Salt & fresh washdown system

[sailingdolphin]

I am working on a salt fresh washdown system.

The idea is to T off from the thru hull that goes to the toilet for my salt water. Also T the cold water that goes to the head sink. Then run both tubes thru the deck lid that is designed to get to the sending unit and into the lazarette.  There I will have a washdown pump 3.o gph with a garden hose y valve so I can switch from salt to fresh water.

The problem so far.  I have a 3/4" hose going from the T at the thru hull and the pump will actually pressurize and stop running  but there is still alot of air in the hose especially when I squirt the nozzle.  When I first squirt it there is the most amount of pressure then I see lots of air in the hose as I am pushing down on the nozzle.

Maybe I should use 1/2" hose. Will that eliminate the air or give it more pressure?

Any Ideas?

[Stu Jackson]

Doug:  The T you have is from the thru hull that feeds the head.  Is there a valve between the head and the T?  If not, it seems you're sucking air back through the head.  I think that's what Ron means.  Changing the hose size wouldn't change the air problem.  I'm not clear where the pump is on the saltwater side.

Stu

[karista]

Doug
I have an almost identical set-up as you have, except I put the T connection for sea-water intake on the sink water drain instead the head-water intake (I was initially concerned of possible air intake) . Like you I also put a T into the fresh water supply line, all connected thru its own shutoff valves with the pump mounted vertically on the inside wall underneath the head sink. I mounted the hose connection on the port side outer coaming wall,  next to the Catalina sign. I use a 25 ft, 1/2 DIA hose that now allows me to clean the entire boat lengh with no air and good pressure. This set-up provides me the option of using sea water or fresh water.
Bernd Mueller

[rxc]

I have a similar system that I use with a hose to clean the anchor/chain.  It is also useful for cooling down while motoring on hot, windless days when we can't swim due to the sea nettles. I have a flush-deck connector from "New Found Metals".

I T'd the saltwater connection off of the engine cooling line, after the strainer.  The strainer is important because you might otherwise suck up particles that could damage the pump.  This caused a problem with the engine, though, because it sucked the water back out of the engine salt-water cooling system when the engine was not running, and gave me lots of air and little water.  I had to install a small bronze check valve in the outlet from the SW pump on the engine.  It is a flapper valve, inexpensive and with low pressure drop, so it doesn't hurt the engine flow at all.  I suspect you need a similar check valve in the line to the head.  You might also consider installing a strainer on the thru-hull, too.

Ralph Caruso
s/v "On y va"  C34#777 Ches.Bay/Magothy

[Glenn]

Doug,
You asked about decreasing pipe size to 1/2" to increase pressure; this won't work.  There are only 2 ways to increase water pressure.  Of these 2 ways, the only way to increase pressure on a boat is by changing the pump. (Just in case you're interested the other way to increase pressure is by raising the water source.  You gain, .433 psi for every foot you raise the source.  This is why the old water systems stored water above ground.  If they pumped the water up 100' they had 43.3 psi at the bottom and no need for pumps.)  

When you down size the hose/tubeing/piping you increase velocity, but not pressure and since we don't want the water to move through the piping any faster than 5 fps, we don't want to downsize too much.  For the length of runs we're talking about 1/2" or 3/4" piping will be fine, actually much larger than necessary.  Again, the limiting factor will be your pump, both in gpm and psi, and not line size.
Glenn