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• ## Re: Boat Offsets and Stitch n Tape

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• Dale, there are free programs like Robert LainĂ© s Carene which rigorously develop conical surfaces. There s also Unfold which does the same starting from
Mar 1, 2002 1 of 15
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Dale, there are free programs like Robert LainĂ©'s "Carene" which
rigorously develop conical surfaces. There's also "Unfold" which does
the same starting from a table of offsets, but it assumes that the
surface is developable. Gregg's "Hull Designer", while much better
rigorous developability check. You have to use common sense when
working on strongly flared sections of the boats (for example,
modeling the Chebacco hull is practically impossible at the bow).
Of course many commercial hull fairing programs do the plate
development.
The best description of the conical development method I've seen is
in John Teale's "How to design a boat". Also Stephen
Pollard's "boatbuilding with aluminum" has a good description.
For what I know, the most used algorithm is the Kilgore's one, first
described in some publication on fishing boats in the third world
(FAO or some similar organization).
Best, Pippo

> Is there a mathematical method to develop the shapes for stitch n
> glue construction of a chined boat from offsets.
• ... I recall reading that Jim Michalak uses the method of joining triangles. I.E. A piece of curved plywood can always be reduced to a grid of ajoined
Mar 1, 2002 1 of 15
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--- In bolger@y..., "rlspell2000" <richard@s...> wrote:
> Why don't you send Jim Michalak an email?
> He developes his panels by
> hand, so he should have the method down.

I recall reading that Jim Michalak uses the method of joining
triangles. I.E. A piece of curved plywood can always be reduced to
a grid of ajoined triangles with the length of the legs determined.
If you flatten out the group of triangles, then you know the shape of
the plywood to cut.

In other words, draw a zig-zag line from edge to edge for the full
length of each plywood panel, each of the spaces between the lines
and plywood edge is a triangle. The length of the lines is the same
whether the plywood is bent or flat. You know the length of each
line, so you can layout your cuts.
• ... It s all based on the concept that the ply doesn t stretch, so the triangles all stay the same size and shape. Suppose you have a chine hull drawn with the
Mar 1, 2002 1 of 15
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> I recall reading that Jim Michalak uses the method of joining
> triangles.

It's all based on the concept that the ply doesn't stretch, so the
triangles all stay the same size and shape. Suppose you have a chine
hull drawn with the usual 11 stations (0-10). To lay out the
topsides, you could start with the triangle that has the base at the
chine and rail at station 5, and the peak at the rail on station 6.
The next triangle would have a base at the chine and rail at station
6 and the peak on the chine at station 5. Etc, both ways.

So far as I know, all the fancy software works basically the same
way, except they use a larger number of stations (and/or cleverly
chosen triangles) to cut down the error.

Exercise for the student: What happens when you try this process with
a "non-developable" hull shape? Hint: the trouble begins because the
straight-line length of a triangle side is not the same as the length
measured along the curve of the hull.

Peter
• I wrote the Hulls program freeware which you can get from carlsondesign.com. You design in 3D and it develops the flat panels to cut. You can model an
Mar 1, 2002 1 of 15
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I wrote the "Hulls" program freeware which you can get from
to cut. You can model an existing design, which you know is
"developable". On a scratch design, my program doesn't restrict you
to conical development (which means each panel lies on a cone or
cylinder), mainly because it doesn't know how. In Hulls, you can
freely bend and twist panels, so they *might* be tortured.

It's a little like bumblees flying - they can't fly, but since they
don't know that, they do anyway. Practically, what happens when you
really bend and twist is that your panel might want to "cup" some
from the straight line segments going around the bulkhead. The
program doesn't know any different. You might see some in short, fat
panels/boats with a lot of torture, though it's generally less
than "manufacturing" (sawing and epoxy-ing) tolerances.

Actually, its a lot more intuitive than it sounds - what looks
pleasing to the eye and streamlined to the water is not going to be
tortured much if at all. So, lots of hulls get built by people who
directly.)

