Why does cooling water exit above waterline?

[Tonali99]

Originally Posted by meirriba

Probably you are mistaken.
I have never seen kitchen sink on a boat to be drained underwater.
You would need a pump to press the discharged fluid into the sea.

A pump? How far below the water line is your sink?

[valhalla360]

Originally Posted by Rob_P

Discharges don't exit below the waterline because you'd have ocean water pressure impeding the discharge flow.

Also, if you are running the heat, odds are everyone has their hatches closed anyway, so they won't hear it.

Even in the summer with air/con...you will likely be the only one without water flowing.

I like a little noise, so if something plugs it up, I know quickly before the unit burns up.

[JimZeigler]

The reason my AC discharges are above the water line is to avoid having to install seacock and also a quick check that the water is running. I have seen discharge that splay the water, but I could not find any on-line.

Each of our AC pumps has a dedicated line. I would be concerned with T-ing any high volume pump into another line. If there is any stoppage downstream, the water can flow back up the other line as several postings have related.

I am not sure, but can think of several issues putting the discharge of an AC under water. Obviously the risk of another thru-hull. (Freezing, breaking, hose coming undone....) The AC pumps are usually small, high volume, low discharge pressure centrifugal pumps. They do not generate a lot of pressure - notice that the discharge stream does not extend very far from the boat. Centrifugal pumps are quirky. They generally do not pull a vacuum, so if the intake side is not full of water, air gets into the rotor and it cavitates. Putting the outlet under water creates back pressure, the enemy of these centrifugal pumps. The backpressure comes from the viscous flow resistance of pushing the flowing water into the still water. Test the difference in pushing air through a straw or hose with the opposite end in water in open to the air. It is more difficult when the other end is submerged. Since the AC pump has to cycle on an off, you want it to have reliable flow.

The discussion regarding the underwater sink drains it more interesting. It is air pressure, not gravity that cause the water to drain out of the hose that ends underway. A column of water 27 inches tall weighs 1 lb /square inch. It is easy to demonstrate - just put one end of a hose under the surface of water in a bucket. Fill it from the other end and all the water will drain down until the water in the hose is the matches the height of the water in the bucket. If you want to empty the bucket, coil the hose in the bucket until it is complete full of water, then place your thumb over one end. Pull that end of the bucket and place it below the level of the water in the bucket. Remove your thumb and the water begins to flow. What is surprising is that while holding that end blow the level of water in the bucket, lift a loop of the hose out of the buck much higher than the rim of the bucket. The water will continue to flow. Does it stop at some point? Run the experiment yourself.

[MAJICDAN]

My 1974 Gulfstar 53MS came from the factory with all three air conditioning water outlets right at the waterline.
No noise. Easy to check if you have flow.

[AKA-None]

Originally Posted by Muaddib1116

Yeah, pretty sure you're wrong. And yes, the drains have seacocks.

Yeap that’s my boat drain below the water line

[Palarran]

Originally Posted by BillKny

Wow! This thread has way more than the usual amount of stupidity... and that's saying something.

Having a water line discharge below the water line does NOT add back pressure--except maybe for the longer hose. If you do not believe this, you need to go back and take Physics 101 again. And maybe a third time. Trust me on this. I have designed piping systems and know how they work.

And the idea that a sink can not discharge to a thruhull below the water line???? OMG!!! Besides showing a total lack of understanding of liquid flow, it also shows a total lack of knowledge and any real world experience with how boats work--ALL boats.

Let's be clear, there are reasons that an AC discharge shouldn't be underwater. It is another constant un-necessary potential leak point and you certainly can't easily confirm water flow.

There are a lot of reasons they shouldn't be high above the water either. NOISE! Hull stains. And the worst of all for noise is directing the water flow straight down under a reverse transom. This is the best way of all to annoy your neighbors. I once watched from afar as an AC water discharge filled and swamped a dinghy belonging to a boat's visitor. There is NO EXCUSE for such crappy design.

The RIGHT way to do it is a discharge through a LARGE diameter fitting (to reduce velocity) right at, or a little above the water level. It will be silent and functional. Yes, you need a seacock. But on any sailboat that heels you need a seacock on pretty much any water discharge anyway.

Any other arrangement is done out of ignorance or cost cutting.

Thank you. Perfect answer.
As an FYI though, I already had a spare below the water through hull when installing the a/c so I ran the discharge to it. Why not? No noise and the hole is already there. Zero performance problems.

[AKA-None]

Originally Posted by Cynara

So you don't get a back siphon/?

From where?

[Corvidae]

I'm wondering if there needs to be a hose attachment you could stick on the side of the boat. Like one of those floppy discharge hoses with a small hole for a siphon break. I've seen there's little lip things you can put under the through hull to hold the drips out from the hull. Maybe a clip on hose to one of them.

[Coolerking]

So you are saying that the depth of the discharge seawater point and the combined head of the entire system have no effect on flow rates?