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Author Topic: Drawing between the lines  (Read 1247 times)

warspite

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Drawing between the lines
« on: September 27, 2006, 08:44:34 PM »

Here is one that may help us newbies,

We get plans for making boats or want to create our own (i will explain below), and the plans show a combined front and rear view of the vessel with lines showing the profile of the vessel at various parts along the hull. On the elevation (side view) it marks these positions and they should correspond, but they are at various distances between these points.

My explanation of why i am posting this question.
I have built a couple of plastic kits (airfix etc) that means the hull etc comes already fabricated (DUH!!) but i want to scratch build a vessel that at present i do not know if the plans are available (the web does not enlighten sufficiently :-\), from photo's available i can approximate what she looks like, and can scale to get a reasonable likeness, in doing so i can create several shapes for the bow up to the point where the hull becomes vertical and i will have the same problem for the stern.

These cross sections are quite a distance apart and it confuses me how you work out how the profile changes shape at regular intervals, to illustrate i have posted below the cad version of the profiles for Warspite from the anatomy of a ship, this was done years ago and never put into practice, each of the profiles i could use for making into the main frames (with a lot of sanding) but to create what i call splitters, frames that go between, to morph the skin between one frame and another is quite hard to figure out.

My only thought about this is to create this 'splitter' as a percentage difference between the frames, i will have to post another reply to explain what i mean by this and create sketches to illustrate it.

Not being a shipwright i was wondering how others do it.
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warspite

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Re: Drawing between the lines
« Reply #1 on: September 27, 2006, 11:02:22 PM »

How to describe this here goes,

In plan you have the centreline or keel, radiating from this are several frames, each from the bow get further and further from the centreline as they get further from the bow, they them start to come back to the centreline as they get closer to the stern, so far so good.

Each frame marks the furthest the hull gets from the centreline and generally is curved and intersects the end of each frame giving the profile of the hull, this curve means that taking different frames there will be a difference in how far the frame is from the centreline.

Frame 1 near the bow is say 100mm from the centreline at deck level, it is also concave to the keel to give the usual knife cutting profile.

Frame 2 is 200mm further back and 200mm away from the centreline at deck level, it is also convex, going flat at the bottom before reaching the keel, bucket profile.

Intersecting the ends on the plan is a curve, now, say you wanted to put a stiffening frame or as I call them, splitters, half way between frame 1 and 2 this would be positioned 100mm after frame 1, it would also have a profile some where between frame 1 and 2, but not a half way version, this is because when you look at the plan, the curve at half way between the frames may not be 150mm from the centreline it could theoretically be 170mm, but the closer you move towards frame 1 the quicker it reduces to near the 100mm mark.

This means that to place the stiffening frame at half way the true distance (ie say the 170mm), will be a percentage of the distance between frame 1 and 2 where 200mm will be 100% and 100mm will be 0%, 170mm would be 70% ( 200 less 100 = 100 divide this by 100% means that for ever 1mm past the frame 1 length will equate to 1%.

So the stiffener will in theory be 70% of the difference between frame 1 and frame 2 so that by the time the morphing gets to frame 2 from frame 1 it will have changed from concave to convex.
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warspite

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Re: Drawing between the lines
« Reply #2 on: September 27, 2006, 11:13:07 PM »

Thats my interpretation of how to do it but is there a simpler way?

I used cad and i am trying to generate frames for a future project, how to create lines for a container / tanker / sub etc.

when i use the polyline function, the form may look right, but it drifts when you want to morph from one profile to another, i created an excel data sheet but the number of points would be excessive for each frame.
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boatmadman

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Re: Drawing between the lines
« Reply #3 on: September 28, 2006, 07:27:49 AM »

Hi,
There is some software called Hullform. Google it. You can get a basic version free, with a tutorial.

Ian
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warspite

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Re: Drawing between the lines
« Reply #4 on: September 28, 2006, 06:08:49 PM »

thanks i will try that
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