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?

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    • #36215
      Ralph Grothe


      I reconstructed the hull lines with DS of a ship which was later lengthened by a couple of meters to increase pax capacity.
      I derived my hull model from a ship’s description published in a naval architecture periodical issue from the year of build that appeared along with a small general arrangements plan.
      This is the state as the vessel was built.
      But suppose the model boater who wants to build a model from the hull lines is interested in the lengthened version.
      The scalar transformation by inserting a scale factor (i.e. Lpp_lengthened / Lpp_built) in x-direction is not viable as it would harmoniously stretch the hull.
      What I require is the insertion of a parallel midship section.

      Is it possible in DS to cut the hull model at the midship section at Lpp/2 in two pieces, and then to extrude the control edges of the midship section of, either aft body positively in x-direction or of fore body negatively in x-direction by the extend of the parallel midship body, and after that to reunite the two pieces (“glue” them together again)?
      That’s the way how I would intuitively procede, and which sounds pretty straight forward with only a few mouse clicks.

      Unfortunately, I cannot sort out from clicking through the user interface of DS if and how this procedure could be followed.


    • #36223

      Hi Ralph!

      Above all, do not cut your ship in halves: Once separated, two parts from a same original net cannot be knitted back together (at least, as far as I know), and this will create leak points. Once the net is broken, you cannot “glue” its parts back together. (You can only replace a missing mesh… this is how to create a window pane B) ). Well, this consideration is valid for FreeShip, and I assume it doesn’t work better with DelftShip…

      Second preliminary remark: Are you sure that only the ship’s length was changed? Length was the most obvious change between the two versions of the ship, then the periodical had to comment this change. But, may be, the same scale factor was applied to the width and to the height, and for they are much less remarkable, these changes weren’t mentionned. Furthermore, I guess that changing the length but not the width or the height can have a negative effect on the ship’s stability.

      Well, now if you’re sure that only length was changed…

      The only way I can suggest you is to move the bow part forward. The result will be not really perfect: the ship’s mid-section will be streched, and this will make a ship with flat flanks, but this can be subdued by moving the neighbouring stations towards the center of this streched part. The model’s meshes are like… errr… rubber-made.

      So, select the ship’s fore part’s polygons edges (only half of the edges are required, either longitudinaly, either lateraly… the whole polygon will “follow” its edges) from the bow to the maximum beam (included or not included, it doesn’t matter). Do not forget the stem central line. The more you drew polygons, the longer the time you’ll spend selecting them, and the more you can forget one of them… Too bad for you… :whistle:

      Then order a forward move for the selected polygons (The 4-directional arrows button, under the the TOOLS tab).

      This will pull the ship’s fore part forward, and strech the mid-section. We’re close to the result you’re expecting, aren’t we? Can you see the ship’s flat flanks? We’re going to subdue this.

      Now, move the stations toward the center of the streched part, and make the meshes interval more regular. (Only the stations’ position, not the points coordinates). A larger move for the closest stations, a smaller move for the farthest stations. At this point, do not change anything to the stem or to the transom. You can check the net regularity by comparing the new ship’s Gaussian Curvature (or Zebra shading) colors scheme with the old ship’s.

      Finally, the bow has been moved away from the stern, but both parts’ profile (stem & transom) haven’t been modified, neither the height, neither the width.

      Note: Instead of moving the neighbouring stations, you can create a series (or many) of new meshes by dividing the “faulty” meshes… and find all the new points’ coordinates… Good luck! :whistle:

      Illustrated example using the Ship Demo 7 (forwared with FreeShip):

    • #36232
      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.

    • #36258

      Sorry, I can’t read german… Well, I probably can read it (awfully), but I can’t understand anything. :whistle:
      For me it’s worse than reading tagalog (Philipinas Islands main language) or chinese (I can read numbers up to 3 😛 ).

      An other suggestion for selecting many points at once could be to draw a box with the mouse. All the points included in the box would be selected. This works on the CAD software I’m used to work with.

      Concerning the stability considerations, I ain’t a good judge: I much more studied airplanes than boats. And boats aren’t planes… except concerning the seaplanes B) But this could be matter for a whole book.

    • #36262
      Jesus Cruz

      There is a way how to choose a set of points, with the ctrl key pressed draw a square from left to rigth with the mouse pressing the left mouse button, all the points and edges located within the square will be chosen and highligthed yellow….

    • #36267

      Thanks for the tip Cruz! :cheer:
      Finally, I’ll probably read the manual once more, there must be lots of thing I left “for later”, but never read again… :unsure:

      And the more I come on the forum, the more I think many of us should do so. 😛

    • #36268
      Ralph Grothe

      Many thanks for this invaluable tip, cruzmayora!

      The draw rectangle selection of objects works like a charm.
      What a dimwit I’ve been to have selected all control points individually.

      Btw, your forum’s nick sounds like a sign in astronomy, but I couldn’t find something in the way like “Grand or Great Cross”.
      There’s only the Crux aka Southern Cross or Cruzeiro do Sul in Portuguese,
      which also happens to be the name of an album of some palatable Bossa Nova.

    • #36269
      Jesus Cruz

      wow is good to know, my forum’s name is actually the combination of my father’s and my mother’s family names, so is a coincidence i guess.

      is good to be helpfull if you have any other doubt do not hesitate to ask in the forum.


    • #36285
      Peter Edmonds

      I’m VERY new to DELFTship, and very much welcome that DELFTship can address the real world of cut and spread vessel lengthening. I throw in some observations from my most recent lengthening experience.

      The subject vessel was 14 m aluminium, monohedron with 2 diesel jet units. She was cut around the engine room forward bulkhead, and a complete new compartment, 7 x 600 mm frame spaces was added. This was primarily to increase passenger numbers through more deck space. we put new fuel tanks in the new space to keep fuel LCG suitably aft.

      The boat has suffered a deteriorating performance as outfit weight was progressively added ost-completion. It was alleviated to a small extent by extending the bottom aft of the transom with non-buoyant planing boards. The loading on the planing surface was quite high. (I use Taylor Model Basin Series 62 data).

      The increased length gave a small increase in lightship (and thus loaded) mass, but a substantial increase in planing area. This generated a small increase in top speed, but a big gain in performance at “cruising” power; say 70% of full rpm.

      I hope this is of value and encouragement.

      Peter Edmonds, Perth, Western Australia

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