Online Since 1997.

Kasten Marine Design, Inc. Logo - Copyright 2017 Michael Kasten


Please see our AVAILABLE BOAT PLANS web page

Frames First... or Plates First...?

A Discussion of Metal Boat Building Methods


There are a number of approaches one can take when fabricating a new metal boat. Among them are methods that erect the framing first and apply the plating to that structure afterward, and others that favor pre-planning the plate shapes in order to create the hull shape before the frames are introduced inside. The following is a discussion of the various approaches taken, their rationale and the pros and cons of each.

With extreme luck, the following will be taken as intended - purely for information's sake. I hope also it will shed light on what has in the amateur metal boat building world become at times a heated debate.

You can review our approach to the various metal boat building methods in our article on Aluminum for Boats where they are discussed in detail, including the various pros and cons of each approach toward fabricating. In that article and among the links provided there you'll also find our thoughts on boat shape itself: single vs. multi-chine vs. radius chine vs. rounded hull, etc.

Rather than to discuss the merits of different hull shapes here, we will limit ourselves to the question of Build Methods.

In this article I will divide the various metal boat building approaches into two broad categories of BUILD METHODS, and then I will address a few basic notions regarding STRUCTURE.

The main thing I hope to accomplish here is to attempt to define and to therefore help the reader to understand the various different terms being used when discussing different boatbuilding methods.

Here are several thoughts that I hope may help clarify the picture...


First, in terms of STRUCTURE, we have in recent years encountered several proponents of so-called "frameless" construction. Unfortunately, when the word "frameless" is used it is commonly mis-construed to imply that a vessel can do entirely without internal structure.

Regardless of the various approaches taken toward fabricating and plate development for boats, there will always be a requirement for hull plate reinforcements, whether they occur in the form of floors, longitudinal stringers, bulkheads, web frames, mast steps, engine girders, tank faces, deck beams, or whatever. These are all various types of internal reinforcements, any of which will legitimately qualify for the term "framing."

The very notion of "frameless" metal boat construction is by definition therefore a flawed and incorrect concept insofar as it refers to the possibility of a completed vessel being able to exist entirely without frames or other internal metal structure.

While it is certainly possible to increase plate thickness in order to increase frame spacing and to therefore reduce the number of frames, it is not at all desirable to eliminate frames altogether. Even with aluminum, the thickness of plate required to completely eliminate frames would end up weighing far too much (and would cost too much) to be practical.

Our approach to the so-called "frameless" metal boat construction methods are addressed thoroughly within our article on Aluminum for Boats, in our article on Metal Boats for Blue Water and below under the heading, The Question of Pedigree...

Build Methods

In terms of BUILD METHODS, we observe the following broad categories:

  1. "Framing First" with the plating being patterned and applied to the already erected frame, and
  2. "Plating First" with the frames patterned to the already erected and welded plating - often making use of temporary moulds to help guide the plate and maintain the intended hull shape.

This latter method is often incorrectly referred to as being "frameless" because the plating occurs prior to the frames being introduced. This is most unfortunate terminology, is inaccurate and therefore misleading, and is all too often cause for misunderstanding and unnecessary argument.

The "Frame First" Method

With the Frame-First method, the hull shape is controlled by first having a rigid "armature" over which the plating is applied, in other words: the frames. The primary advantage of this method is that it allows exact control over the shape. As a result it is by far the most common approach to metal boat building, whether being used for ships (nearly 100% used) or for yachts (possibly 90% used if Europe is included).

We'll limit our discussion to small yachts here (boats under around 60 feet), so we need not involve methods used for larger vessels such as modular construction.

In order to achieve a fair exterior hull surface, the "frame-first" method requires that the builder be skilled in the lofting, the set-up of the frames, the patterning of the plate, and the final weld-up of the hull. It is of course exacting work to achieve this level of precision and fairness in the completed hull, but the attention to detail is well worth it in the end..

The question then becomes, "How shall we save the builder time...?"

With the "frame-first" method, quite a lot of the builder's fabricating time can be saved by having all the frames and plating pre-planned by CAD and pre-cut by NC Cutting via plasma or water jet. What is NC...? It literally means "Numerically Controlled."

The high level of precision offered by NC Cutting takes the traditional "frame-first" method to the next level... When using pre-cut metal parts, there is no lofting needed, and there is no cutting required for the frames or plates or other key structures. NC Cutting therefore provides substantial efficiencies to the builder, thus considerable time savings, in addition to offering a degree of accuracy that is simply unachievable by manual lofting and cutting.

While NC Cutting can effect substantial labor savings in the hands of a professional builder, it has the potential to save even more time in the hands of an amateur builder. Why? Mainly this is due to the elimination of quite number of "What do I do now?" questions, and the relatively huge amount of time expended on them - inevitable for a first time builder when having to loft, plan, spile, and cut every part of the structure.

