# Pitch mass moment of inertia

Discussion in 'Stability' started by Rhys2908, Oct 26, 2016.

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### Rhys2908Junior Member

I've been given a coursework from one of my lecturers on the seakeeping of a monohull vessel.

The exact task is 'describe how you would conduct an experiment to measure the pitch mass moment of inertia of a monohull vessel'

I've been searching the library and online for hours but can't find any previous experiments!

Can anyone advise how to do this experiment or where to look?

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### TANSLSenior Member

Pitching occurs about a transverse axis passing through the center of gravity of waterplane area. Therefore, you must calculate the moment of inertia of this area with respect to that axis. I know how it is calculated but do not know of an "experiment" that would calculate it.

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### jehardimanSenior Member

Virtual or Real pitch mass moment of inertia?

FWIW, this is a thinking exercise.....So; what is similar to the question asked.

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### jehardimanSenior Member

Actually, that would only give him the water plane inertia, not the mass inertia.

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### TANSLSenior Member

Yes, you are quite right but I thought the OP was making a mistake because the moment of inertia of the mass of the boat I do not know why he needs it for (certainly nothing related to the pitching motion of the boat) and, in any case, about which axis?.

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Well, since it is your course work, it is not for me to give you the answer. However, ask yourself the question (taking JEH’s lead), what is inertia? Think of Newton’s first law of motion….then think about how this relates to a straight line motion and one that is not a straight line, a change in direction or velocity. And then think about a pendulum and a swing with small angles of or rotation and simple harmonic motion.

And I don’t know why you keep answering questions you obviously do not know the answer to and even state so yourself afterwards!

Last edited: Oct 26, 2016
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### TANSLSenior Member

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### Rhys2908Junior Member

I think I've simplified the problem to down to an experiment now, but am unsure of the governing equation.

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### gonzoSenior Member

Look up pendulum with damper. Remember that the minimum to have an oscillatory system is a mass and a spring.

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### TANSLSenior Member

In an oscillatory system it is not always necessary a spring.
Considering the boat like a pendulum, what do you think that would carry spring functions?

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### gonzoSenior Member

Gravity.

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### jehardimanSenior Member

Since this is a seakeeping lecture, think about the motions of the floating free body. There are six motions, three (including pitch) are oscillatory, two are controlled, and one is unbounded. Look at the other two major oscillatory motions. One is similar, one is not. How are the three linked or not?

What text is the lecturer using, perhaps I could point you to the relevant section so you can read and study it?

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### Rhys2908Junior Member

The lecturer has included this as extra research and has not included such tests in the notes directly, but has made numerous references to Faltinsen, Bhattacharyya mostly

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### jehardimanSenior Member

FWIW, the answer can be found in Ch 4 of Dynamics of Marine Vehicles by Bhattacharyya or Ch 2 of Theory of Seakeeping by Korvin-Kroukovsky. I haven't seen Faltinsen's Sea loads on Ships and Offshore Structures or Hydrodynamics of High-Speed Marine Vehicles.

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### TANSLSenior Member

Ad Hoc, I did not understand your aggressive tone for what I have done a brief search and I found that indeed Rhys2908 was talking about a subject I had no idea. I apologize for intervening in a conversation to which I have no preparation. Just trying mistakenly to help.
Thanks to Jehardiman I could leave my error.

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