Panel Development

[smjmitchell]

True. Kilgore and Michelsen detail the condition for developability.
For practical purposes if the projection of each of the lines onto the cartesian planes results in a line where the curvature (I.e. second derivative) has the same sign along the length of the line then the surface is developable. However this is not absolute and some lines with inflection points (I.e reversal of curvature) maybe developable. Michelsen details a graphical method of establishing developability in the more general csse. There are of course mathematical methods for establishing developability as well but that is outside the scope of the current discussion on graphical methods.

 

[philSweet]

Steve, there isn't all that much that is special about designing a boat to be developable. It is just some additional constraints to deal with. The important issues of the hull design don't change. You must get the displacement curve and centers correct for the application. The parts of Kilgore's and Rabl's methods that may seem approximate or arbitrary are probably due to the fact that they assume you are familiar with conventional lofting practices. So for most boats, the hull design begins with a midship section, and that really does take a lot of the options out of the lines of a chined hull. You want the flow to follow the chines. In a sailboat, that include various angles of heel. You also want a certain amount of positive curvature on the frames to help resist hard spots, so the ruling lines shouldn't be transverse. Same with stringers in a runabout. This is a bit more problematic since the runs tend to be quite flat overall, but if the framing is longitudinal, use more transverse rulings even if the curvature is tiny. It's the almost-flat panels that are the most annoying. If you start with a design that you know is a good boat, and is close to being developable, then these methods work just fine. If you are starting with a blank sheet of paper, don't worry about developablity. Get the midship and transom sorted and then look at the panel widths and frame spacings and plate thicknesses you need. Then you want to to resolve material waste vs chine position as you select the chines for the hull. Strongly tapering panels are a nuisance from a bending perspective. So in the end, precise developability only becomes a concern quite late in the program, where you maybe trade off a bit of waste in order to fair up the chines and manage the buildability of the design. You might decide that a twisted plate that flattens into a panel with a straight edge is worth the trouble and you can live with the visuals. Or you might decide that you can scarf the panels to create a big banana shape with no twist and get better looking chines. Either way, after you have decided such matters, you go do Rabl's and develop your frame shapes and CHECK the developability of your design. Have faith, if it is a good design, it can be executed as a good developable design with out any great trouble.

 

[ImaginaryNumber]

Hello,

I have been researching various approaches for lofting flat panel developments for plywood and metal boats. I am trying to teach myself how to do this using traditional drafting / lofting methods.

Whilst I have a few papers and books that discuss the issue none of these really provides a sufficiently complete discussion of the methods used by practising naval architects. Two references keep popping up in my reading, that are long out of print, but which seem quite practical. I have not been able to locate a copy of at a reasonable price. These are:

Aluminium Boats (Kaiser Aluminium Co) 1978 - Appendix D
The Plywood Boat Builder (Vol 41 of Motorboatings Ideal Series)

A previous poster back in 2003 mentioned that there is only about half a dozen pages in each book that are relevant.

Does anyone have a copy of either book that would be willing to scan or photograph the relevant pages and email them to me ?

Thanks,

Steve

A 1964 edition of Aluminum Boats can be purchased for $38USD (located in UK, shipped to US).

http://www.abebooks.com/servlet/SearchResults?kn=Kaiser&sortby=17&tn=Aluminium+Boats

 

[ImaginaryNumber]

maybe of interest?

True Round Metal Boat Building: Bezier Chine Design (Amazon $30)

Traditional true round hull 'plating methods' have always been arcane and obscure in nature. Plating a true round or wineglass styled sailboat hull design, to date, has been an artistic metal forming process. The purpose of this book is to demonstrate a simpler approach to plate a true round hull. One that reduces the time, effort and the special skills required. The book begins with a discussion about the theory of ‘Bezier Chine’ by relating it to long-established architectural sheet-metal layout principles. ‘Bezier Chine’ design and construction uses high tech computer design. This reduces construction to time-honored architectural metal-working procedures that are methodology, consistent and predictable. Every part of the hull is fully developed and clearly described. This includes never-failing techniques to pre-form all the shell plating and position it on the hulls corresponding framework in a fair and seamless manner. There is nothing vague about the process. "True Round Metal Boat Building" by D.L. Schaffer, simplifies construction of true round hulls for every boat builder. The ‘Bezier 12.5” - a classic 16 foot aluminum tumblehome hull design is used to demonstrate the principles of ‘Bezier Chine’ design and construction.

 

[Tanton]

Unfolding panels.

Get Rhino.

 

[Mr Efficiency]

True. Kilgore and Michelsen detail the condition for developability.
For practical purposes if the projection of each of the lines onto the cartesian planes results in a line where the curvature (I.e. second derivative) has the same sign along the length of the line then the surface is developable. However this is not absolute and some lines with inflection points (I.e reversal of curvature) maybe developable. Michelsen details a graphical method of establishing developability in the more general csse. There are of course mathematical methods for establishing developability as well but that is outside the scope of the current discussion on graphical methods.

For "practical purposes" on a simple planing hull, you can easily go about your task once you have the chine more or less decided. You simply use that as the base to draw your generating lines to a point, one "cone" ( an oblique, non-circular one usually) for the bow topsides and one for the forefoot, with a common generator passing through the chine peak, can be sufficient. You just change the apex points of the cones to change, e.g., the shape of the forefoot, or the rake of the bow, till you get something you like. The very rough attached drawing should illustrate the general idea.

