# KN curves problem

Discussion in 'Stability' started by kaptcatb, Jan 27, 2013.

1. Joined: Sep 2011
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### TANSLSenior Member

I think you're not understanding the method that I describe (which is not mine). The method is completely accurate. Now if the method you use to calculate the volume is not accurate, the data you get will not be accurate. The "accuracy of my method" depends on the accuracy of the integration method you use.
If you hold the AutoCAD, I can show you how to use it, without doing you any mathematical operation, calculate KN values​​.
To answer your second question: The KN has nothing to do with the GZ. The dependent KN boat forms and GZ of the position of its center of gravity. That is, the values ​​of KN are "fixed" for certain forms of the boat, many times that the calculations always find the same values​​. In contrast, for the same boat, if you change the vertical position of its center of gravity will change GZ values​​. This is obvious but you do not seem to have it clear. And no, I do not know the Vlasov curves for integration.
Trust me, using "my method", and forget about weird stuff that life is simpler than some we endeavor to make.

2. Joined: Jan 2013
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Location: Bulgaria , Varna

### kaptcatbJunior Member

Hello again
The only thing in your method is how you set your new WL ? Arround which point you rotate you ship in autocad ? Cuz if you see different ordinates for the cross point ( w1l1 x w0l0 ) you get different submerged area , volumes , centers and etc . Will be interested to explain this part of the task .
I'll be interested to explain this part of the task .
In following link i set Q=30 for 2 different cross point ( different Yf , ah this Yf ) . Hope you understand my logic
http://store.picbg.net/pubpic/7E/23/31732f6e6af97e23.jpg

3. Joined: Sep 2011
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### TANSLSenior Member

Could we, please, comunicate via e.mail?. Mine : 657677483@orange.es
It would be much better to interchange information. Thanks

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### maxxwellNew Member

Why you dont use software?
the ORCAD for RHINO and maxsurf calculate KN very fast

5. Joined: Sep 2011
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### TANSLSenior Member

Before using any software must know what is being done, how it is made and what it does. Unfortunately, it's going to a good user of software is believed ready to design boats and this is a mistake. Some people believe that a beautiful thing in Rhino, is a good boat just because it has be drawn in great detail. But how to find out, with Rhino, if the boat supports flood compartment, without sinking?. How to tell Maxsurf applying a stability criterion if you do not know what that is?. One must know how to do things "by hand", before using any software.

6. Joined: Jan 2013
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Location: Bulgaria , Varna

### kaptcatbJunior Member

Hello all
thanks to TANSL for helping .

I finished my calculations for KN . Now my teacher want to generate ship model ( i dont know the reason .. i have already calculated in excel GM , KN , GZ for different wave crest and different ship direction in sea ) .
I have little expirience with Maxsurf . From excel i generate ~ 600 markers , put in text filie and load it in Maxsurf . Then i select all markers -> group , sort markers by station . Then i need to fit curve to markes but dont have this option ( no Curve menu in main window ) . How to solve the problem ?
I use this video from Formsys channel for "teacher" -

7. Joined: Sep 2011
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### TANSLSenior Member

Unfortunately trying to define a surface in Maxsurf which adequately fit the markers is, in my opinion, impossible. You can make something that looks good but you will never have the assurance that passes through the points.
For me the best is:
- Draw transversal sections and longitudinal profile with AutoCAD.
- Place it all in his true position in space, working in 3D.
- Use the "_loft" command to generate surfaces forming the hull.
- Creates a file .igs thereof.
- Import this file in Maxsurf.
You can also make a model in Rhino, and import the file .igs in Maxsurf.
If you have problems, send me your body plan to see what can be done. But first, you try it.
Good luck.

8. Joined: May 2020
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Location: Pakistan

### UmairJunior Member

Hello sir,
Can you share your work with me?

9. Joined: Sep 2011
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### TANSLSenior Member

Do you know how to calculate center of buoyancy position?.

10. Joined: May 2020
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### UmairJunior Member

Yes that's easy, but when inclined it thats make difficult to calculate anyhow I got the KN by finding the transverse buoyancy point BB'cos(phi) but it's little different like I got 2.34 and from software I am getting 2.30. is there any why I can get more precision.

11. Joined: May 2020
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### UmairJunior Member

Is there any software or algorithm type thing from which I can direct find BB' (transverse center of buoyancy)…?

12. Joined: Jan 2013
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Location: Bulgaria , Varna

### kaptcatbJunior Member

is it good idea to restore 6 years old thread? Method I have used is clearly described in post #12.

As far as I remember -
for method I used you need CAD program / Excel to integrate values . If youre good with MatLab it can also help.

Draw all your frames, find volume of underwater part, CoB
Draw inclined WL -> find volume of underwater part. If it dont match with volume in upright position move WL up or down until volume match.
With massprop command you can find centroids of new underwater shape ( new CoB location ) , then I think its easy ..
Repeat for all angle of heels - it may take alot of time so I suggest you to look for another method.

Thanks Ignacio for great help

Best regards
Todor

TANSL likes this.

13. Joined: May 2020
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Location: Pakistan

### UmairJunior Member

Thanks sir, this is what exactly I did in Solidworks. But it's way too time consuming. Any recommendations?
Also recommend me some books to study KN curves, because no one explained it in details like some manual methods etc

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