Actual Drafts for ships with design trim

[Sergio Diaz-Ibieta]

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.
Sergio

[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.
Sergio

[Marven]

Hi Sergio,

Of course it’s possible to extend the software with a feature like the one you’re describing. It’s just that I don’t fully understand your problem. Perhaps you can post an image (or email) of what you’re doing in autocad?

[Sergio Diaz-Ibieta]

Hello Martijn,
I’m not sure I can point this out in a good manner, but just by looking definitions for drafts in the manual you will see that these are related to ships with design even keel, i.e. where the base line is the same as the moulded keel line (z=0). In fact you have a phrase in the Delftship report where Drafts are measured with respect to Z=0.
Please find attached Autocad file with the case of a vessel we are building that has design trim by the stern. In this case the baseline does not pass through the moulded keel line and therefore the real drafts cannot be measured with respect to Z=0. I now a few Stability programs that work with both drafts (hydrostatic, where the reference is the baseline Z=0 and real drafts where the reference is the moulded keel line). Best example, though not best software, is Autohydro from Autoship Corp.
I hope you will be able to find what I´m trying to establish and give me a solution or otherwise solve my possible missconcept here.
Very best regards,
Sergio

draftmarksvsmouldeddrafts-Model.pdf

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