Open thru hulls

[doctorbill]

So this seems to me to be a simple question of physics, but here goes.

When I open a below waterline thru hull with a hose attached to it, how much water goes into the hose? I assume it levels off at the waterline? I ask because my cockpit scuppers are usually kept open, and it only recently occurred to me that there is a permanent water level in the thru hulls and hoses. What about my holding tank macerators? Can there be something like an air-lock that prevents sea water inflow?

 

[CharlzO]

Not sure on the last bit, but yes, it basically will level off at the waterline. In my case, my cockpit drains are barely above it, so there are times you can actually heel enough to get it to splash slightly, but usually goes away. I actually will close that seacock at times when I know there's no rain forecast while I'm around, just to avoid that sort of thing.

 

[dlochner]

So this seems to me to be a simple question of physics, but here goes.

When I open a below waterline thru hull with a hose attached to it, how much water goes into the hose? I assume it levels off at the waterline? I ask because my cockpit scuppers are usually kept open, and it only recently occurred to me that there is a permanent water level in the thru hulls and hoses. What about my holding tank macerators? Can there be something like an air-lock that prevents sea water inflow?

Water will always seek its own level. If there is an open seacock, as in a sink drain seawater will rise to the waterline level.

If you have an open seacock that is sealed above the water line, such as might be the case with a macerator pump, the water will rise until the pressure of the water going up equals the pressure of the air in the line pushing down. This may or may not be at the waterline. The deeper the through hull, the greater the water pressure will be so it will push up into the through hull until the air pressure in the closed fitting is equal to the water pressure.

 

[kloudie1]

The tall vented loops in the sanitary system are air locks to keep the seawater from siphoning into the boat, but the sea side of the loop has water in it up to the waterline level (if the seacock is open). The last diagram in this article shows the concept.
http://www.westmarine.com/WestAdvisor/Selecting-a-Sanitation-System

 

[Joe]

I think your concern is that even though you close the seacock, there will be water in the line, since the valve is below the waterline. The water will never go below the waterline unless there is some kind of pressure to push it down.

So the question might be: Is the standing water between the seacock and the waterline a problem?