Inside/Molded or Outside/Displacment - Where to Design

Inside or outside? Watercraft have been designed using both the outside of the hull surface and the inside of the hull plating/planking as the primary reference. An individual may view one as “correct” based on their background and experience. Metal vessels usually are designed using the inside of the plating. “Fiberglass” boats almost always are designed using the outside of the hull surface. Wood boats in the past frequently used the inside of the planking though the more common practice for many years appears to be to design to the outside of the planking.

Below are some excerpts from references on naval architecture and yacht designs on the subject.

Principles of Naval Architecture: Volume I, Stability and Strength: Ship Geometry, Hamlin, 1988 pp1-2

The line in Fig. 1 represent the molded surface of the ship, as surface formed by the outer edges of frames, or inside of the “skin,” in the case of steel, aluminum and wooden vessels. In the case of glass reinforced plastic vessels, the molded surface is the outside of the hull. (The term molded surface undoubtedly arose from the use of wooden “molds” set up to establish a surface in space to which frames could be formed when wooden vessels were being built.).

The shell plating of a steel or aluminum ship constitutes the outer covering of the molded surface. The shell plating is relatively thin and is formed of plates that are usually of varying thickness, causing some unevenness, although the molded surface is generally smooth and continuous.
The thickness of planking of a wooden boat is relatively larger than the shell thickness of a steel vessel, and it the usual practice to draw the lines of a wooden boat to represent the surface formed by the outside of the planking, since this gives the true external form. However, for construction purposes it is necessary to deal with the molded form, and therefore it is not unusual to find the molded from of wooden vessels delineated on a separate lines drawing.​

Basic Ship Theory, Volume 1, Fourth Edition, Rawson & Tupper 1994 p9

The Shape, lines, offsets and dimensions of primary interest to the theory of naval architecture are those which are wetted by the sea and are called displacement lines, ordinates, offsets, etc. Unless otherwise stated, this book refers normally to displacement dimensions. Those which are of interest to the shipbuilder are the lines of the frames which differ from the displacement lines by the thickness of hull plating or more, according to how the ship is built. These are called moulded dimensions. [Comment: This is the only use of the term “displacement” to describe the outside surface which I encountered in a reference.]​

Yacht Designing and Planning, Revised and Enlarged Edition, Chapelle, 1936, 1964 p152

It is now the custom to draw the lines to the outside of the planking; in former times the lines were drawn to the inside of the planking. The latter type of drawing is best from the builder’s point of view but makes calculation difficult and requires a knowledge of loft work that few amateurs possess.​

The Principles of Naval Architecture Series: The Geometry of Ships, Letcher 2009, p34

Regardless of the means of hull surface definition, the construction material, or the construction method, hull geometry is generally defined in terms of a molded surface or molded form. In some cases, the molded surface will be the actual exterior surface of the hull, but more often it is a simplified or idealized surface whose choice is strongly influenced by the construction method. The molded surface usually excludes any local protrusions form the hull, such as keel, strakes, chines, and guards. When the vessel is constructed as a skin over frames, the molded surface is usually defined as the inside of the skin, outside of frames; this is the case for both metal and wood construction. When the skin thickness is constant, the molded form will then be a uniform normal offset of the exterior surface (and vice versa). For a molded plastic vessel the molded form is often taken as the outside of the hull laminate (i.e., the exterior surface of the hull, same as the interior surface of the mold), but it could also be the outside of the frames used to construct a male plug, or the inside of frames used to construct a female mold.

The important point is that the relationship between the molded form and the hull surfaces varies; it needs to be made explicit and taken into account in all aspects of the design and construction process. [Bold emphasis added to last sentence.] ​

 

The important point is that the relationship between the molded form and the hull surfaces varies; it needs to be made explicit and taken into account in all aspects of the design and construction process.

Exactly David....there is NO easdy answer to the question, it is a matter of understanding the current situation, some do, some don't, but as long as it is identified it does not matter whetjher it is inside or outside when building from plans.

The moulded dimensions have caused many an amateur builder to either come to grief or build not to plan, I have seen two boats done "wrong", it took some sorting out to fix em.

 

Caveat Emptor eh.

