Re: Actual Drafts for ships with design trim

DELFTship forum Feature requests Actual Drafts for ships with design trim Re: Actual Drafts for ships with design trim

Sergio Diaz-Ibieta

Hi Martijn,
I hope you have considered my request to study and implement since it is very important to get real drafts from DS and not to calculate them any time one runs a loading condition for ships with design trim. I think it’s not difficult to check the real drafts in the position of the draft marks, since you already have the hull geometry. Please give a thought on this.
Best regards.

Dear Martijn,
Most software for stability calculations give you the chance of drafts outputs with respect to the Baseline (Z=0), which we normaly call “hydrostatic drafts”, and/or also with respect to the underside of the keel plate, which we normally call “real drafts” (or simply “drafts”). Hydrostatic drafts (wrt Z=0) are used in the perendiculars and real drafts (wrt Keel line) in the draft marks.
For the case of ships with design even keel (design trim =0°), where the Baseline corresponds to the moulded keel line, the calculation is quite easy and it’s the method used in Delftship. However, when you have a ship with design trim, then the calculation of the real drafts has to account the design trim as well. Definitions for drafts in Delftship Manual are not applicable to ships with design trim and it would be quite necessary to implement a small routine/algorithm to solve this problem.
In order to get real drafts for the Stability report I take the Delftship drafts with respect to Z=0 and draw the Waterline in Autocad. Then I draw a line parallel to the baseline in the intersection of the keel line and the draft mark in order to measure from this point to the waterline. If there is no “keel line” in the position of the draft mark, which is the case of the aft ship, then one measures the projection of the closest lower point in the hull, parallel to the baseline.
This is maybe difficult to understand because of my very poor English, but please give a thought and revert with your comments or questions.
I take the opportunity to wish you and your family the best of the best for this coming new year.
Very best regards.