Help Identifying the Correct Value for Inertia

[yorkhighschool]

Hello. My students are participating in a high school boat competition. The short of it is I need some help identifying some "Inertial" information from our Inventor program. We are specifically asked in the competition rules to find the value for the "Moment of Inertia about the X-axis" or Ix and also the "Moment of Inertia about the Y-axis" or Iy. 

I am not sure which is the correct value in Inventor and it is critical to our math calculations that we get it right. Attached are some screenshots I made of a sample "Barge" boat design. The barge part was is aligned so that the front bow of the boat rests on the origin and the length of the boat goes down the positive X axis. Height is on the positive Y axis. Width or Beam is symetric about the X axis. 

When I go to the IProperties and update the physical properties, I see some values for I1, I2, & I3 under Principal Inertia and Ixx & Iyy under Global & Center of Gravity. The units for all values are in lbsmass / in^2.  Which of these values are the Ix and Iy that we need for our math calculations? Does anyone have any idea? Thanks!!

[yorkhighschool]

One more pic.

[yorkhighschool]

Not sure what happened to my pics. Here they are again.

[yorkhighschool]

Pic 3

[yorkhighschool]

Last one.

[yorkhighschool]

I think I need I1 & I2 from the "slide2" picture as I think they are the Ix & Iy I need. I was reading the help topic and it said the Principal Moment was rotated about the X, Y, & Z axes. Can anyone confirm?

[swalton]

Which Moment of Inertia are you looking for?

Area Moment of Inertia, used to calculate beam strength?  Or Mass Moment of Inertia, used to calculate rotational motion?

See http://www.engineeringtoolbox.com/area-moment-inertia-d_1328.html  and http://www.engineeringtoolbox.com/moment-inertia-torque-d_913.html

Inventor will calculate both, but the process is different for each property.

What you have found so far is the Mass Moment of Inertia.

[yorkhighschool]

Mass Moment of Inertia, used to calculate rotational motion....I'm pretty sure this is it. I'm still a little fuzzy about what the competition guidelines call for and have some calls and emails into our liason to verify which I need. But I do know it is rotated around the X and Y axes which lends me to believe it's the one for rotational motion.  

[yorkhighschool]

SWalton....I did confirm with the folks running the competition and I was wrong about my previous post. I am looking for the Area Moment of Inertia. Can you tell me how ot go about doing this in Inventor please? Thank you.

[karthur1 · Accepted solution]

In the Inventor help, search for "Measure Region Properties" (I know, that makes alot of sense).

Kirk

[yorkhighschool]

Major progress Karthur! Thank you! Please see attached.

One question / or problem remains though. Is there a way to calculate these values for an entire part or, in this case, the entire boat design? The procedure you gave me calculates these values for the individual sketches or plates of the hull of the boat. I would then have to add them all up to get overall values for the entire boat (not that that's impossible or we do not want to do it...I just want to make sure our final calculations are accurate). Your thoughts? 

[yorkhighschool]

I figured it out guys. Well, at least I figured out a way to do it. I made a 3D sketch and associated intersection curve that represented the design waterline. I was hoping this 3D sketch would be recognized by the region properties but it was not. So I made a regular 2D sketch, projected the geometry of the 3D sketch, and drew lines outlining the border of the waterline plane / boat hull intersection. I was then able to run region properties and get the subsequent Ix & Iy values. Thank you Kirk and Swalton for your posts as they led me to the answer.


Cheers!

[karthur1]

For most beam/deflection formulas to work correctly, they are under the assumption that the section has a continuious cross section. That means that the "I" value is the same from one end to the other.  If the inertia varies along the length I am not sure how to handle this.  I dont know if you need to give them the maximum or the minimum value.

If you just "add up the values" you have to make sure its about the correct axis.  To add them up, you have to use the "parallell axis therom" to get the correct value.

Kirk

[yorkhighschool]

"To add them up, you have to use the "parallell axis therom" to get the correct value."

Excellent! That's exaclty what it says in the competition guidelines.