 

No, there IS only one correct way. The problem is there are 2 totally separate issues at play which are being fused into one meaning/understanding. And its intent is "lost in translation". What is a lines plan for?:-

1) Hydrodynamics
2) Fabrication.

The Lines plan, is always, always, drawn using the moulded line. This is “the datum”. This dictates all that is required by the NA to ensure the shape is as required/designed.

Thus, when tank testing, or performing any other hydrodynamic analysis, the thickness of plating is totally irrelevant. It does not play any part - other than the difference between moulded displacement and “full” displacement, which is almost insignificant on “smaller” boats anyway, certainly at model scale.

To fabricate the boat requires a structural design. If the hull frames are calculated as say 200mm deep, on a given displacement/speed/SOR etc, then the structural design can be completed and the frame depth, 200mm in this case, refers to that design using that hull. The hull plating is determined by the frame spacing and the longitudinal stiffener spacing ( and Class rules as default min’s).

So if that hull has now added say 20% to its displacement for a different design SOR, if for fabrication purposes, the frame and stiffener spacing is maintained, clearly the hull plating will be change as will the other scantlings. The lines plan does not change; it is exactly the same Lines Plan. All the scantlings though change i.e. the depth of the frame, for more stiffness, and the hull plating. So the lines plan is not redrawn because the structure is now different, because the inside “moulded” line is used to define the hull shape.

If this is now built with composite, the procedure remains the same. Whether a ‘female’ (inside) or ‘male’ (outside) construction if the “outside line is used”, where does this line come from?...the Lines Plan + the structural design (offset by "x" amount from the Lines") . It is not the Lines Plan only.

The lines are then “offset” from the datum line, that being the Lines Plan. Why, because of the structural design. And, as before, if that same hull increases in displacement by 20% does the Lines Plan change. No. It is still the same hull. The hull shape, in terms of the lines, never changes. However, depending upon which method of construction, note construction, and the effects of the new structural design has on the hull plate thickness (and any internal framing) compared to the original hull, one is now using a different offset from before simply to account for the structural design. So the mould/plugs required to build the boat do change, but the hull lines do not change.

The Lines + offset, which include the final design layup, is not the same as the Lines Plan only. The Lines never change, only the “offset”.

Thus Lines Plans are always, always moulded lines, never anything else. It is used to create other forms required to build, such as where to offset from etc, but the datum lines never change. Thus do not confuse the 2 very separate issues and join them into “one process”, because it is not.

 

This is a point that I have been pondering over since the strange discussion on mould lines in my Rhino thread.

The art of ship design has changed dramatically over the last 5 years, with access to 3d software that has never been so powerful in the history of boat design.

Obviously, the 'outer limits' of the hull have to be defined, as they not only dictate the displacement, but also many times, the design has to be confined. In the case of many boats, this outer skin will also be 'unrolled' for 2d measuring purposes, maybe even automatic cutting.

Once this outer limit has been defined, it is a simpler matter to create objects like female moulds and support structures, referencing that outer limit.

Now, to get the frame dimensions, if an inner 'thickness', or multiple 'thicknesses' can be applied to the 'skin', laying up the frames using the appropriate inner shell line is made much easier.

Then there is the fittings between frames. If you have a frame line 2 metres apart, its no use having the partition, bench or whatever cut to 2 metres long, as it has be be 2 metres minus whatever the frame thickness is.

This is all a fact of life if you use real time 3d cad.

The statement
"The lines are then “offset” from the datum line, that being the Lines Plan. Why, because of the structural design. And, as before, if that same hull increases in displacement by 20% does the Lines Plan change. No. It is still the same hull."

works for large traditional craft, no doubt. It has no relevance for a hull that has 20mm clearance to the inside of a container, and is not satisfactory for generating female moulds for ply or foam layup.

Its all about getting the most efficient way of assembling a digital version of the hull, and if it doesn't match traditional methods, that is pretty much irrelevant at $120 an hour.

 

Time has a way of changing glass boats !

When making racing boats the skin and glass sheathing thicknesses was always reduced off the frame sizes when they are cut by cnc so the outside surface is the dimensions off the plans !!almost !!.
Have always done this for as long as I can remember building boats !
Moulds taken off wooden race boat hulls were usually made a little thinner but had stronger steel work outside so the shrinkage would not affect the correct shape to much over the years and the older and newer boats were as close to being the same as possible .