Needless to say, the NC approach requires a high degree of skill and actual building experience on the part of the vessel's designer in order to be able to pattern all the frames, plates, and other parts correctly. It also requires a high degree of accuracy on the part of the builder who must then place everything as intended, i.e. exactly where it belongs.

Ordinarily when using NC Cutting, aside from just the frame shapes being pre-defined, the frames will also have mouse holes for weld-throughs, and will have notches pre-cut to receive the longitudinal stringers. Additionally, there will be all the other parts such as the engine girders, tank faces and lids, stem and horn piece shapes, mast steps, deck beams, bulkheads, and of course all the exterior plating - all of it pre-cut to an accurate fit-up.

The result of these efforts is that since all parts are machine-cut to an exacting shape, and the hull can be erected with precision, the builder - amateur or otherwise - can avoid the distortion problems associated with poor fit-up of plates.

For further information about the NC Cutting process and how NC Cut Files are developed, we have posted a number of NC cutting articles online.

The "Plate First" Method

With the "Plating-First" method, the plate outlines are precisely defined. They are either developed manually, or from a 3D physical scale model, or they are defined using a 3D CAD model. Then the plates are cut to their perimeter outline, and arranged so they can be pulled into place one by one and tacked together, and finally the plates are welded up. This much can sometimes be done without using internal or external moulds as a guide, but more commonly moulds of some sort will be used. The moulds can be frame segments or they can be other types of temporary guides to the shape.

After the hull plate seams are welded up, the frames are patterned to the interior of the hull plating, then the frames are installed and welded in place.

The primary advantage of placing the frames afterward is to allow the plating to be welded up first without there being any potential distortion introduced by the presence of a relatively un-yielding frame inside. This can produce an extremely fair hull, and can do so even without there being much skill involved on the part of the builder, thus although it has enormous appeal to amateur or back-yard builders, it also has substantial appeal to many professional boat builders. It is for example the most common method in use throughout the Netherlands, where there is a very highly developed metal boat building industry.

The main disadvantages of the "plate first" approach are:

This "hull shape" restriction is the only significant drawback to the "plate-first" method. It just means that the designer must use fully "developable" hull forms. Though this limits what is possible aesthetically and in terms of being able to optimize the underwater hull form, it is fortunately not a crippling limitation...!

When we are discussing any of the various "plate-first" methods, it should be recognized that this approach is really only applicable to the hull bottom and side plating, possibly including the transom. This method however is generally not applicable to appendages such as the keel or rudder, nor ordinarily to the deck and house structures. Therefore, really only about 35% to 50% of the vessel's total plating surface is even under consideration when referring to any of the plate first methods.

Just as with the "frame first" method, in order to address the issue of accuracy, the "plating first" method can take excellent advantage of CAD for patterning and NC cutting. This approach will yield an extremely precise plate definition and consequent cutout, and therefore will provide a much more accurate as-welded hull shape.

By using NC cutting, the frames too can be pre-planned and pre-cut, making allowances at their joins for the inevitable small variations introduced by the weld shrinkage during the weld-up of the plating prior to the frames being put in place.

It should be mentioned that even when using the "plate-first" approach, it may be advantageous to attach a number of internal frame members to the plating prior to it being offered up to the boat, in particular this will often apply to the longitudinal stringers.

This kind of "plate-first" approach is rather common among professional builders in the Netherlands. Often, frames are placed as there are opportunities to do so in order to retain the overall shape. For example, once the bottom plating and longitudinal stringers are in place and welded to the keel sides, internal bottom frames can be introduced while the structure is easily accessed, then the topside plating attached and welded up prior to introduction of the side frames. This results in an extremely fair hull, as well as a highly accurate shape.

In my view, this hybrid strategy has the most to offer, especially when used with NC cutting. In order for the designer to plan the shape and the NC cutting so that construction can proceed smoothly, it must be determined in advance just what sequence the builder will use to assemble the plates and frames.

Variations on a Theme...

Within the "plate-first" approach, there are two main divisions:

  1. The "Pre-Cut-Plate" method as described above, and
  2. The "Folded-Plate" or so-called Origami method.

With the "Pre-Cut-Plate" approach, the plating is all planned for developability (curvature in one direction only, i.e. not saddle shaped or dome shaped). Here, the plating is all pre-cut, pulled into place - ordinarily over a mould or temporary supports - then stitched together along the seams. This is essentially the "plate-first" method described above.

Taking this pre-cut-plate approach one step farther, we have the "Folded Plate" or Origami method, whereby as many of the hull plate weld seams as possible are eliminated via an ingenious layout of the seams and a shape that allows there to be a number of "pre-joined" areas.

