's builder, Hopcar.

 

[Britannia]

Some of this is ringing a bell from dive class years and years ago.

But I was talking about not a pump pumping X amount of water up an empty hose but having to pump up against a hose already filled with water before it starts to pump. And up above sea level.

But maybe it makes no difference that one size hose would hold more water by volume and mass than the other and I'm just having a slow day.

Although I believe there is a difference between salt and fresh water due to their differences in density.

My description was for a hose that was full of water. It's correct - you just have to trust me!

Salt water is denser than fresh - so the pressure is greater at the same depth. The difference is only about 2-3% however.

Richard

 

[Peter B]

I think this whole thing has been a bit over-analysed, but Mark, like Hopcar said, that would work, but you'd have to close the seacock after each shower use - from your usage frequency, not a huge issue. However, a better one would be a Whale type Gulper pump, which won't jam with hair etc, as it does not have a blade, and leading the waste into the sink outlet (if easy to do) with a T or Y connection, (preferably a Y, so sink water does not just fill the shower sump, as Northern Spy pointed out), to avoid an under waterline thru-hull, and obviate the need to remember to open said seacock for each shower. Matter of priorities I guess, and just how accessible that seacock is.

 

[Capt.Bill11]

My description was for a hose that was full of water. It's correct - you just have to trust me!

Salt water is denser than fresh - so the pressure is greater at the same depth. The difference is only about 2-3% however.

Richard

OK, OK, I trust you and basic physics.

 

[ranger58sb]

Yes - but with a hose that's twice the diameter it only has to move the water at a quarter of the speed - so the same power pump will work as for the narrower hose where the water has to move faster. The only thing that matters is the hydrostatic pressure at the pump (ignoring friction). That's determined by how far below the waterline it is.

Richard

Hmmm.... digesting...



-Chris