Does the sea suck?

[Malabarista]

Probably a really daft question and my ready made excuse is a severe lack of education.
I have been pondering and googling but i an unable to discover how, if a through hull fitting is below the waterline, does the water actually get out? Surely the immense water pressure outside the boat would keep the exiting water in the pipe.
I can understand how a self bailer works on a dinghy with the forward motion causing a 'suck' due to a pressure difference caused by the flap. But no through hull i ever saw has anything like this.
Would some enlightened person please explain this to me?
Thanks

 

[boggybrn]

The water stays in the pipeline up to the waterline of the boat - unless you pump it out...

 

[prv]

I have been pondering and googling but i an unable to discover how, if a through hull fitting is below the waterline, does the water actually get out? Surely the immense water pressure outside the boat would keep the exiting water in the pipe.

"Immense" is a bit strong unless you're in a ship a deck or two below the waterline

You get approximately one bar of pressure per ten metres below the surface. It's rare to find a skin fitting more than 50cm below the waterline on a normal yacht (most are less), so a maximum of 0.05bar or 0.7psi.

More importantly for your question, that pressure exists whether the water is in the sea or in a pipe inside your boat - if the top of the pipe is open to the atmosphere (for example your sink drain) then the water doesn't know the difference. So the pressure (whether "immense" or not) outside the seacock is exactly balanced by the pressure inside it. If you empty a pan of water into the sink and fill the pipe higher, the pressure at the bottom of it is greater, greater than the pressure of the sea outside, and out it flows. Once the level in the pipe drops to the level of the sea, the pressure is equal once more and the flow stops.

Except for temporary dynamic effects, the water in the pipe never drops below the level of the sea. This is why sinks that can go below the waterline when the boat heels are bad news, and toilets (generally the lowest-mounted open-topped fitting connected to the sea) need special care to prevent back-flowing.

Pete

 

[RichardS]

The water stays in the pipeline up to the waterline of the boat - unless you pump it out...

I had a real life example of that when I poured hydrochloric acid into my port rudder stock tube from the top to try and dissolve any crud and free up the jammed rudder.

It didn't work so I disconnected the rudder and after another night at anchor and a day's sailing we arrived back at the marina to be hoisted out.

One of the yard guys pulled out the rudder and the residual seawater flooded out .... except that it was acid and his shirt fell apart as we worked.

I did warn him that I had poured acid down it the previous morning. :ambivalence:

Richard

 

[Malabarista]

I am now enlightened. Please excuse my ignorance and thanks for not mocking it.
David

 

[Seajet]

You might get a partial ' self bailer suck effect ' under way by fitting a half shell fairing over hull outlets - also Whale make various sizes of one way valves for bilge pump pipes etc.

 

[Kelpie]

I remember my Dad trying to explain to me why the dinghy we were in didn't sink via the centreboard slot. I was just amazed that there was this hole in the middle of it and I could see the sea. I'm sure he tried his best to explain it but I remember being thoroughly confused and determining that it was some form of magic.

 

[AndrewB]

Whirlpools suck. Well, the scientists say its the effect of highly aerated water, which causes the yacht to be less buoyant. But it feels like being sucked ... down.

Dent Rapids, B.C., "Devils Hole".

 

[dolabriform]

I remember my Dad trying to explain to me why the dinghy we were in didn't sink via the centreboard slot. I was just amazed that there was this hole in the middle of it and I could see the sea. I'm sure he tried his best to explain it but I remember being thoroughly confused and determining that it was some form of magic.

Everything can be categorised as some form of magic, even when we understand how it works

 

[Pye_End]

Depends if the boat is moving. It is the 'Venturi effect' which produces the 'suck', so you would expect the water in a tube from the skin fitting in a moving yacht to be lower than if the yacht were stationary (ie with the water).

Apart from that, if you poor water down the sink, the level in the pipe try to remain the same level as sea level. Put a litre down the sink, and a litre will come out the skin fitting - might not be the same litre, but a litre none the less.

 

[Malabarista]

Everything can be categorised as some form of magic, even when we understand how it works

This magical effect is what i had presumed from the start. However i have been properly informed now and no longer believe in magic…well apart from all the other systems i don't understand like the batteries…
Thanks to all

 

[ex-Gladys]

I'm old enough to remember the predecessor to the Elvstrom style self bailer, which was a bronze affair consisting of two concentric cylinders. The inner one had an aperture cut in the back, and when lowered, the reduced pressure aft pulled the water out... skin fittings aren't like that...

 

[johnalison]

I thought this was going to be a thread from someone who had fallen out of love with sailing.

 

[LittleSister]

I'm old enough to remember the predecessor to the Elvstrom style self bailer, which was a bronze affair consisting of two concentric cylinders. The inner one had an aperture cut in the back, and when lowered, the reduced pressure aft pulled the water out... skin fittings aren't like that...

But only when you were going fast enough. Slow down, and it starts coming in! (Or at least that's what used to happen on my old Firefly with similar fittings.)

 

[Malabarista]

I thought this was going to be a thread from someone who had fallen out of love with sailing.

??

 

[Habebty]

Bit of thread drift, but consider how you get water into the pressurised boiler of a steam locomotive - using only its boiler pressure.

Oh, and my old National 12 had those cylindrical plunger bailers that only worked if you you were moving too

 

[Habebty]

I'm old enough to remember the predecessor to the Elvstrom style self bailer, which was a bronze affair consisting of two concentric cylinders. The inner one had an aperture cut in the back, and when lowered, the reduced pressure aft pulled the water out... skin fittings aren't like that...

Actually, the laminar flow over a moving hull will "pull" some water out of a skin fitting, or prevent efficient intake, especially if it is a raw water intake near a saildrive skin fitting. I had overheating issues until I increased the internal diameter of the hull skin fitting.