Why does cooling water exit above waterline?

[MarkI]

MY AC/heater uses sea water for cooling/heating transfer. The sea water enters a through hull, goes through a pump, heat exchanger, and then all the way aft to an above the waterline exit (through hull). There are threee such exits with a lot of hose in a new boat, there has to be some reason.

So when the heater is on (winter, or AC in summer), there is a constant splashing, that makes much more outside noise than the generator. It makes me shy to use heating when at a marina, or rafted up, or even perhaps at anchor (which I have not done yet).

Why don't they just exit the water out a nearby through hull that is there anyway to for instance source water for the head? It would be quieter, AND use less plumbing (this is a new boat design, 2019).

Is there a reason not to reroute the water exit to a nearby through hull (that is spaced away from inlet thru hull)? I realize it would be harder to confirm the pump is working, so I would use a Y valve to allow routing to original exit.

I am worried there is some problem with a slat water LOOP, as I have found no other such loop. Engine coolant goes through an air separation via its mixing with exhaust, same for generator, and head uses go through either black water tank, or sink drain.

Would I create some sort of corrosion path if I have a salt water loop? The pumps appear to be centrifugal, and would not separate the water path.

[Cheechako]

Originally Posted by MarkI

MY AC/heater uses sea water for cooling/heating transfer. The sea water enters a through hull, goes through a pump, heat exchanger, and then all the way aft to an above the waterline exit (through hull). There are threee such exits with a lot of hose in a new boat, there has to be some reason.

So when the heater is on (winter, or AC in summer), there is a constant splashing, that makes much more outside noise than the generator. It makes me shy to use heating when at a marina, or rafted up, or even perhaps at anchor (which I have not done yet).

Why don't they just exit the water out a nearby through hull that is there anyway to for instance source water for the head? It would be quieter, AND use less plumbing (this is a new boat design, 2019).

Is there a reason not to reroute the water exit to a nearby through hull (that is spaced away from inlet thru hull)? I realize it would be harder to confirm the pump is working, so I would use a Y valve to allow routing to original exit.

I am worried there is some problem with a slat water LOOP, as I have found no other such loop. Engine coolant goes through an air separation via its mixing with exhaust, same for generator, and head uses go through either black water tank, or sink drain.

Would I create some sort of corrosion path if I have a salt water loop? The pumps appear to be centrifugal, and would not separate the water path.

Really? You must have a huge AC unit!

[rslifkin]

I do hate boats that discharge A/C water 2+ feet above the waterline. That ends up making a ton of noise for no reason. Personally, I like the discharges to be just above the waterline. Not too much noise, easy to confirm waterflow and less fittings that are permanently submerged (although on a sailboat they would be when heeling, so they should still have seacocks).

[Rob_P]

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

[Muaddib1116]

I'm pretty sure my refrigeration and ac raw water is discharged under the waterline through a thru-hull shared with the kitchen sink.

[MarkI]

It seems like two issues, one being pressure, especially differential pressure due to depth of each through hull, and resistance the water will present.

For the pressure issue, having the exit higher than the inlet, will aid that issue, good point.

For The resistance of the water, the alternative is lifting the water 2-3 feet and running through an extra 25' of hose.

i assume the worry is using more power, and perhaps needing a slightly larger pump, though that seems less likely to me.

Thanks.

[MarkI]

I am sorry but I do not see what prompted that response (big AC unit). For details, they are 1500watt or less AC units, three in a 46' sailboat. I don't think its unusual, but again I am not sure what point led you to that conclusion.

Perhaps it is the noise? The pumps do move an excessive amount of water, but I have seen that on other boats as well. The designers do not add complexity to adjust water flow to the needed amount, just "lots of water".

[Coolerking]

The btu rating is directly tied to the gallons per minute the pump provides.
Approximately 4 gpm per 12k btu/hr.
The reason you have 3 discharge thru hull is that the builder send the water overboard very close to where the self contained unit is located.
I usually will oversize the discharge fittings to slow the exit flow rate down and this also keeps noise down.
I see lots of underwater exits on the European built boats I work on, and they seem to always have issues, not the least of which is the fact that you can't tell if you have any water flow at all without getting wet.
They are also more prone to air lock, ideally the ac unit is located higher than the inlet and outlet, which means that it is self draining to the waterline, but in reality it's hard to do.
Some I've seen don't even have a ball valve to close down...not good.

[Lodesman]

Originally Posted by Rob_P

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

And fouling with aquatic shrubbery.

[Sailingsolo]

Confessions of a boat builder..... 10 yrs ago I wondered the same thing and decided to multi task the through hulls. The aft salt water toilet had its own vented loop and a 5 foot drop to a Y..... Which connects to the washing machine. Which has its own vented loop with a 5 foot drop. 8 years later ...
.. A new brand of toilet paper clogged the through Hull ball valve..... Unknown to me.... I kept using the head. The black water easily went up the washing machine pipe and the black water filled the washing machine.

What a mess.... And smell....
Another mess.... The shower grey water shares a through Hull with a bulge pump... The below waterline through Hull clogged by marine growth and the grey water backed up and exited through the bulge pump in to the bulge.

Dam.... Sharing through hulls..... My mistake.

[Cpt Mark]

When I ran my heat pump the water exited the aft of the boat which is far from the living/sleeping quarters. When I 1st installed it neighbors told me my bilge pump was running a lot. After they learned nobody thought anything about it and nobody complained or even commented on the noise which was the same as my old AC water cooled refer system.

If it really bothers you maybe try a screen over it or something to spray the water instead of a solid jet of water??

[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.

[Muaddib1116]

I'm pretty sure most monohulls have the the sinks drain below the waterline.

[Howler]

To complement the physics on other answers, it's also useful to be able to see the exhaust as a way to diagnose problems. For example by observing flow rate (or lack thereof), color, smell, temperature, accompanying smoke, etc, you can tell a lot about the health of your system (diesel engine, air conditioning, plumbing, etc.)

[HeinSdL]

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.

Apologies in advance, my sense of humour is not great, but this is meant to be funny/sarcasm, right? I have never seen a kitchen sink drain above the waterline. Of course, the kitchen sink would need to be above the water line which I can only HOPE it is...!

Anyway, on the topic of (neighbour's) airco's running at night and draining above the waterline, I find it a highly irritating noise. In any case when it's one of those very peaceful nights, no wind, etc...


Edit: above or below the waterline is probably a cat vs mono difference. I know nothing about cats...