1. Junior Member
Join Date
Aug 2020
Location
Clackmannanshire, Scotland
Posts
15

## Conic Projection

Dear members,

Does any-one have a suggestion for a source of information about conic projection - the method of designing the forward bottom of a stem and keelson area, and for deciding the amount of round on the forward frames/moulds?

I have John Teale's book on small boat design, and though he covers this, and though I understand each and every word he says, I cannot make it work for me. Tainternet searches turn up lots of complicated maths and cartography, nothing useful for a bodger like me.

Thanks,

Collin

2. ## Re: Conic Projection

FS Kinney's Skenes Elements 8th (1973) edition has a section on it. https://www.abebooks.co.uk/servlet/B...srp1-_-title15

A how it works picture.

3. Junior Member
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Location
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## Re: Conic Projection

Thanks Peerie Maa, I'll be getting a copy shortly.

Keep safe,

Collin

4. ## Re: Conic Projection

Originally Posted by CollinR
Thanks Peerie Maa, I'll be getting a copy shortly.

Keep safe,

Collin
Are you designing from scratch or trying to loft an existing design?
If it is a new project and you are comfortable working on a PC screen try Freeship
https://sourceforge.net/projects/freeship/

Lofting an existing design will require a lot of trial and error to find where the generator locus sits. So it may be easier to input the stem-keel and chines into Freeship anyway.

5. ## Re: Conic Projection

So Nick, please expand... the distance of the locus from a fixed point on the hull denotes fineness or otherwise of the bow?

Although I can imagine a drawn out mathematically (and therefore dimensionally) cone to be a reasonable entry and easier to build that an asymmetrical form, is all there is to it hydrodynamically? A rule of thumb to fall within the limits construction materials, build time and method habits?

6. ## Re: Conic Projection

Originally Posted by lupussonic
So Nick, please expand... the distance of the locus from a fixed point on the hull denotes fineness or otherwise of the bow?

Although I can imagine a drawn out mathematically (and therefore dimensionally) cone to be a reasonable entry and easier to build that an asymmetrical form, is all there is to it hydrodynamically? A rule of thumb to fall within the limits construction materials, build time and method habits?
First off it is not a true cone with a circular base. The conic part is that the straight line generators all sweep around a common locus that would be the a[ex of the surface, or can be parallel, and sweep along the CL profile or the chine. On a true cone, the keel profile would be part of a parabolic or hyperbolic curve.
The fullness or fineness is affected by the position of the locus. The farther forward/lower, the finer the bow. The further to the side/higher, the lower the dead rise of the bottom and forefoot. Obviously he keel/stem profile has a profound effect.

7. Senior Member
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## Re: Conic Projection

developable-surface-boat-designs.blogspot.com

It is a subject I have been playing with for years.

8. ## Re: Conic Projection

Nick's How it Works picture doesn't quite give the whole story for me, nor does Skene. As I was initially a little confused, I decided to mark up the diagram to show how to use the cone to get the actual offsets. The green lines represent one particular mold or frame. The solid blue are the two views of a line on the cone. From these you can lift off the offsets (red). I hope this helps others understand the process as it did me. What is not clear to me is how one chooses the vertex of the cone. It appears that the further forward and higher the position, the softer the sections. And, the further down and out from the center line the harder (straighter) they are. I assume choosing the optimal position is a matter of experience (and experiment).

conic development (1).jpg
Last edited by akitchen; 11-21-2020 at 04:15 PM. Reason: Diagram unclear, added to description.

9. Junior Member
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Location
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Posts
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## Re: Conic Projection

Peerie Maa, this is the continuing development of a "new" design ("new" as in about ten years from conception!). I tried Freeship, but I will stick to pencil and paper, I can "see" things more easily.

W Grabow, despite the note about Freeship, above, I'll have a look.

aKitchen, you seem to have been in the same place as me, I understand theoretically what I'm supposed to do, but so far when I try to draw it, it all goes to nonsense. Thanks for your additional information, I'll keep trying with more hope of success.

All - keep safe and thanks.

Collin

10. ## Re: Conic Projection

Originally Posted by CollinR
Peerie Maa, this is the continuing development of a "new" design ("new" as in about ten years from conception!). I tried Freeship, but I will stick to pencil and paper, I can "see" things more easily.

Collin
Freeship really is better, easier, than paper and pencil.

• It is less wasteful of your time when iterating the design. You do not need to start over from a blank sheet of paper.
• You can rotate the model on the screen to see it from any direction.
• It has routines that prove the developability of the panels as well as visualising their curvature.
• It will calculate displacement and stability.
• It will output the shape of the developed panels.

11. ## Re: Conic Projection

Originally Posted by Peerie Maa
Freeship really is better, easier, than paper and pencil.

• It is less wasteful of your time when iterating the design. You do not need to start over from a blank sheet of paper.
• You can rotate the model on the screen to see it from any direction.
• It has routines that prove the developability of the panels as well as visualising their curvature.
• It will calculate displacement and stability.
• It will output the shape of the developed panels.
Nick,

I realize that what you say is true. But I'm so old fashioned, I just love the visceral feel of drawing lines with pencil and paper. Funny, because in a previous life I taught computer science.

Andrew

12. ## Re: Conic Projection

Originally Posted by akitchen
Nick,

I realize that what you say is true. But I'm so old fashioned, I just love the visceral feel of drawing lines with pencil and paper. Funny, because in a previous life I taught computer science.

Andrew
Fairy Nuff. Can I suggest that you stick with round bottoms, and do not go down the hard chine conic projection route?

13. ## Re: Conic Projection

Originally Posted by Peerie Maa
Fairy Nuff. Can I suggest that you stick with round bottoms, and do not go down the hard chine conic projection route?
That's my plan. Next project is Doug Hylan's "Siri', and no designing required on my part.

14. Junior Member
Join Date
Aug 2020
Location
Clackmannanshire, Scotland
Posts
15

## Re: Conic Projection

My copy of the eighth edition of "Skene" finally arrived today (delayed due to Covid, presumably), what a disappointment! Very little information on conic projection, and the notes on the drawing are barely legible, as well as irrelevant as to the "How do you do it?" question.
On the other hand, there's loads of other stuff of interest, some of which is likely to be directly relevant to my project.
I've been thinking about this some more, and I'm going to try to get the pen and paper method again with what I've now got in my head, and if it works, I'll post up a "How to" guide here in the next few days.

Dwedais"Gwirion", nid "Twp"

15. ## Re: Conic Projection

You might also look up the work of Harry Schoell, and his delta-conic and duo-delta-conic hull forms.

Here's a good article from our host's sister pub:

http://trojanboat.com/wp-content/upl...ll-article.pdf

Kevin

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