{That's the difference between engineers or "applied scientists"
(like me) who design to +/-10% and get it built, and scientists who
enjoy rigor. But, I will concede here that I will NOT be volunteering
for the Mars shot that makes it 90% or so of the way back to earth;-}

To output, you can 1.) send the hulls output to a sailmaker who has
one of my plotting machines and would put it on paper for you, 2.)
find a draftsman with a big roll paper plotter and give him the HPGL
or DXF output to plot, 3.) use the handplot.txt and tape meausure to
loft out the panels, or 4.) under the hulls nesting feature, nest the
panels and plot them directly, X by Y, onto the plywood as shown by
the program. The optical method sounds like a lot of extra work (?)

Gregg Carlson
www.carlsondesign.com

--- In bolger@y..., "steelcb" <steelcb@y...> wrote:
>Don't know if this is what you have in mind, but I have been using
>to develop some interesting hull shapes which I modeled in cardboard.
>
>After printing them out, I scanned in those images and inserted
>scanned images into Micrsoft Powerpoint slides. I adjusted these
>inserted images to take up whole page and superimposed a gridwork...
• Greg, I ve enjoyed your hulls software immensely. It provides such scope for the imagination. I don t worry about how it works, I just use it for hours on
Mar 1, 2002 1 of 15
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Greg,

I've enjoyed your hulls software immensely. It provides such scope
for the imagination. I don't worry about how it works, I just use it
for hours on end pulling and pushing and contemplating the new hull
shapes. It works well for designing decks and structures, wings, and
fuselages too. Thanks a million.

Tom Pannell
Tulls Bay, NC

--- In bolger@y..., "ghartc" <gcarlson@c...> wrote:
> I wrote the "Hulls" program freeware which you can get from
> carlsondesign.com. You design in 3D and it develops the flat panels
> to cut. You can model an existing design, which you know is
> "developable". On a scratch design, my program doesn't restrict you
>
• THE book on developing panel shapes is: Ship And Aircraft Fairing And Development for Draftsmen and Loftsmen and Sheet Metal Workers by Sam Rabl It s available
Mar 2, 2002 1 of 15
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THE book on developing panel shapes is:

Ship And Aircraft Fairing And Development for Draftsmen and Loftsmen and
Sheet Metal Workers by Sam Rabl

It's available for reasonable prices used (see http://www.abe.com), but
I've also heard that it's been reprinted recently (or maybe it never went
out of print?).

Yes, that's the same Sam Rabl who wrote Boatbuilding in Your Own Back Yard.

--
John <jkohnen@...>
Nobody ought to wear a Greek fisherman's hat unless they meet two conditions:
1. He is a Greek
2. He is a Fisherman <Roy Blount Jr.>
• Westie Farmer had some great tales about Sam s experience in designing a flash amphibian aircraft, worth looking up at the library ( My Old Boat Shop ). Good
Mar 2, 2002 1 of 15
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Westie Farmer had some great tales about Sam's experience in designing a
flash amphibian aircraft, worth looking up at the library ('My Old Boat
Shop'). Good book anyway, no matter what Thomas Firth Jones says..

David Romasco

-----Original Message-----
From: jhkohnen@... [mailto:jhkohnen@...]
Sent: Saturday, March 02, 2002 1:21 PM
To: bolger@yahoogroups.com
Subject: Re: [bolger] Re: Boat Offsets and Stitch n Tape

THE book on developing panel shapes is:

Ship And Aircraft Fairing And Development for Draftsmen and Loftsmen and
Sheet Metal Workers by Sam Rabl

It's available for reasonable prices used (see http://www.abe.com), but
I've also heard that it's been reprinted recently (or maybe it never
went
out of print?).

Yes, that's the same Sam Rabl who wrote Boatbuilding in Your Own Back
Yard.

--
John <jkohnen@...>
Nobody ought to wear a Greek fisherman's hat unless they meet two
conditions:
1. He is a Greek
2. He is a Fisherman <Roy Blount Jr.>

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