Offset Surfaces to the inside of the hull

DELFTship forum Feature requests Offset Surfaces to the inside of the hull

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    • #37355

      I was really surprised after Downloading the new professional version of delftship I recognised the button “offset surfaces” that’s a important feature for me that I missed before. I need it for generating the outer contour of the bukheads and stations of my boats. After playing around a little I realized that it just works to the outside of the designed boat. When I changed the surface normals to the inside it automatically changes the direction to outside. As a workaround I tried to choose a negative thickness, but the result was really strange. So how can I get a inner offset of the hull surfaces?


    • #37394
      Ken Snelgrove


      I’m interested in this feature as well. I’d like to be able to generate a new surface for mould frames based on hull model I’ve developed. The surface for these interior frames would be offset 3/4″ from the hull surface I have.

      The “offset surface” feature seems like the tool to do this but I have not been able to find any documentation to give me some confidence about what the tool does and how to use it.


    • #37395
      John R. Coil

      It took me a while to realize that DELFTship adds thickness to the model, and if it had not been for an accident in assigning plate thickness (missing decimal point) who knows how much longer it might have taken?

      I must admit that I’m quite curious about changing surface normals, something I had not even considered possible, much less doing (I missed that in my skimming of the manual) until reading your post. [/Homer Moment]

      Thanks for that! 🙂

      I would that I could return the favor and give you an actual solution besides suggesting trying undersizing your model just enough so that the added thickness brings the now unseen outer surface up to where it should be, but by then you might as well have exported it to AutoCad … though I guess you might try using scale once you’ve got it looking right … but unless you are using a uniform thickness shell (it could happen) that isn’t necessarily much of a solution either. ***

      I wish you well and will return to see what someone who actually knows may tell you.

      *** For example, per Gerr scantlings rule the .8″ thick topsides ply on what I’m currently playing around with would give (4-0.0667)/4 = 0.9833Y and (39.9167-0.1333)/39.9167 = 0.9967X) and saving that as a different model. Of course THAT gets anyone trying this into the whole: well the sheer strake is 0.9″ and plank below BLH – see the formula in the fiberglass section of the book for how high above the waterline that is – is 1″ thick except for the garboard which is 1.1″ and the 3.1″plank keel … but, hey, that’s what lots of added tiny hard chines are for, right? So my proposed so-called “solution” is more like another form of self abuse … which should be just about right coming from someone wanting to design and someday build their own 40′ boat…. 😉

    • #37459
      Peter Edmonds

      I am very new to DELFTship, but can add something to this topic.I haven’t got anywhere near exploring the DELFTship offset surface. It appears attractive, and I await further expereinces..

      Scaling a shape is fraught with issues, in that you are looking for an absolute shift (normal to local surface), and would get proportional shifts with scaling. I don’t think this is worth exploring. For a quick and dirty fix just ignore the shell thickness; do it properly if the project is fussy.

      I am a Naval Architect, working mainly in small commercial vessels, with SolidWorks as my man design tool. I’ve moved into DELFTship to make use of its capability of generating as IGES surface, which I can then move into SW. I’ve done this once so far, in a simple environment.

      One approach I intend to use is to generate a solid block model in SW for the hull, and then make use of SW “shell”, to hollow it out, leaving a thickness inwards from the IGES surface (which has become the outside of shell surface). This will be comfortably representative of a GRP hull out of a mould. I can then go on and add other stuff.

      The SW “Shell” allows me an open shell (no deck) illustrated in the SW course notes example.

      Another approach I have started on is to transfer the IGES surface as the shape of the temporary moulds, plus stem, keel and transom, as for a small clinker planked boat. I was then able to emulate the plank edge ribbands over the electronic copy of the hull moulds, create cross sections, plank by plank,and mould by mould, to generate the solid bodies for each plank.

      At the DELFTship stage your surface choice is governed by your source data. For this example as above (14 ft) I had lifted offsets from the full size mould templates. For another example (again 14 ft) I had a range of offset points lifted from the inside of the empty hull mould. Points in an offset table may be for outer shell surface, or for inside of shell planking/plating, and outer edge of framing.
      Rurudyne appears to have the objective of generating a moulded (inside of shell) surface. If I were working from design offsets to outside of shell, I would convert all half breadths and heights bu subtracting the shell/planking thickness, with a cosine factor for oblique surfaces. This would maintain the longitudinal positions for all transverse section offsets, the heights for waterlines, and the half breadth (lateral offsets) for buttocks. Centre profile, rabbet and sheer lines, and diagonals if used need to be thought through.

      This approach would enable a “moulded surface” to be generated and faired, and used directly to generate inside of planking building moulds if required.

      The inwards offsets would be selected to give the appropriate design moulded surface – frame line/inside of planking or plating for planked or plated boats, outside of mould shapes if a batten mould for cold mould ply or foam sandwich construction, to get the outer face of the shell at its desired location.

      Peter Edmonds
      Naval Architect
      Perth, Western Australia

    • #37963
      Lewis Ardron

      I worked as a boat builder 50 years ago, so computers are new to me. I thought by using a negative offset I could generate measurements for a set of building molds but it doesn’t work. Small clinker boats used to be drawn to the inside of the planking and I believe ships are also drawn to the inside of the plating. It seems that Delft-ship originated as a big ship program and as yet does not have provision for doing this.

      Delft-ship is by far the best program I have come across (free or not). As a working Naval Architect CAD programs and spreadsheets must save you hundreds of hours, back in the day we did everything on the drawing board and on the loft floor, we didn’t even have calculators!

    • #44318
      Matt Tucker

      I’m not sure if this will help since it seems that you had trouble with the ‘offset surface’ option but I managed to generate what appears to be the offsets for the frames (inside of hull).

      I first created a new layer and made sure it was selected in the layer selection then used the offset surface option and put in -.007 (7mm hull thickness). It created a new surface inside the original one but the offsets still were for the outer surface so I turned off all the outer surface layers and exported the offsets again and it seems to have worked.

      Since I couldn’t find any info online about how to do this I hope this helps someone.

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