# How to derive KN value

Discussion in 'Stability' started by Adeyele, Aug 11, 2012.

1. Joined: Jun 2012
Posts: 48
Likes: 0, Points: 0, Legacy Rep: 10
Location: Lagos

Dear all, I want to calculate GZ prior to plotting a curve of statical stability. I know the formular as GZ= KN - KG sin tetha. but i dont know how to obtain the KN value from the KN curve plotted on the graph.

I was given KN values for each angles of heel and displacement.

Angle of heel are: 10, 20, 30, 40, 50, 60, 70, 80, 90.

displacements are 130, 170, 210, 250, 290, 330, 370, 410, and 450 tons.

I have ploted this values with the KN given to me; but i dont know how to derive the KN value on the graph from the KN curve plotted for each angle of heel.

Please any idea on how to derive this average KN that will be used to find GZ.

Regards
Mike

2. Joined: Aug 2004
Posts: 3,260
Likes: 698, Points: 113, Legacy Rep: 2040
Location: Port Orchard, Washington, USA

### jehardimanSenior Member

Ok, you should have 81 KN points. 9 angles x 9 displacements. For each displacement draw a line through the 9 angle points, this should give you 9 KN curves, 1 for each displacement.

What you need now is a displacement and a KG associated with that displacement.

For a given displacement, you interpolate between the KN curves you have. So for a displacement 350 tons, you use the average between the 330 and 370 ton KN curves.

To get KG you need the load plan. Lets say lightship is 130 tons with a KG 5m, then you bunker 60 tons of fuel with a KG of 1 m and load 160 tons of cargo with a KG of 4m. As loaded KG is then (130*5+60*1+160*4)/(130+60+160) = 1350/350 = 3.86m.

You also have to do that for the last day of the voyage (i.e. burned out). If we burn the 60 ton bunker, the KG at the end of the voyage would 4.44 which is significantly higher and the displacement is only 290 tons meaning that a whole new GZ curve is required but is easy to find using the KN curves.

1 person likes this.
3. Joined: Jun 2012
Posts: 48
Likes: 0, Points: 0, Legacy Rep: 10
Location: Lagos

Dear Jehardmab, sorry for late reply and many thanks for your contribution. I actly did it d way u said; bt am nt gettin ow to pik d KN value. shld i trace out d mean displacemnt for all heel angles and take dem as my KN value.

Regards
Mike

4. Joined: Sep 2012
Posts: 3
Likes: 0, Points: 0, Legacy Rep: 10
Location: Suzhou City China

which software do you use? many design software can export data easy.

5. Joined: Jun 2012
Posts: 48
Likes: 0, Points: 0, Legacy Rep: 10
Location: Lagos

Kn Curve

Helo all, kindly check the attached file that contains the KN data with the curve.
At full load, the vessel can carry 280 Tonnes of cargo. with the KN data and the curve attached, I was asked to prepare a GZ curve for the vessel.
I need to find the GZ curve for this data. I was given many displacement; so i dont know whether am to find the GZ curve for each displacement; If am to do that, i will need to find different KG for each displacement before drawing the curve.
Any idea of on how to find the GZ curve?

#### Attached Files:

• ###### KN CURVE WITH DATA.pdf
File size:
30.7 KB
Views:
772
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.