How to perform a metacentric height lab experiment

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1. Nov 10, 2014

PrincessIceFall

I need help in understanding how to conduct this experiment. I found this lab report online but it is confusing

http://site.iugaza.edu.ps/mymousa/files/Experiment-2hydraulics-lab-.pdf

I am finding it difficult to visualize the experiment. Also, I asked my university lab technician and he said that our university does not even have the apparatus. But he said I can actually fabricate the apparatus by myself, but I am not even sure how to do that. Please if anyone has any clue about this experiment than please tell me about it.

2. Nov 10, 2014

SteamKing

Staff Emeritus
The basic experiment requires that you have a floating box of known dimensions. The outside of the box should either have a scale attached to it of some means to measure the draft of the box when it is floating. The scales should be attached to all four corners of the box.

You will also need to fabricate a box which is deep enough so that it floats without filling with water when it is heeled to one side when carrying a set of inclining weights and a mast and pendulum for measuring the angle of heel when the inclining weights are shifted to one side. Additional weights may be required to trim the box to obtain level drafts before the inclining weights are moved to one side.

You will also need to have a tank large enough which can be filled with water so that the box can float and heel from side to side without coming into contact with the sides of this tank.

This video shows a basic setup with a description of how to take the measurements of the vessel and other data:

There are other videos of similar experiments posted on YouTube.

3. Nov 10, 2014

PrincessIceFall

how are they measuring the center of gravity? I know they use the knife edge support, but like I still don't get how they are measuring it. Also, if my university does not have a knife edge support then how would I measure the COG? Is there any other way?

4. Nov 10, 2014

SteamKing

Staff Emeritus
That's the purpose of this experiment. It's to determine the location of the CG of the floating box by measuring the change in heel of the box as a known weight is shifted from side to side. The height of the metacenter above the bottom of the box is based on the geometry of the box itself; the unknown height of the CG can be determined by knowing the relationship between the amount of heel produced by a known transverse moment.

5. Nov 10, 2014

PrincessIceFall

If I am not mistaken. The distance of the center of gravity from the base of the pontoon is one of the parameters needed to calculate the metacentric height. If you read step 2 of the procedure in the lab report, it says to weigh the pontoon and to measure the CG by using the knife edge support.

6. Nov 10, 2014

SteamKing

Staff Emeritus
The location of M above B is determined by the waterplane characteristics of the pontoon and the volume of displacement. Similarly, the location of B above the keel (bottom) of the pontoon can be determined from the shape of the submerged portion of the pontoon. After all, the weight of the pontoon is equal to the weight of water displaced when the pontoon is floating.

In a regular inclining experiment as performed on a real ship, the location of G is unknown. You cannot take an ocean liner or aircraft carrier out of the water and balance it on a knife edge in order to find its center of gravity.

By shifting the movable weights back and forth to heel the pontoon, you can plot the heeling moment of the pontoon against the tangent of the heel angle. If you have done the experiment correctly, a straight line should be obtained. Knowing the amount of heel produced by the moment of the shifting weights, the GM of the pontoon can be calculated. Having determined the location of M, one can work back geometrically and find the location of G.

According to the dimensions of the pontoon given in the procedure, the expected location of G will be somewhere within the hull of the pontoon, so your knife edge will require special construction to support the pontoon and mast by sticking into the hull a ways.

7. Feb 13, 2015

PrincessIceFall

Hi, I haven't posted here in a long time. I just want to give an update that I have since my last post successfully done an experiment on a flat bottomed pontoon of known dimensions and have also done the calculations to obtain a set of reading and coded them into matlab. However, the challenge now is that I want to perform and experiment and do calculations on a pontoon that is differently shaped. My aim is to perform experiments with different shaped pontoons until I can work my way up to a pontoon that very closely depicts a real boat/ship. So right now, I want to do an experiment on a pontoon that may not be flat bottomed. And this is proving to be difficult. I wanted to ask if you know of any kind of books that may help in the understanding of calculating the metacentric height of a more complex shaped model? Or do you have any advice on how to carry out such an experiment or calculation? Thank you

8. Feb 13, 2015

SteamKing

Staff Emeritus
Yes. The calculation of the stability characteristics of a vessel is covered in many texts on naval architecture.

The attached guide illustrates some of the stability calculations for a sailboat:

http://www.sailboatwayra.com/Pdf/Hydrostatic and Stability.pdf

These techniques can be generally applied to almost any vessel with a single hull. Take some time to review it, and then come back with any questions you have about the things you don't understand. Pay close attention to calculating the volume of displacement and the transverse moment of inertia of the waterplane.

Good Luck! :)