 

This is a total misnomer. The method of drawing lines and fabrication has not changed. Only the speed at which it is done. And by users sat behind a monitor, rather than old men in dusty overalls marking out and refairing a lines plan on a scrieve board at 1:1 in the mould loft shed.

A lack of understanding/experience does not alter the way boats are made from today to 5 years ago nor to 50 years ago. A computer is only a tool and replaces a job previous done my hand. That is all. Nothing "dramatic" or "revolutionary". These claims are always made by those with no shipyard experience.

 

Exactly. And if you were cnc'ing a plug to create a mould, would you use he 'mould line', or the outside skin dimensions ?


"lack of understanding/experience does not alter the way boats are made from today to 5 years ago nor to 50 years ago"

yes it does - 3d cnc cutting has become much more common in the last five years, as the technology has become more pervasive. The productivity increases of drawing up hulls, taking off frame sizes, internal fitout, even down to artists impressions for customers, has never been greater.

I am not against time honoured standards, especially for the more traditional boatbuilding techniques, but insisting that it is the only way to do the job is the height of silliness.

 

CNC , or 3D or whatever has absolutely nothing to do with this at all. You are barking up the same wrong tree as many before you. Since all you are demonstrating is a lack of knowledge and understanding of how boats are built from design information.

 

My original thread was how to do calculations in a cad package for cnc.

You already have one long time professional say that they dont use the 'mould line' method in some cases, and i have given you several good reasons why you need some designs with both the mould line and the outer skin dimensions.

You can go on saying I am 'barking up the wrong tree', but you need to actually address the issue and explain why you think its the only way in the face of the evidence.

 

There are no "calculations" for a cad package or otherwise.

As for the rest, you’re putting the cart before the horse and fusing the two vey separate issues into one. Not sure why..but that is your prerogative. And as such continue to misunderstand the whole process.

 

All you are saying is

"The Lines plan, is always, always, drawn using the moulded line. This is “the datum”."

What is wrong with calculating the 'moulded line' from the 'displacement line' ?

That is the preferred method in many cases


as defined by :

Yacht Basic Ship Theory, Volume 1, Fourth Edition, Rawson & Tupper 1994 p9

The Shape, lines, offsets and dimensions of primary interest to the theory of naval architecture are those which are wetted by the sea and are called displacement lines, ordinates, offsets, etc. Unless otherwise stated, this book refers normally to displacement dimensions. Those which are of interest to the shipbuilder are the lines of the frames which differ from the displacement lines by the thickness of hull plating or more, according to how the ship is built. These are called moulded dimensions.

 

There is no calculating at all, never! It is a line, that is all.

You are constantly referring to CNC method of calculating a line; whatever that means??. As soon as you begin cnc or anything else, you have introduced a secondary process, i.e. what is done with this design/datum line to get a finished boat. That is where the fabrication takes the design data and make into a production process, the 2 are not the same. Yet you consistently fail to grasp this for some reason.

Happy calculating..

 

Obtuse logic. I cannot follow your premise or your point. Lets leave it at that.

 

Yep, John has it, but it's an experience thing. The Hydros need to be done to the outside of the planking, for obvious reasons, but the wise designer also understands how the puppy will be built, so station molds will be to the inside of the planking on most build types. One of the first thing I look for in a new set of plans, is some of these experience based arrangements. Subtracting planking thickness when doing mold loft/setup, is a pain in the butt and shows the designer hasn't built one of his own boats yet, other wise, he'd realize this. On the commercial side, the same is true, with production requirements usually being different than the hydro side. Another example is the mold layout. Are they evenly spaced along the LWL, likely because someone just took the stations and just made them molds or are they more appropriately arranged. In commercial work, this is a given, but in pleasure designs, I've seen all sorts of crap, like this. Real plans usually tell you right away, about the designer's abilities and experience level. A lot of new CAD based designers have popped up in the last 10 years, but they've no idea which side of the line to cut on. This is apparent very quickly and easily seen, once you get some experience. Lastly, the computer doesn't care; an arc segment, a line, a fill, it literally doesn't know or care.