# Quick Water Ballast Question

Discussion in 'Hydrodynamics and Aerodynamics' started by Southern Cross, Oct 11, 2017.

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### Southern CrossSenior Member

1. Will a water ballast tank below the water line drain with the scupper open while the boat is moving? Air valve open.

2. If the 1 1/2" scupper is 2' below the water line for example, how fast would the boat have to move for the water to drain out unassisted?

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### TANSLSenior Member

I guess the tank is a normal tank and it is full of water because otherwise the effect of free surfaces can be very damaging (a ballast tank should never go unfilled). In that case, if the tank top is below the water plane, the tank will not release even a gram of water. If the top is above the water plane, water will come out until the surface of the liquid inside the tank is at the same level as the water plane.
When the boat is tilted everything will depend, as before, on the relative position tank-water plane.
But perhaps if you show us a scheme of the case you want to study, we could answer more accurately.

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### Southern CrossSenior Member

Thanks Tansl;
This goes more to an argument than a case study.

A sail boat failed to transfer it's windward ballast to the leeward tank before a tack on gravity fed tanks. The crew subsequently bailed the tank manually. I had never seen this before as many systems have pumps or the water just is drained out.

I understand that the tank top or portion of (if healed) must be above the waterline for the tank to drain. does this include the air vent? If the the air vent is above the waterline, does this change anything?

Also, doesn't water passing over the scupper from the boat moving forward "suck" out the water like a scupper on a dinghy? For example, water pressure at 2' is approx 15.5 psi at a stand still. With a combination of air pressure (vent open) and boat moving forward (greater water pressure) wouldn't the ballast tank drain?

I'm just curious at this point. Thanks again.

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### Southern CrossSenior Member

PS. When does the Venturi effect come into play?

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### TANSLSenior Member

The pressures inside the tank (water + vents) should be much greater than the seawater pressure at the height of the scupper for the tank water to come out.
The venturi effect only works when there is movement of a fluid through an orifice. It has no influence in this case, imo

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### Southern CrossSenior Member

I don't want to test your patience on this but my understanding is the working mechanism behind the Venturi is the pressure difference across the thru-hull - where the pressure in the tank is basically atmospheric (open vent) while the pressure on the downstream side of the thru-hull decreases as the speed squared. And the greater the pressure difference (the faster the speed) the greater the flow of water from the compartment. So this combined with air pressure should drain the tank no?

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### TANSLSenior Member

I am going to copy for you the explanation of the Venturi effect that I found in Wikipedia: "Venturi effect consists of a phenomenon in which a moving fluid inside a closed conduit decreases its pressure when it increases the speed when passing through a zone of section. In certain conditions, when the velocity increase is very large, negative pressures will occur and if, at this point of the duct, the end of another duct is introduced, aspiration of the fluid from this duct takes place. it will mix with the one that circulates through the first conduit. "
So I continue to say that the venturi effect has nothing to do with this matter. Now, it is clear that the movement of the ship can create overpressions in some points and depressions in others. As I say, it's all a matter of adding up the exterior and interior pressures and seeing which is larger, which will determine in which direction the liquid flows.
And do not worry, you do not bother me at all, I like to help, if I can, within my limitations.

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### Southern CrossSenior Member

Is it possible that the introduction of the scupper into the flow of the water decreases the water pressure? I may not have made this part clear. See below:

Bernoulli's principle. This is the principle which underlies the Venturi effect: increasing the flow speed leads to a drop in pressure.
• The constriction in this case is between the wedge of the autobailer and the water further away from the hull of the boat: when the boat passes over a given 'parcel' of water, the wedge applies a force forward and downward (perpendicular to the plane of the wedge). The fact that the water is incompressible means that to pass the wedge, the flow relative to the boat must speed up.
• It doesn't matter whether it's the boat or the water that's moving; only the relative velocity between the boat and the water is important in this case.
• The pressure of the static sea water at some distance from the boat can't be lower than the air pressure, but that in the thin layer which is accelerated by Bernoulli's principle can be the same as the air pressure, meaning that the suction will continue until there's no water left in the cockpit.
Let's look at the situation with the aid of a diagram:

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### TANSLSenior Member

Have you taken into account that the fluid is incompressible ?. The pressure essentially depends on the height of the water on one side and the other on the orifice and, if there is an overpressure or depression, due to I do not know what effect, in an incompressible fluid would be negligible. IMO.

