# Pressure Underwater of Ship

Discussion in 'Hydrodynamics and Aerodynamics' started by waNsi, Apr 17, 2016.

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### waNsiJunior Member

I am an scholar in University and my supervisor wants me to measure pressure at depth of 20m beneath the huge ships (approximately 150m length). I want to know that is it possible to get accurate data by using a towing tank and scale it? And if it is, which towing tank do you suggest?

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### TANSLSenior Member

The water pressure at 20 m depth is the same, whether or not there is a boat. I think this is a trick question.

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### Mr EfficiencySenior Member

Even if not a trick question, what possible use would it be to have the information ?

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### waNsiJunior Member

Thank you TANSL

For huge ships, the dynamic pressure is considerable and can affect hydrostatic pressure and change it. This paper and other works by NavLab company (http://www.navlab.net/) can explain it perfectly "Filtering Depth Measurements in Underwater Vehicles for Improved Seabed Imaging" (http://www.navlab.net/Publications/...ater_Vehicles_for_Improved_Seabed_Imaging.pdf).
So, the question is can I use towing tank and measure pressure in the depth of 1-2m and scale it to the depth of 20m? or not?

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### TANSLSenior Member

My previous answer seems to be a bit away from what you needed to know.
If you want to take into account the dynamic or viscous effects of a phenomenon, you should consider another many variables (outside the scope of my knowledge).
In any case, whatever it is the study you want to do, it is very likely to be carried out in a tank test. The scale factors to be applied only know the tank's technical people. There are many tanks of experience throughout the wide world and many universities also they have them.

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### waNsiJunior Member

The dynamic effect is apart from viscous effect of water. The first one is related to the ship movement and the wave which is created by it and the second one is related to the water characteristics.
In the case which I want to investigate, the depth of measurement (20m) is enough so that we can neglect the effect of viscosity and consider the pressure with potential equations (Laplace equation). I do not know that can we do the same in towing tank or not. I will appreciate it if anyone could help.

Thank you TANSL

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### gonzoSenior Member

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### TANSLSenior Member

Does anyone know the formula: p = ro * g * h?
p = pressure
ro = specific gravity of the fluid
g = acceleration of gravity
h = height of fluid.
According to that formula, the sequence of values "2 atm at 10m, 3 atm at 20m, etc." is not always correct, albeit approximate

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### Mr EfficiencySenior Member

I don't get how knowing pressure changes 20 metres below the hull, helps any.

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### upchurchmrSenior Member

It helps him answer the professor.

Practically I don't understand either, but who am I?

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### waNsiJunior Member

Thank you gonzo for your pdf file. I should read the references carefully.

Considering the real situation of ocean, which we want to measure, we have a formula for estimating the pressure related to depth which is CTD (Conductivity, Temperature, Density), the Conductivity is related to the water salinity. Also we have waves which can increase the water height and should be estimated as a hydrostatic pressure. So, we have 4 parameters for estimating the pressure related to depth, and maybe it is possible to exert them on the towing tank.
But the problem is we have a moving huge ship (150m length approximately) in reality (it creates dynamic pressure addition to the water static pressure) so that it can create waves in the range of swell waves (9-15s period, 3-6m wave height, 60-100m wave lenght and 10-15 m/s wave speed) by its movements. We want to measure the pressure in the depth of 20m under the hull. For measuring this pressure we want to use a towing tank and scale the situation. The important thing for us is the pressure in that depth, not maneuver or drag or anything.

The first question is: can we use towing tank? 2nd: what is the dimensions? 3rd and most important: can we neglect the effect of the capillary waves (the effect of viscosity) in our model, considering the fact that they are in the range of our towing tank depth (max 5m). While in reality we can omit them because they will damp in the depth of 4-6m.

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### TANSLSenior Member

I think you should direct your questions to the technicians of a towing tank or the like. Most of the answers we can give you here, I fear, will be assumptions and probably incorrect.

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### baeckmoHydrodynamics

I'd guess that your professor is not interested in "the pressure.....", but the total pressure; ie the sum of atmospheric, hydrostatic and dynamic pressures.

With the hull at rest, the dynamic is zero, and the pressure at the measuring position is atmospheric pressure plus "rho x g x h", where g is the Earth acceleration and h is the depth at the measuring Point.

As soon as the hull is moving, some of the hydrostatic pressure is "sacrified" in order to move water past the moving obstacle (ie increasing the dynamic part of the total pressure). The result is a change in trim and/or draft.

And, yes, you can use a towing tank to study this, but remember that the outcome is a function of the degree to which the hull is blocking the transverse area of the tank.

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### waNsiJunior Member

Thank you very much baeckmo

You are right, but there are a little more. We are only interested in dynamic pressure because we think that it can help us to define ships and ... (the dynamic pressure is crucial for us . When the hull is at rest, the hydrostatic pressure can vary with the waves' height but can be defined.

In the case of measuring the dynamic pressure in a towing tank there is a problem. If we measure the pressure at the reality we can omit the capillary waves, which were created by both waves of ocean and hull, because they will be damped and can be neglected after the depth of 3-6m (due to the effect of viscosity), and the waves with long period (big wave length in the range of > 9-15s period, 3-6m wave height, 60-100m wave length and 10-15 m/s wave speed) will effect the dynamic pressure in the depth of 20m. While in the towing tank, if we scale the waves to the real ones, we do not know that would it be correct or not; because the nature of these waves in towing tank are capillary waves or viscous waves which is completely different compared to waves with long wave length.

This question is a big obstacle for us because we want to buy a towing tank and its not cheap.

So, please consider capillary waves again, and if possible, shear it with some expert friends and guide me. Thank you again.

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### gonzoSenior Member

It would be cheaper to install pressure sensors in the hull of the ship being studied. Even if you go ahead and build a tank, comparing your test tank results with data taken from a ship will either validate or prove wrong your calculations.

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