Calculating kn values with maxsurf and AutoCAD

[Siddarth Amaravathi]

Greetings for the day. I was recently given an assignment to find kn curves of a perfect cuboidal shaped vessel. I was given range of heel angles and displacement ranges.I gave the inputs (displacement range, angle of heel) in maxsurf and it gave me the in values for every angle I wanted. Apart from maxsurf I tried manually extracting the kn value for 1 particular displacement and heel angle in AutoCAD and then checked with maxsurf. Surprisingly I've found variation between what I got manually and what I obtained in maxsurf (variation is -0.4m). One more surprising thing is I tried it using formulas and that gave me a different value but nearby what I got in maxsurf (variation is 0.3). So I just wanted to know why is this happening, why is there a variation?

One more question, can someone please help me out with formulas too find out kn value for angle ranging from 10 degree to 90 degree with 10 degree interval
(I've read that wall sided formula works between 20-30 degree only, so how to calculate kn for 40-90 degree)

Thank you in advance

 

[TANSL]

Welcome to the forum.
The formula to obtain the value of KN is the same for 10 degrees as for 15 than ... for 123.45 degrees. It does not depend on the heeling of the floating object.
Regarding the values that you obtain, if the model that you have made in maxsurf is correct, the values that Maxsurf gives you will be correct. Do not hesitate. Regarding the values that you obtain with AutoCAD, you can be sure that you are doing something wrong. AutoCAD, and most CAD / CAM programs, is amazingly accurate. One of my programs performs the naval architecture calculations with AutoCAD and I am totally sure that it works very well, if the user knows how to use it.
Review the procedures you use and make sure you use them well.

 

[Siddarth Amaravathi]

Welcome to the forum.
The formula to obtain the value of KN is the same for 10 degrees as for 15 than ... for 123.45 degrees. It does not depend on the heeling of the floating object.
Regarding the values that you obtain, if the model that you have made in maxsurf is correct, the values that Maxsurf gives you will be correct. Do not hesitate. Regarding the values that you obtain with AutoCAD, you can be sure that you are doing something wrong. AutoCAD, and most CAD / CAM programs, is amazingly accurate. One of my programs performs the naval architecture calculations with AutoCAD and I am totally sure that it works very well, if the user knows how to use it.
Review the procedures you use and make sure you use them well.

Thank you for the response. I will try once again on both softwares and check for equality

 

[Ad Hoc]

.... I tried manually extracting the kn value for 1 particular displacement and heel angle in AutoCAD and then checked with maxsurf. Surprisingly I've found variation between what I got manually and what I obtained in maxsurf (variation is -0.4m). ...

So, you need to calculate what volume you get using the programme at the angle you have selected and then, what volume you get by your hand calculation.

Then you need to consider, what are the other variables?
Then check what you have against what the programme output is showing...

 

[Siddarth Amaravathi]

Welcome to the forum.
The formula to obtain the value of KN is the same for 10 degrees as for 15 than ... for 123.45 degrees. It does not depend on the heeling of the floating object.
Regarding the values that you obtain, if the model that you have made in maxsurf is correct, the values that Maxsurf gives you will be correct. Do not hesitate. Regarding the values that you obtain with AutoCAD, you can be sure that you are doing something wrong. AutoCAD, and most CAD / CAM programs, is amazingly accurate. One of my programs performs the naval architecture calculations with AutoCAD and I am totally sure that it works very well, if the user knows how to use it.
Review the procedures you use and make sure you use them well.

Sir i'm not understand where am I going wrong, I'm still getting the variation. If possible, whenever you're free could you please calculate kn value in both maxsurf and autocad for the following and share the method to me.
Rectangle cuboid of:
L : 85m
B :14m
D : 8m
Dwl: 5m
Density 1.025
Angle 10°
Displacement 1000

Thank you in advance

 

[TANSL]

I assume that you are studying an 84 m x 14 m x 5 m rectangular parallelepiped and that the displacement is 1000 tons. In that case, the draft should be 0.82 m.
Reconsider the calculations you are doing before doubting your software.

 

[Siddarth Amaravathi]

I assume that you are studying an 84 m x 14 m x 5 m rectangular parallelepiped and that the displacement is 1000 tons. In that case, the draft should be 0.82 m.
Reconsider the calculations you are doing before doubting your software.

Thank you for the reply. The barge is not a parallelpiped but a cuboid. Draft I got is 0.82 for 1000 displacement. Attaching here the assignment question I got. Please do check it and help me out on the variation

 

[TANSL]

What you have drawn is a rectangular parallelepiped, but it doesn't matter.
With a 5 m design draft the displacement is 6098.75 metric tons.
Please clarify at which displacement you want to calculate the KN

 

[Siddarth Amaravathi]

I want to calculate kn value for displacement 1000 and angle of heel 10°. We actually have to calculate for displacement ranging from 1000 to 8000 and angles from 10-90°( as shown in the table) but I'm just trying to calculate one value and compare it with that of maxsurf's. So please try calculating for 1000 displacement with 10 degree

 

[TANSL]

For displacement = 996 tons I get KN = 3.271 m


Edited at 8:44 AM
For 1000 tons ..... KN = 3.265 m

 

[Siddarth Amaravathi]

For displacement = 996 tons I get KN = 3.271 m

Can you please show me the calculations sir.

 

[TANSL]

Here you have AutoCAD's file

 

[Siddarth Amaravathi]

Here you have AutoCAD's file

That's great sir.I was rotating my vessel keeping lcf as center But how do you get the same value manually with formulas. I think I'm too many doubts. If you don't mind could you please help me get the same value with formulas and share it with me.

Thank you in advance

 

[Siddarth Amaravathi]

And one more doubt sir. Whenever a ship heels, does it rotate about lcf? Or lcg?

 

[TANSL]

could you please help me get the same value with formulas and share it with me.

I am delighted to help you but before answering your question, let me ask you another: do you know the relationship between the value of the KN and the position of the center of buoyancy of the boat?.