need help -- inclining experiment calculation

[jerryah]

Dear all,

Few questions regarding the inclining test calculation puzzles me for a long time. I need your expert direction now.
I try to work out test report of a 180m (Lbp) cargo carrier, the vessel have a 0.2m hogging and 1.35m trim (by strern) during test period. The draughts at midship & fore/aft perpendiculars are Df, Dm & Da. The sea water specific gravity is just equal to 1.025 (I need not consider the correction of different specific gravity, so luck), below are steps of part of my calculation.
1). do a hogging correction by (Df+6*Dm+Da)/8=Dmn
2). use even keel hydrostatic data sheet to find out the displacement (Disp1), TPC, MTC, KMT and LCF coresponding to Dmn.
3). do a displacement correction due to trim by (LCF/Lbp)*Trim*TPC*100
4). got the actual displacement under inclined condition

Ok, my question is
1) is that correct to use (Df+6*Dm+Da)/8 to get Dmn, because I've ever saw another formula (Df+4*Dm+Da)/6, which one is correct or both correct?
2) is that correct to get TPC, MTC, KMT and LCF corresponding to Dmn? The Dmn is draught at LCF or at Lbp/2? If it is draught at Lbp/2, need I corrected it to draught at LCF coresponding to actual trim?
3) is that correct to use even keel hydrostatic data to get these parameters mentioned in question 2) or I should use trimed hydrostatic data by interpolate method to get those parameters?

Thanks for any reply!!!

[jerryah]

surprise! nobody knows?

[JLL]

Hi,
In my experience, I never use the Dmn. In the input data of my Hydrostatics software that I used to work , I introduce Df&Da , and the software runs with trimmed hydrostatics to calculate CoG
You could run with even keel hdrostatics at Dmn and compare with the trimmed hydrostatics. Differences is expected to occurs.

[jerryah]

Hi, JLL, thanks for your reply. But I'm really doubt that how your software consider the hogging correction. You know, for small vessel L<50m, normally they don't have obviously hogging. but for big vessel L>150m, 0.1~0.3m hogging is very normal.

[JLL]

"have a 0.2m hogging " means initial list? If so, IMO procedure for inclining test accept a initial list not exceeding 0,5º. Due that we correct list by list weights in order not exceed above value, we introduce the mean value of port and starboard deflection. Our range boats are up to 30m length.

[Guillermo]

Originally Posted by JLL

"have a 0.2m hogging " means initial list? ....

JLL:
Hog means : "Quebranto" and Sag: "Arrufo", in Spanish.


JERRYAH:
As a 1.35 m trim is less than 0.01*180, you may use the level hydrostatics.

I do not know about your Dmn calculation for hogging, either with 6 or 8 times the Dm.

I know about this correction:
"In cases the level trim hydrostatic data is used, a correction shall be made to Disp derived from the mean of Df and Da, as follows:
Cd = 0.75*(Dhs*TPC)
being Cd the correction to be found, and Dhs the measured hog (-) or sag (+) in centimetres." (TPC you already know)
I think this correction is all you need for your case.

I hope this may be of help.

[JLL]

Guillermo: Thank you. I didn´t know the term. ¿Where are you?

[jerryah]

Originally Posted by Guillermo

JLL:
......
Cd = 0.75*(Dhs*TPC)
being Cd the correction to be found, and Dhs the measured hog (-) or sag (+) in centimetres." (TPC you already know)
I think this correction is all you need for your case.
I hope this may be of help.

Hello, Guillermo,

it's pretty good to see your reply. I think you are very good at such calculation. regarding the "hogging correction", actually Cd = 0.75*(Dhs*TPC) is the exactly same correction way as "(1+6+1)/8.

I check a lot of class rules and IMO regulations. they all said if the trim is less than 1%Lbp, even keel hydrostatic values are acceptable for calculation. I want to get more accurate result. They suggest me to use ship model and loading software, but I don't understand, need I find out all hydrostatic data (LCB, MCT, KMT, TPC, LCF) base on the actual inclining condition?

[Guillermo]

JERRYAH
Yes, if you want to be more accurate, you should obtain the hydrostatic data at the inclining experiment condition, using the trimmed waterplane. But really, using the level trim data is good enough for your case's trim.