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### Southern CrossSenior Member

It's getting over my head but I see what you're saying.

This looks like the same effect a wing has on air pressure to me. The water, unable to be compressed between the static sea water and the hull, has to move quicker and so changes the pressure. Bernoulli?

This is how I imagine it. Say you were able to equalize the pressure in and outside the tank so that there was no water ingress. Next you open the scupper. Water fills the scupper but does not escape because of the equalized pressure. The water in the tank makes contact with the sea water. Maybe it mixes a little. Now you start to move forward and the water outside the hull flows aft taking the water it is attached to from the tank with it. That displaced water is replaced by more water from the tank except that this is happening fluidly and eventually the tank is emptied.

So back to the original question, if there is enough atmospheric pressure at the top of the tank (the vent being above the waterline) and the boat is moving quickly enough, a tank below the water line should drain, no?

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### Mr EfficiencySenior Member

I wouldn't worry about atmospheric pressure, if it is vented, that is enough. It depends on where your outlet is, and what the external water pressure is at that point, at the speed of the boat, it will only drain out as far as the inner and outer pressures equalise. If you are running the drain through a transom, and the transom is "dry", it will all drain, if the outlet is as low, or lower than the tank bottom.

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### latestarterSenior Member

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### TANSLSenior Member

Yes, I'm aware of self bailers and how they work, but if you think I said something wrong, I'll be happy to discuss it with you and acknowledge my mistake. This will also help the OP not to get an incorrect idea of its problem. I respect your opinions a lot, but keep in mind that I have always referred to closed ballast tanks. I do not think that what I have said can be applied to scuppers in general, holes that pass through the helmet, etc.

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### latestarterSenior Member

To address some of Southern Cross' points:-
The air vent needs to be above the waterline otherwise the boat will flood.

TANSL is right in saying Venturi applies to tubes.
Applying Bernoulli other than in a tube can be contentious. If you scroll down your link you get to misapplications and misunderstandings.
This is an interesting site I came across Incorrect Theory https://secretofflight.wordpress.com/incorrect-theories/

As I said previously the atmospheric pressure has no effect as it is on the water in the tank and the sea.
It will all depend on the shape of the outlet through the hull, whether it draws water out or pushs water in.
A current example is planes that fight fires, they skim across the water with forward facing scoops to fill their tanks, the exact opposite would be self-bailers that suck the water out.

Another way of looking at it is a building with a high wind hitting the front, high pressure on the windward side low on the leeward. A self bailer is similar to opening the back door.

@TANSL
In reply to post 8 which shows a diagram of an Elvstrom bailer you wrote

It appeared to me you were saying that it would not work. It could be argued as to why it works but work they do.

When you wrote helmet did you mean hull?

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### TANSLSenior Member

I think at some point we are saying the same thing. Due to the ventilation pipes, some overpressure could occur at some point but I think it will always be negligible compared to the height of the water over the exit orifice of the tank. And, of course, putting the vents below the water plane would be nonsense. In addition, it is mandatory (or at least good practice) to set them at a certain minimum height above the main deck. (It is very possible that in some countries, for some type of boats, there is no regulation that "obliges" to do things correctly. I do not know).
I'm afraid I do not understand this phrase.
HELMET : Yes, I use Google translator and one possible translation for the spanish word "casco" (hull) is helmet. One always need to check automatic translations. We use the same word "casco" for the hull of a boat as for the helmet of a driver.

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