The advantage of the "Folded Plate" method is that with an accurately pre-planned outline that's cut out of plate, the entire hull plating can first be laid out flat - port and starboard - welded where necessary to create the sizes and shapes required, then it's all pulled together and stitched into place. Using this method, once the plate shapes have been determined, the hull plating can be erected in a very short time - often in a matter of days.

Of course this looks impressive...! It actually is impressive! Naturally this concept has captured the imagination of the amateur metal boat building community, thus a possibly significant contingent among potential owner-builders.

With the Folded Plate / Origami method however, one must realize that the designer is unfortunately extremely limited in terms of the possible hull shapes that will actually do this trick. Try it with paper cutouts and you will be immediately convinced. You can achieve a few minor variations and still get shapes that will fold together, but regional subtleties of hull form are just not possible. If a different type of hull form is desired, then quite a lot of trial and error time must be spent - usually by making actual trial cutouts and seeing if they will fit together in an attempt to discover a totally flat plate layout that will provide the intended shape when folded together.

This is not only a severe limitation on the designer - it also restricts the builder who may as a result have only one basic model to offer. In other words, variations to the hull shape are difficult and time consuming to create, so the vessels are limited to being either larger or smaller, fatter or more slender, taller or shorter, having more or less sheer, yet essentially the same in their general shape and appearance.

Further, it must be kept in mind that just as with the "pre-cut-plate" method, the "Folded-Plate" or Origami method is generally only applicable to the hull plating itself, and not to the keel, rudder, deck, superstructure, nor to the equipment, rig, joinery, systems, etc. In other words, though it should be accomplished as efficiently as possible, erecting the plating is only a small part of building the hull, and a very small part of the whole picture.

We therefore observe the following disadvantages of the "Origami" method:

  1. Only a limited portion of the total plate surface will be addressed by the Origami method;
  2. The variety of hull shapes that are possible both aesthetically and functionally are quite limited;
  3. There will be quite a lot of fussing around with trial shapes prior to achieving the desired result;
  4. There will still be internal framing... actually quite a lot of it in the form of girders, tank faces and tops, bulkheads, sole flats, deck beams, etc.

As a result of these factors, I have not been tempted to pursue the Origami approach in my design work.

Except for the initial "wow" factor, which holds a certain well deserved appeal among amateur boat builders, I don't see much advantage to it, especially in a professional boat building context. In particular, this is so due to the extreme restriction on the variety of possible hull shapes that can be offered. The result is that the hull shapes become extremely alike, therefore ordinary and uninteresting.

Ask any of the proponents of the Origami method how many truly "different" hull shapes they have been able to design or build using that approach (hulls which are not simply stretched or squished versions of the same thing), and I believe you'll immediately see what I mean.

As an extremely viable alternative, one can just as easily make use of the "pre-cut-plate" approach and have considerably more freedom with subtleties of hull form.


If one is able to begin with a blank sheet, in other words if one is able to create a new custom yacht design, it becomes possible to choose between a frame-first vs. a plate-first building method. In this case, the first task in the design cycle belongs to the owner, and that is to find a designer who can bring about the owner's vision and purpose for the vessel they have in mind.

The designer's role is to act as the owner's advocate throughout the whole process, attempting to meld their requirements / requests with what is practical / achievable / safe / etc., at the same time as attempting to achieve the aesthetic, the layout, and the performance being sought. Then once the vessel has been designed, to follow through during the construction of the vessel, first to connect the owner with a builder who is suited to the task, and then to follow through during construction to assure that what has been designed gets built as planned.

Occasionally this order of events gets turned around, and the owner first finds a suitable builder, then together they forage for a design that the owner likes and that the builder wants to build. While this can often result in great success, it can also result in great compromise. However if the compromise is not too great, and the cost is attractive, then a deal may be struck that is satisfying to all involved.

More often than not though, the builder or the owner will want to introduce changes to the design. Subtle variations to the interior, usually introduced by the owner, are to be expected and are usually not of any consequence.

Major changes to the layout that involve changes to the structure, or that involve relocating tanks, bulkheads, engines, major machinery, masts, etc. are very often sought. However, any of these kinds of changes must necessarily involve the designer. Or if a stock design is entirely suitable as is, or if minor changes are all that's needed, then certainly the designer will be able to "customize" that design to suit - it is all part of a designer's usual routine.

What is not often realized is that collectively, these changes can quickly eliminate any possible advantage to having selected a stock design. At this point, it can become advantageous to begin from a blank sheet - even if it is largely based on a prior design. It is for this reason that I have been nearly 100% focused on new custom yacht design.

