Re: Howto lengthen a hull by inserting a parallel midship section in DELFTship?

DELFTship forum Hull modeling Howto lengthen a hull by inserting a parallel midship section in DELFTship? Re: Howto lengthen a hull by inserting a parallel midship section in DELFTship?

Ralph Grothe

Hi Icare,

many thanks for your thorough explanation.

Actually, I meanwhile found out myself how to do the lengthening, although I used a slightly different approach.
It was really straight-forward why I wonder that I had to bother you.
Just like you described I marked all control points from the midship section in the foreship half of the hull.
I think this would have been less cumbersome had I separated aft and fore ship in two layers.
When all points shone yellow I just entered the coordinate pop-up for a selected point, that shows up in the upper left corner,
and entered in the X-coordinate text field the length of the parallel midship section (i.e. 6.44m) the shipyard inserted later on conversion of the vessel.
But to indicate that the value should be taken relatively rather than the default absolutely I prepended the “@” sigil.
Submitting the intended change with a click, voila my hull got lengthened.
The hardest part was only to click all control points manually beforehand.
I wish DELFTship had a command line input interface where I could have entered a logical expression like e.g. select all points if point.x >= L/2.

To see what was done you could follow this link to my post in a German RC model boat forum.
I’m afraid, my posts are in German there, but the images should be self-explanatory.

Sorry, I cannot follow you when you write that I should be wary because not only the length could have been changed during the vessel’s conversion but also other of the main particulars, such as beam or draught.
This assumption is unrealistic as the ship yard virtually cuts the ship in two pieces and welds in a fully fabricated and equipped parallel midship section.
This certainly can only change the ship’s length while beam an draught remain the same.
Well, as for the draft, since the vessel after such conversion has an increased displacement owe to the added extra volume,
usually the draught will decrease a little as long as no extra cargo is taken aboard, which was the reason why the ship’s operators underwent the fuss of lengthening her.

As for (transverse) static stability it usually should increase from the lengthening because of the increased moment of inertia of the increased waterline plane.
As you will remember the metacentric radius BM is given by the ratio of transverse (i.e. with regard to the Y-axis) area momentum of inertia divided by the displacement volume.
However, things may slightly shift when dynamic stability is considered, especially with modern hull shapes of RoPax and Container ships with their pram like aft bodies and extreme square transom sterns.
Often accompanied with extreme flare of bow sections.
This may in waves lead to drastically changing waterline areas between ship on wave crest and wave trough.
Especially if the wavelength is nearly the same as the ship length and if the wave is traveling from stern.
If this is superposed with heavy rolling severe sudden loss of stability can occur in ship on crest situation which may even lead to capsizing.