[Plugin] Center of Gravity

RBarone

Thanks jgb, much appreciated. C of G being the same as C of B makes sense to me. Question: how would you go about using SU to determine underwater portion? Would you do the same thing in the video or would it be everything below the C of G? I appreciate the advice very much.

RBarone

One more thing....I am trying to work of the C of B of a simple spar buoy cylindrical in shape.

Dave R

RBarone, I think I would do what you are trying to do in DelftShip or Free!Ship. You could get all the hydrostatic calculations done easily and accurately. If you want a SketchUp model of the buoy, that's fine but it probably makes sense to get the data you need from an application that is designed to do it.

Out of curiosity, how are you determining things like the waterline of the buoy?

RBarone

I was using an existing larger concept model I found online and drawing a similar version in SU. Then scaling it down to a smaller size as a start, with the thought that the weighted portions, waterline, and buoyancy would scale down proportionately. I guess its pretty clear by now I am no naval architect. So any input or advice is well received. I was hoping I could determine the waterline as shown in the video clip mentioned above. I'll checking out DelftShip and Free!Ship now. Hopefully they let me make such a simple shape. If they calculate everything that would be great.

jgb

@rbarone said:

Thanks jgb, much appreciated. C of G being the same as C of B makes sense to me. Question: how would you go about using SU to determine underwater portion? Would you do the same thing in the video or would it be everything below the C of G? I appreciate the advice very much.

Obviously you need to know and be able to draw the buoys shape reasonably accurately in SU.
Then you need to know, calculate or estimate where the waterline should be on the shape. That would be the hard part, because the total weight of the buoy is far more than just the buoyancy chamber. Assuming you know the total weight, then a solid rendition of the SU model will give you the maximum floatation available, because SU will calculate the solid volume of the buoyant part.

Then using a simple ratio of total buoyancy to total weight will give you a starting point to finding the waterline. Lets say for example, the weight is 60% of total buoyancy, then the waterline will be at 60% of the volume. That, like I said, is a starting point.

Make a note of that volume. That is the target you are shooting for. Entity Info will tell you that.

I don't know what level of expertize you have with SU, so I will explain in some detail what happens next. Ask for help if you need it.

Make sure you have a solid component of the buoyancy chamber.
Make a copy and move it away from the original. Make it "unique". That way you don't mess up the original which you may need several times over doing this.
Draw a square face larger than the expected waterline size on the buoy, and group it.
Move the grouped waterline face over the copy at about the 60% point. Make sure none of the buoys lines touch the edges of the waterline face. Make the waterline face larger if that is the case. It doesn't matter how big it is, just has to be bigger than the buoy.
Edit the unique buoy and select only the faces that touch or cross the waterline face. You can also "select all" of the buoys faces if you wish.
Do an "intersect with model". That will transfer a line where the waterline face crosses the buoy.
Now select only the portion of the buoy that is "below" this waterline and group it.
Edit this group, and select any line that is the waterline and "face" it. This will close the shape and form a solid. If it does not face, you have a missing fragment problem that happens often in SU. You will need to analyze every line and vertex on the waterline and fix those errors manually. Post the model here if you can't solve this, it can get hairpulling at times.
Close the group and check the volume statistic in Entity Info. Compare it to the target volume.
If it is "close enough" (you decide how much is close enough) then that IS your waterline.
Use CofG to find the center of buoyancy.
If the volume is not close enough to target, then do a number of "Undo's" to return to where you were at step 5 before you edit. Or delete the buoy copy and get a new copy from the original, and again, make it unique. Then move the waterline face up or down a bit and repeat steps 5 to 10 till you are happy.

It is not as complex as you think. Steps 5 to 10 should take about 1 minute total, unless you have waterline gaps in step 8.

If you can't make that work for you, we can take it to the next level.

RBarone

Ok, lets see how I did in the attachment. Please see attached and let me know if I am close on the C of B and C of G. If so, then I've got it. I am pretty sure I am grasping the concepts here. If I made a mistake, I suspect its got something to do with either including or not including the volume of the ballast section. I appreciate the help very much.

Work in progress - practice determining center of buoyancy with sketchup

jgb

Please save and re-post it in Ver 8 format. Then I can check it out.

RBarone

Sorry about that. Here it is in v8.

SU v8 format

jgb

Here is the solution.

Your error in computing the Center of Buoyancy was not including the ballast section volume as part of the underwater section. That was a 3 1/8 inch difference. The ballast weight does not factor into the CofB.

The displacement value (density) should be 62.4 pounds/cu.ft (fresh water) which determines the maximum weight that will float at your defined waterline. The displacement is .05 cu.ft or 3.12 pounds. This is the total maximum weight, including buoy shell and ballast to float at your defined waterline.