The Question of Pedigree...

All the building methods mentioned above can be made to satisfy the structural requirements of the ABS or other rules - with the exception of the so-called "frameless" building method, which cannot. In considering any existing design, one should inquire as to whether the structure has been designed according to the standards of one or more of the yacht classification societies.

For motor vessels, I calculate structure per the requirements of the ABS Rule for Motor Pleasure Yachts, or for sail boats, the ABS Rule for Ocean Racing Yachts, or both, taking the most conservative result from each, then adding our own factor of safety. I also consult applicable portions of the German Lloyd's rule and other classification society rules such as Lloyd's Register wherever they may be appropriate, such as for spars and rigging or for wooden structures, etc.

Aside from the structure, when inquiring about any stock design or new custom design it will be prudent to inquire about the stability compliance of the vessel being considered. For example, I impose the EU Recreational Craft Directive stability requirements on my designs - both sail and power. Even though there are no legally imposed stability standards for pleasure craft built and registered in the US, I feel this is quite important, therefore the EU-RCD is the base-line standard that I use.

These standards are outlined within ISO-12217-1 for motor yachts and in ISO-12217-2 for sailing yachts, and are applicable to recreational craft (private yachts) up to 25 meters (79 feet) overall length. For larger yachts, both power and sail, the British MCA Large Yacht code is the best guide.

You can read about my rationale for use of these standards, what they mean, and how they apply by reviewing our various articles related to "Boat Design" on the Articles web page.

In terms of safety equipment, while I very much advocate the use of the ABYC guidelines for safety and systems, there are a few specific areas where I disagree with the ABYC recommendations. Mainly this is limited to those chapters where the ABYC guidelines are at present inadequately developed and are rapidly changing - in particular with regard to bonding and electrical isolation on metal vessels. These matters are thoroughly addressed in the Vessel Specification which accompanies each of my designs, often amounting to some 50 to 80 pages..!


A number of my designs are fully developable, and are thus directly adaptable to the "pre-cut plate-first" approach, in particular if NC cutting were to be employed. Examples that come to mind are the 36' ketch Grace and her larger sisters: the 42' schooner Highland Lass and the 42' ketch Zephyr, which have fully developable hulls.

Many of my other designs, while largely developable, have intentionally violated developability locally in order to achieve the right aesthetic shape or the right distribution of displacement or the right waterlines or buttock lines, etc. Examples include the 44' schooner Redpath or a few of my other designs for which a rounded hull form was preferred such as the 36' cutter Fantom, the 40' schooner Benrogin, the 48' junk rigged schooner Jasmine, or the 50' schooner Lucille.

Although I have no particular interest in pursuing the "origami" approach as such, I do believe there to be considerable merit to the "pre-cut-plate-first" approach. More particularly I have observed big advantages to the hybrid "plates-first-then-frame-as-you-go" approach mentioned above. Even though hull shapes are thus limited to only what is developable, there are innumerable good shapes that one can achieve which are aesthetically pleasing and that have a water-friendly shape.

To be a success, the pre-cut plate shapes must be precisely planned and cut, but this is not at all difficult when combined with CAD driven NC cutting. I have developed NC cutting files for a number of our designs to streamline their construction.

Further Considerations...?

I believe that metal is the ultimate boat structure. To review the metal boat designs that I've created, please see the Sail Boats Gallery and the Power Boats Gallery. I have also created quite a number of Prototype Designs, most of which are also intended for metal structure. Sail or power - mono or multi-hull - if the structure is well-designed and well built, the resulting boat will be excellent.

I am often asked about one metal vs. another - most commonly steel vs. aluminum. I do not have a distinct preference. There are so many varying factors that will contribute to making that decision for each boat, and for each owner. Some boats are designed for one material only, other boats can make use of either.

In general, any of my designs that have been developed for steel can easily be re-specified for construction in aluminum. The design conversion from steel to aluminum is done at no extra cost, unless of course there are other changes to be made to the design. Where NC cutting files exist for a steel boat, they will need to be re-done in order to work for aluminum structure, and there will be a cost incurred for that conversion.

Designs that were originally developed for aluminum structure are not usually readily converted to steel, since they will ordinarily have been designed specifically to save weight. Therefore, to convert an aluminum design to steel structure will ordinarily require a re-work of the hull shape in order to support the extra weight of steel.

For more information about any of the above options please feel free to contact me.

Other Articles on Boat Structure

Metal Boats for Blue Water | Aluminum vs Steel | Steel Boats | Aluminum for Boats
Metal Boat Framing | Metal Boat Building Methods | Metal Boat Welding Sequence | Designing Metal Boat Structure
Composites for Boats | The Evolution of a Wooden Sailing Type