2.1 One off construction over male moulds and the evolution of kit building.

Strip planked construction rapidly gained popularity in the 1970's and 1980's as builders realised there was a big saving to be made in build time by eliminating the timber stringers and frames that typically formed the permanent structure over which the hull shells and decks were planked for stringer frame construction. 


By using temporary mould frames for the form the strip material, usually western red cedar at the time, could be laid up over the male mould frames, glassed externally, removed from the building mould and glassed internal, leaving a clutter free interior to be fitted with bulkheads and floors as called for by the design. This effectively created a sandwich structure of timber core and glass or carbon skins.


As cedar became more expensive, and with a trend towards lighter construction, PVC foam cores began to replace the cedar strips. The foam on its own is too flexible to use for strip planking, even with quite closely spaced mould frames and so it became common to glass the foam sheets both sides with a light unidirectional reinforcement and create foam glass strips by joining the panels and then ripping them into strips. The final integrity of the sandwich laminate was achieved with one or more additional layers of glass inside and out after the planking operation was completed and the hull faired ready for the final laminates.


Over time this system was further refined by incorporating some or all of the permanent bulkheads in the mould framework. The strips became wider in areas on hull sides where there was little or no compound curvature. Designs evolved with larger areas of flat panels, and so the panel stiffness increased as the need for panel support was reduced.


So the need for temporary mould frames was reduced or entirely eliminated, except for rounded hull bottoms which in most cases were, and still are built separately with temporary mould frames to allow for the compounded curves.


Eventually whole hull sides were being laid up in full size panels that incorporated the final skin laminates. These panels did not require additional skin laminates for their integrity and only needed to be tabbed to adjacent components to achieve laminate integrity after the completion of planking.


ATL Composites began laminating custom panels under pressure to achieve high fibre ratios. They also developed a proprietary system for joining CNC panels on site. And so the Duflex kit system came to be.

Catamaran hull being planked with foam glass strips

Planking a catamaran hull with wide strips of pre laminated foam/glass sandwich. With this method an additional layer of glass is applied inside and out to complete the structural laminate.

This is how cats were typically built in the 1990's before the right way up method became popular.


Note that the ingdeck was planked first so it could be used as a working platform to plank the inboard hull side.

2.2 From Upside Down to Right Way Up

I'm not sure who did it first but I'm pretty sure it happened in Australia. Someone figured out it made good sense to avoid the turning over operation and build the boats right way up once the hull bottoms had been been planked, glassed, faired and rolled right way up.

The roll over operation could be risky and typically required a lot of temporary bracing because the boats are floppy and liable to twist before the decks are attached.

The added advantage of building the boats this way is that the outboard hull sides can be left off until the inside of the hull is fitted with engines, tanks, plumbing and other systems.

The right way up construction method is especially suited to the DuFlex kits

The upside down method before the advent of the DuFlex kits. This cat is has all of the permanent bulkheads and temporary frames in place ready to start the planking. The pale green panels are foam glass epoxy panels from ATL Composites. The darker coloured panels are MDF. All of the panels have been CNC cut. Note that with DuFlex panels much less support framing is required and so the temporary mould frames can be eliminated from the hulls.

Turning a cat planked with foam glass strips using two cranes

Cut Loose turn over-1
Cut Loose turn over-2

Cut Loose being turned over end for end using two cranes as was typical for this operation. Turning over sideways could be done with one crane but with no laminate on the inside of the planking it put too much twisting load on the structure and using two cranes was always the safer option. Either way the turning operation was nerve wracking. Cut Loose was planked entirely in cedar strips.

Building the hull bottoms separately and turning them over before building the topsides and decks is much easier than building the hull and wing-decks upside down because it avoids having to turn the combined hulls and wing-deck before there is any interior laminate to hold the structure rigid. This is the way most one off cats are now built.

2.3 DuFlex Kit Construction

The Duflex Kit was developed by ATL Composites in Australia. The kits are sold world wide by ATL Composites and in Europe by VDL Composites. A kit can be configured to any design that your designer is willing to create cutting files for, monohull, catamaran, trimaran. It will usually  incorporate all of the flat panels in the design, material for strip planking if there are compounded surfaces, and even a set mould frames and strong-backs with notches accurately located to stand the mould frames.

The panels are laminated in a press, the shapes are CNC routed to the cutting files provided by the designer and the panels are joined on the build site with ATL's proprietary "Z Join" system.


The standard kits we supply for our designs have glass reinforcements and PVC foam cores. Paper honeycomb cores are sometimes used for internal furniture and other non structural panels.

Duflex Panel Image
Representation of Panels for a DuFlex Kit for a catamaran

2.4 Resin Infusion under Vacuum

In parallel with the developments in kit building in the 1990's and early 2000's, resin infusion was developing as a method for professional yards working with female tooling. The main advantages of resin infusion is that you can achieve a consistent high fibre ratio laminates with minimal chance of air entrapment, thereby achieving consistent high quality laminates with improved strength and stiffness for a given laminate weight. 


Resin infusion also provides a cleaner working environment because multiple layers of dry reinforcement can be stacked dry in the mould. The resin is injected under vacuum pressure and cured in enclosed environment minimising styrene emissions in the curing process.


Apart from production boats resin infusion is also used for one off custom boat construction but because of the cost of building plugs and moulds that will later be destroyed it is not as broadly deployed as kit construction.

Resin infusion is also more technically demanding than wet hand lay laminate or vacuum laminating over a male plug. It requires good planning and worker training.

2.5 Kerfing

Catamaran wingdecks are often built by kerfing flat panels and bending them to the curve required over male formers. 

There are two methods.


1. Glass the outside surface and cut the kerfs in the foam core from the inside face. Fill the kerfs with an epoxy filler and push it into female receiving frames to achieve the shape. Remove the excess resin and then glass the inside surface.


2.Glass the inside surface, cut the kerfs from the outside face and bend the panels over male formers. Fill the kerfs and glass the external face.


The second method is preferred.

2.6 CNC Machining

While Computer Numerical Control machining is now commonplace for cutting laminated panels and mould frames in many small to medium sized boat yards, the larger five axis machines that can cut large 3D forms for hulls and decks are still very expensive to instal and operate.

Their benefit lies in accuracy and labour saving, but the the build still has to build the plugs to create the basic form, or use very large blocks of foam which are expensive and have to be disposed of once the plug and mould work is complete.

Five axis routing is being used more to make plugs for rudders, daggerboards and other complex shapes like trimaran beams but at this time is not broadly employed for hulls and decks.

In this shot a male plug is being strip planked for a production plug for a trimaran. This plug will be glassed, coated in a tooling compound and then CNC routed to create the final surface. The plug will be destroyed after the female mould has been taken from the plug. 

This plug has been surfaced with a tooling compound and the five axis router is making a first pass over the surface. Accuracy of the final surface depends on the speed of the router and how many times it is passed over the surface.

First pass of the router blade over the foam plug for a trimaran float half shell. The float was carved from a solid block of foam but the main hull was built over a strip planked plug as shown in the above photo.