Both weight and weight in water also Cof G and C of B

[stephengibson76]

Im sure I can work around but I was wondering if anyone has done this type of thing before and how they go about doing it.

Problem:

We overboard equipment from a ship, for the lifting process we need to know the overall weight and the C of G, that part is easy.

Once the equipment submerges we need to know the weight in water and the new C of G.  There will be some components such as syntactic foam that will have a weight out of water and a negative weight in water or buoyancy.  Also fluid in tanks and pipework will be slightly lighter than seawater so that will also effect the C of B and weight for specifying the subsea winch

I know I can do it manually but was wondering if there is a quick way to do it and keep it updated automatically.  iLogic may be an option assigning custom properties to materials and having it switch them between the 2 environments...

Any suggestions will be gratefully recieved, I do appreciate the efforts people put in on this forum

[graemev]

Umm... tricky.

Assuming you can assign correct material properties to each component of your assembly, you can get Inventor to calculate the correct mass and CofG.  So far so good.  For subtracting the displaced volume of water (make sure you have the correct density - salinity and temperature variation may be significant) try this:

Derive a new part, making a union of the entire assembly.  Assign the correct mass properties to the new (single) part as if it were water.  The CofG and mass should be correct for correction factors for the original assembly CofG and mass data.

I'll make the bold assumption here that since you design marine equipment you know about righting couples, stability, etc.

[stephengibson76]

Thanks for the reply, I know a little aboaut stability but im not a naval architect.  Tell me if you think these are fair assumptions provided all of the materials and weights are correct

C og G and mass in air easy!

C of B as you suggested with resulting mass considered in the opposite direction for stability calcs

C of G in water, a little more tricky.

I was thinking of having 2 seperate material masses for everything, (hopefully second mass derived from a seawater density variable)

All materials will weigh less in water, more obvious materials like syntactic or pressure vessel voids will have a negative mass property thus give me the correct C of G in water

If that sounds right my next thing to look at is how to produce a negative mass automatically.  I know I can manually enter it into part properties but it would be nice if I could assign a negative density to a material but it does not seem possible

[stephengibson76]

Ok, so speaking to real naval architects... C of G does not change above or below water, the weight in water is simply the weight in air minus the displacement... that sound about right?

[graemev]

Ok... CofG vs. CofB...  (scratches head)  Let's see if I remember this right.

The CofG is as per Inventor iProperties.  Easy, as you pointed out.  Cool.

The CofB is the CofG for the water volume displaced.  For an anchor (of uniform and solid material), this is the same location, so the result is simply the weight of the anchor minus the weight of the water displaced for the solution.  For a boat (non-uniform material and plenty of voids) the CofG for the displaced volume of water (a boat displaces is own weight of water while floating) will be directly over top of the boat's own CofG if it is in a stable orientation.  If the boat's CofG is over the displaced volume's CofG the craft is unstable and will capsize, seeking a stable orientation.  Note that as a boat rolls, the CofG for the displaced volume shifts to the lower side, causing what is known as a "righting couple" - a moment equal to the weight of water displaced multiplied by the horizontal distance between the two centers of gravity.

A cylindrical ship will roll without effort.  (See: log rolling.)  A flat bottomed boat will right itself quite readily.  (See: barges.)

For your specific problem, assuming the complete assembly is bouyant over all, you may need to rotate the assembly iteratively (in both axes) to find the proper stable orientation and waterline.  If it is not bouyant overall, merely rotate the assembly to have the vector between the two centers of gravity vertical in the water column with the water's CofG higher than the assembly's.

[stephengibson76]

Thanks again, the component is for sub sea and I am pretty sure I have got my head around it now...  the CofB of the assembly is the derived part which is what i was strugling with initially

[Allen.Bai]

Hi Stephen,

I noticed you want to set a negative Density in material properties.Unfortunately, from Inventor 2013, all materials are using Protein library and negative Density value is not allowed (actually there is a range, seems 0.001~25000?). In real case, there should be no negative Density material, but I can understand it is reasonable from software perspective.

We are trying to do some changes now. Here, I want your opinion - do you just add a negative symbol (-) when change the value? do you think we still need a range for such kind of properteis with larger range (saying -25000~25000?), or not necessary to have range at all?

Thanks,

Allen

[stephengibson76]

A negative density would be useful so we can work out the C of B in water without having to derive an assembly into a single part.  Problem is it would need 2 different density values for the different environments, ( in air / in water )

Also density of sea water changes around the world as well as fresh water and salt water so I am not sure if an enviroment setting may be more useful to a wider range of engineers, some of the different environments I can think of

air at sea level

fresh water

salt water

air at plane altitude

space

surface of the moon

soup, (volcanic, nuclear, mud)

gas environment

sub terainian

just a thought, see how the same components act in the different environments