To determine the total weight and the resultant CofG, you need to know the weight of every component in the buoy; shell, batteries, radio, antenna, ballast and wiring, and their location in the buoy.

The CoG must be BELOW the CofB in order to float upright. The greater the distance, the more upright it will float and the faster it will upright when knocked over.

I hope this helps.

Solution to Center of Buoyancy problem.

RBarone

Thank you jgb. I understand. This is extremely helpful for this an future projects and very much appreciated.

Phono

Hi,
I have a problem with the composite C of G.
I am designing a houseboat, and I have designed the hull and the superstructure in two different files.
In each file I have calculated the individual C of G's and blended them all to get a global C of G. This worked well.
Now I need to merge the two files, to calculate the C of G of the whole boat. What I do is :

Open one of the files (e.g. the Hull)
import the other file (e.g. the superstructure).

The imported items are automatically grouped as a component. I explode it.

When I try to combine the C of G of the Hull with that of the superstructure, I get a composite C of G very close to that of the hull, with the weight of the hull only.

I have tried to do it the other way round :

Open the superstructure file
Import the hull

If I try to produce the composite C of G of both, I get a C of G close to that of the superstructure, with the weight of the superstructure only.

To make a long story short, apparently the addition of the weights (and the calculation of the location of the composite C of G) fails with the imported C of G.

Could you help me?
Jean-Marc

jgb

I think your imported file is still grouped. Ungroup the 2 sections so that all the groups and comps that you want to do a combined CofG are at the same "level". CofG does not look inside groups to compute each sub-grouped CofG.

==================

Also, an easier way to bring another SU sub model into the main model is to open both the main and the sub in 2 separate SU sessions. Then, in the sub model select only the parts you want (groups/comps) and Copy to the Clipboard. Copy them all at once, not one after the other. It can be slow.

Then go to the main model and "Paste in Place". This will bring in those items as a single group to explode. The main advantage here is the main model contains the sub components in the same spacial relationship as they were in the sub model.

If you do an "Import" you have the problem of properly positioning it with the main model.

I do this a lot in my airplane and other complex models, where the main is basically a skeleton or simplified group of objects. Then I open a clean sheet, Clip copy then Paste in Place only that section I want to add significant detail to. (ie: the wing structure).

Then any other sub drawings are positionally in proper relation to each other as well as to the main master drawing. One of my models has 14 sub models, all in proper position to each other.

Phono

jgb : thanks for the advice. Unfortunately, the pasting way is not successful either.
In the meantime, I have also tried doing this :

Open SU, a new design is created (an empty one)
import one file
import the other file

It does not work either. One intersting thing is, like in my above post, that the total weight is that of the C of G that belongs to the file imported FIRST. T can confirm this, as I tried both ways.

Still at my wit's end...

Phono

I have done more experiments.
In the last one, I open both files, each in a separate session of SU.
I open a third session of SU, to have an empty workspace.
In the Superstructure file, i have produced four separate composite C of G's, related to four major classes of objects. I copy only these four composite C of G's, and paste them in place in the empty design.
I do the same for the Hull file. I have there 2 composite C of G's that I copy, and paste in place into the new design.
Thus the new design only contains six composite C of G's.
When I try to combine these composite C of G's in the new design, I can only get a result with the four C of G's of the superstructure file. The two from the Hull file won't combine when I select them and choose the Composite C of G menu option.

If I do it the other way round, and copy the Composite C of G's from the Hull file first, only those would combine. I am unable to combine those that were pasted later.

Is this a clue to the reason of the problem?

jgb

Phono

I am no expert on the CofG plugin. This is one problem that Tig needs to weigh in on.

But, anyone trying to help further will need to see your 2 models (.SKP's).
There may be something innocuous you are missing, or some condition CofG cannot handle.

TIG

Are you sure all of the CofGs are in the same 'entities' context.
If some are inside a group then they can't be combined with the others?

If you run it with the Ruby Console open are there any error messages?

Can you post [or PM to me] the file containing the apparently problematical CofGs...

Phono

Hi TIG,
I sent you a PM some time ago. Did you have a chance to have a look at it?
Thanks
JM

TIG

Work on a copy.
Delete all unneeded things - like text and dims.
Purge the model.

I see you have many nested parts, which will stop the object being solid ! Even if they themselves are all solids [but some are not!],,,

Explode the sub components / nested groups...
If the result is not a solid, then work on it until it is.
There are tools to help identify the issues causing non-solidity...

Now find the CofG of the now solid parts.

Combine then as desired...

TIG

There's a new future-proofed update.
http://sketchucation.com/forums/viewtopic.php?p=229401#p229401

TIG

Here's an update with a fix for the FR lingvo file to make it v2014 compatible...
http://sketchucation.com/pluginstore?pln=CofG