 

[SukiSolo]

Good illustration Mr E. In practice it is the forefoot and lower bow area that is the hardest to resolve to get a good hydrodynamic form. A small amount of panel bow (transversly) may help and also in practice you can get a small bit of 3 D curve. Don't push the 3D though or the ply will split.

 

[smjmitchell]

Get Rhino.

I have Rhino. I have used this a little for sheet metal developments on things other than boats. It seems to work OK for simple lofts but there are issues at times in more complex situations. I don't like software where it is impossible to know exactly what is going on inside. For various reasons I have decided that I prefer to design a developable surface for my project that I truly understand the shape of rather than just accepting what a computer spits out.

I started doing developments for my design with Freeship and Delftship but I am suspicious of the results around the bow. I have made largish cardboard models and these confirm my suspicions. Also control of the panel shape seems difficult - particularly if you are trying to enforce a developable shape. Also there is no explanation exactly how these programs are developing the shapes .... you just don't know exactly what you are getting. I have spent hours trying to generate the shape I want but never seem to achieve it.

I have also tried lofting the panels in Solidworks and that is a debacle. The people who write the Solidworks code for sheet metal developments are clowns. They simply do not understand the basics of lofting shapes more complex than a cone in an airconditioning duct .... Solidworks just spits out a development based on a simple ruled surface without positioning the ruled lines to connect points on the defining lines with parallel tangents.

 

[smjmitchell]

For "practical purposes" on a simple planing hull, you can easily go about your task once you have the chine more or less decided. You simply use that as the base to draw your generating lines to a point, one "cone" ( an oblique, non-circular one usually) for the bow topsides and one for the forefoot, with a common generator passing through the chine peak, can be sufficient. You just change the apex points of the cones to change, e.g., the shape of the forefoot, or the rake of the bow, till you get something you like. The very rough attached drawing should illustrate the general idea.

This is the simplest approach to a conic loft and may well work (I need to send some time in AutoCAD to know for sure). Teale, Kinney and Lord, to name a few, detail this approach in their books. This approach requires some trial and error to locate the apex (or in some cases multiple apexes). Also, it can only fit one of the lines exactly and you need to fiddle with the apex location to get the best match to the other line that you can. So the development drives the shape of one of the lines to make sure it lies on the conic surface. Having said that some nice shapes can be developed with this approach (see Kinney for example). Whether it will work for me, time will tell.

The most general approach is the multiconic method which builds up a general panel shape with a number of different conic surfaces joined together. This lofts a surface between two predefined lines. The Rabl approximation is the simplest. The Kilgore method (actually methods) is fairly exact but more complex. I have spent some time looking at the Kilgore methods today and they do not look that bad once you get more familiar with the approach (he claims 2 1/2 hrs to develop the lines of his example hull). OK ... so it takes me five with the learning curve - that is not too bad. So my plan is to use both the Kilgore and Rabl methods and compare the results to see what sort of error is implicit in the Rabl method. If the error is not significant it is clearly a simpler and quicker method.

The more I read on this, I get the feeling that there is no one method that is best. There are actually 3 or 4 methods that can be used to generate conic surfaces on a drawing board and often hulls are lofted using a combination of two or three methods - basically choosing the method that is best for different areas of the hull.

It is interesting trying to relearn a long lost art .......

 

[smjmitchell]

True Round Metal Boat Building: Bezier Chine Design (Amazon $30)

Interesting .... before I buy the book, does anyone know how he forms the panels ?

 

[smjmitchell]

Either way, after you have decided such matters, you go do Rabl's and develop your frame shapes and CHECK the developability of your design.

Thanks for the practical design tips.

Are you saying that you have used Rabl's method to loft boats that you have built ? If so, do the panels fit as designed ? Do the frame shapes mate properly to the skin when it is bent into shape ? Plywood or metal ?

 

[Mr Efficiency]

You are a very methodical sort of person, I'd say, but you really have to apply due weightage to getting the right boat shape, as much as a precise definition of the lines. No good having a thing built to fine tolerances that is found wanting because the basic shape is flawed. Do you feel competent with that ?

 

[smjmitchell]

You are a very methodical sort of person, I'd say, but you really have to apply due weightage to getting the right boat shape, as much as a precise definition of the lines. No good having a thing built to fine tolerances that is found wanting because the basic shape is flawed. Do you feel competent with that ?

Well as competent as someone can be who is designing his first planing hull. I have read a huge volume of stuff on planing hull design and I have no shortage of actual hulls to study. So yes I feel pretty confident I can get the lines right. But, as they say, the proof is in the pudding and you learn by doing and reflecting on what could be improved. My underlying concern is getting the subtle details of the hull loft right so it looks like a professionally designed shape ... how many homebuilt boats do you see that clearly look homebuilt because the designer lacked good lofting skills ?

 

[Mr Efficiency]

Homebuilt, home-designed small craft are a bit of a rarity these days, especially in ply.

 

[smjmitchell]

Homebuilt, home-designed small craft are a bit of a rarity these days, especially in ply.

True .... but I am not aiming to revolutionise boat design nor get rich. I just want a simple project I can build in my home workshop to learn about hull design and build confidence that I know what I am doing. The next project will be more ambitious ..... kind of like stepping stones to something more worthwhile. I would build in aluminium if I had better welding skills but I don't .....