# Generating Offsets

Discussion in 'Software' started by rangebowdrie, Dec 16, 2022.

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### rangebowdrieSenior Member

I have a set of lines drawings for a sailboat.
Quite complete with Stations/Water lines/Buttock Lines, and Half Breadths
shown in plan and profile.
Also known are LOA/LWL/Beam/Draft.
Can they be scanned, the known dimensions entered, faired, and a Table of Offsets generated
which would allow lofting to commence?
I suppose that volume/displacement could also be calculated as well as CB/CG?
I am, shall we say, "computer challenged".

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### TANSLSenior Member

I suppose that what you need is to vectorize some drawings on paper, to fair the shapes and deduce the table of offsets. This can be done, although it is not easy and is quite laborious. The process begins, of course, with a careful scanning of the drawings, avoiding their distortion.
Once vectorized, a lot of information can be obtained, but not the center of gravity.
If you need help to carry out this work, please tell me what information you have and what help you need, if any.

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### rangebowdrieSenior Member

Thanks TANSL, I'm going to reflect on this for a bit.
I mis-typed, I should have said 'find the Center of Buoyancy of the immersed volume.'

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### rangebowdrieSenior Member

I should add that I guess there's nothing wrong with just taking the scale ruler and doing it the old-fashioned way.
Measure out from center line to section at each WL at every station to get the half breadths and measure up from the baseline to each section on each WL line for every station to get the heights, using the Buttock lines as a "check line".
I guess in my naivetÃ© I thought a scan job and a few mouse clicks, and 'presto', a T of O.

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### TANSLSenior Member

You are partly right. The table of offset can be obtained as you say, even directly from the paper, without the need to scan anything. It all depends on the degree of precision you want to achieve and how fair you need the lines and surface of the hull. The normal thing is that the paper is deformed, which will produce some discontinuity in the forms and, in addition, the precision that was obtained with the drawings by hand may not be optimal for your needs. If you need something of quality, what I would do is scan your drawings and then, with a CAD program, draw each of the lines and proceed to fairing the shapes to obtain a good match between all the lines on the hull and avoid excessively large lumps on its surface.

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### TANSLSenior Member

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### gonzoSenior Member

What size boat?

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### TANSLSenior Member

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### SkyakSenior Member

It can be done. Back in the day, the company I worked for converted literally tons of drawing to digital. Depending on the size and quality of your drawings it might be possible to do it with a camera. As you convert to 3D from 2 you will find significant error in projections and curvature. If your intension is to use the CAD files to eventually drive CAM production, I advise you to NOT use scan files. Getting all the errors out is more effort than creating from scratch and CAM glitches are costly.
If this is a wood boat that will be built without CAM, don't invest in conversion to a medium you aren't proficient at -save it for the best builder you can afford. They do their own loft and fairing to appropriate accuracy.
If you just like to design and aren't averse to learning, by all means, use this design as you first project. CAD is awesome.

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### KayakmarathonSenior Member

Lines drawing are for finished dimensions. Offsets take into consideration the tangent of the lumber in addition to its thickness. This was one of the mistakes I made when learning about boat design and building. It wasn't until I was nearly done with the woodstrip canoe that I saw hollows in the bow and stern waterlines. To avoid major disappointment, I would recommend you try generating offsets as you envisioned, but build a 1 meter model using balsa strips. Learning about building is also part of your journey. Most of us don't build a masterpiece on our first try.

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### rangebowdrieSenior Member

Yes, I'll go ahead and sit down with the scale and generate the numbers.
It's not hard to do, just tedious, now realizing that there wasn't going to be any magic way to do it with my limited computer and software knowledge.
I've done lofting and building before, as someone said, "Lofting is the essence of boatbuilding".
With the numbers laid-out full size, a good batten and a good eye shows the bumps and inaccuracies of your offset table, where the thickness of a pencil line can easily lead to a 1/4>1/2" discrepancy.

Yes, lines are to the outside of the hull surface, but they do not consider the "tangent" of the thickness.
For wood boats, (where hull thickness and/or frame dimensions can introduce a substantial difference,) the lofting shows that up, especially near the bow where the angles are large.
That's one of the reasons that the molds that are forward of amidships are set on the front of the stations, and the ones aft of amidships are set on the aft side of the stations.
That allows the molds to be "finish faired" with battens.

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### wet feetSenior Member

That is an interesting approach,over here we set up frames in that manner but moulds are the other way round i.e aft of the section in the bow and forward of the section from midships to stern.It shaves fairing and if the lofting was thorough there should be almost no fairing in any case.

As for the OP,I find it odd that a designer should supply a design that doesn't include offsets.It certainly wouldn't have been drawn to offsets as they normally are extracted from a complete set of scale lines that have been faired on a board.There often was a scale along the lower edge of one of the views that allowed for direct scaling with a pair of dividers-Howard Chappelle often did this- and this could be the basis for a table of self derived offsets-which obviously would need proving on a conventional floor.

I would respectfully suggest that the OP has the perfect justification for learning the basics of 2D CAD drafting and there are numerous programs at price points ranging from free to many thousands of dollars/euros/pounds/rials/shekels.A simple program in which sections can be plotted in 2D will make it easy to extract section areas and if the same program is used to create a bar chart with station spacings correctly scaled it is easy to use the area below the line as another exercise in determining an area and this area will equate to the displacement.Better yet,the ratio of the area to the area of the bounding box will tell you the prismatic coefficient.A few hours of exploring the technology will probably amount to less than the time taken to lay out a grid for traditional lofting and its much less arduous for the knees.There are many useful introductory CAD tutorials on youtube that may help.

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### rangebowdrieSenior Member

You're right, I completely mis-spoke, (easier to do as you get older),
Yes, the molds set-up as you said.
I don't have a set of plans, I have a set of lines only, actually have had them for many years.
This is an exercise for a winter project, something to do when the weather is bad, on a boat that I'll not
be building unless I win the lottery, and then I would have it done.
Yes, familiar with those scales that Chappelle often used.

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### DCockeySenior Member

Lines published to illustrate a type of boat may never have been faired with waterlines, stations and buttocks reconciled.

Designers have been known to deliberately distort lines in "study plans" and similar to prevent someone from extracting the design.

rwatson likes this.
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