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3.1 The Datum Waterline - The vertical reference point for everything to come

What's the first thing the designer does when they start a new design? They draw straight line on the profile drawing if its a 2D drawing, or make a flat plate if its a model.  It's the foundation stone for everything that gets incorporated in the plans and the model.

Its the reference point for the strongback and mould frame setup, for everything that gets installed in the boat. Its the reference point you use to establish where the boot strip goes and where the antifouling comes up to on the hull when you're ready to launch.


Never lose track of your datum line. Mark it clearly on the mould frames, transfer those marks to the hull, inside and out. Establish you datum height on a fixed object such as a support column or on the workshop wall.  If something doesn't fit or doesn't look right you have a reference point on your plans, your patterns and the designer can reference your problem to the model if need be.

Laser levels are inexpensive, easy to use and a great aid to accuracy in the build, a highly recommended tool to add to your kit.

3.2 The Strong-back


The strongback provides the base to attach and align the mould frames.

 Create a solid foundation for the project. Make it strong and anchor it well in the work space. The strongback can be made from metal or wood and it can be ordered and supplied as a component of the CNC kit for the temporary mould frames.



Leveling the strongback timbers. A level strongback assures a minimum of work adjusting the height of the mould frames when standing them on the strongback.
Leveling the strongback timbers. A level strongback assures a minimum of work adjusting the height of the mould frames when standing them on the strongback.
Image of catamaran Strong-back
Strong-back for a catamaran well secured and braced

3.3 Mould Frames

The moulds are built by setting up a strong back and then standing the mould frames on the strong back at the required spacings as described in the plans.

The mould frames can be all temporary (usually MDF or plywood), they can be made up of permanent BH’s that will stay in the boat (as is often the case with a Duflex Kit), or they can be a mixture of permanent and temporary frames.

How to make the mould frames? In days long past we used a table of offsets to mark out the hull lines and frame definitions on the shop floor, a very time consuming process called lofting that required a great deal of patience and skill. The advent of computer drafting, large scale plan printers and CNC has made that process redundant. 


Rather than a table of offsets your designer will provide you with an electronic cutting file that can be read by a plan printer or a CNC machine. If you print the lines to paper or vinyl you then have to transfer them to your frame material which is usually MDF or plywood. On the other hand if you have your frames and bulkheads CNC'd then the part gets delivered cut to shape with no transferring of lines required.


The zero point for the fore and aft location of the mould frames is usually the transom. The reason for this is that it's much easier to catch the tape measure on to the transom edge rather than measure from the bow. Each BH or mould frame will be numbered according to its distance from the zero point and the dimensioning is usually in mm. So BH 4650 is positioned 4650mm from the zero point.


Be sure that your patterns or cutting files have the correct offset for your build method. If you are making frames for a hull to be strip planked directly over the frames then the lines for the bulkheads and temporary mould frames need to be offset to reduce the lines for the planking thickness. 


 If the mould frames are to have stringers over them as might be required for sheet foam construction, then the patterns need to be reduced for the thickness of the stingers as well as the foam core. If you are building in a female mould and and the planking material is going directly against the mould then no offset is required at all.


It is conventional practice to set up the mould frames for the forward hull sections on the aft side of the setup line, and the aft frames on the forward side. By following this procedure the contact edge of the frame will not be intruding on the planking line and forcing an unfairness in the hull shape. The drawings should indicate if there is any exception to this practice.


If you are building from a kit it is important that you position BH's and floor panels in accord with the designer's drawings. otherwise you might find panels dont fit exactly.


The process is the same for permanent and temporary frames. Two optional methods are described below.


1. Print out the shapes onto paper or vinyl using a plan printer. Most small to medium size towns have a plan printing service where you can have large size prints made. The designer will supply you with a pdf drawing at 1:1 scale for this purpose. For a small boat the patterns are usually clustered onto just one large drawing, but if there are a lot of shapes they are sometimes broken up onto separate sheets for clarity. You can put each pattern on a separate sheet but this requires a lot of paper, so it is common to combine a number of patterns onto just one sheet. The patterns are then transferred from the paper or vinyl  pattern to the mould frame or the bulkhead. 


2. CNC machines have now become standard equipment in many boat yards and the mould frames or composite components can be cut directly from the electronic file  provided by the designer. This will usually be a dxf file which can me read by the CNC. It can also be imported into drafting software to be viewed, manipulated and modified as required. The designer will normally provide a print of the file in pdf format so you can visualise the content of the dxf without the need for drafting software.

If you don't have a CNC there are various companies that will do CNC machining for you and ship your frames and other composite components to order.


Which method is best? Method one is more time consuming but saves the cost of CNC cutting. Method two is increasingly more common, and convenient. It’s very accurate and saves a lot of labour hours.

Note that if you are building from a Duflex kit then it is likely that all of the mould frames and bulkheads are included in the kit and you can start to assemble your building moulds from the moment the kit is delivered.

Corresponding slots in the strong-backs and mould frames allow for accurate alignment of the building mould with a minimum of adjustments.

Catamaran cabin top on strongback
Catamaran cabin top and decks on a steel strong back mounted on wheels so it can be moved to different parts of the workshop depending on space requirements.

3.4 Mould Types

This is a stringer- frame mould as typically used for foam sheets or strips which can be applied in much wider strips than conventional strip planking because the strips are wider and they have more support from the longitudinal battens. The foam strips are sometimes laid vertically but more often at an angle to minimise the plating effect you can get from the vertical strips.

Note that for this construction method the mould frames have to be reduced for the thickness of the stringers and the planking material.



This is the frame mould for the R42 trimaran supplied by ATL Composites. All of the frames and the strong-backs have been CNC cut. The mould frames and the strong-backs have matching slots to allow for fast and accurate assembly.


The material thickness needs to be verified before the cutting files are prepared.

3.5 Planking with Strips

One thing that is common on all strip planked hulls and decks is that a planking transition line is established before the planking begins. 

On this hull the lower planks (in the photo) have been trimmed to a line more parallel to the keel line, a new set of narrower planking is started and then trimmed again on the keel.

Some builders will determine where the trim line is and set a longitudinal plank into the mould frames to provide a solid support for the trimming operation.


Roughly one third of the planking material needs to be cut to a narrower width for the bilge curve. About 25mm to 35mm wide is typical for the bilge curve and about 50 to 70mm wide for the flatter sides.


Don't be tempted to use wide planks. It creates less join lines but more filler and more sanding to get a fair surface.

3.7.(i) Rudders and Steering Systems from Jefa

3.7.(ii) Hydraulic Systems from Hydrive




Floats and decks for the R42 trimaran being strip planked with a PVC foam core and a light layer of uni carbon on both faces. A second layer of carbon will be applied inside and out once the planking is finished the hull faired.

3.6 Planking with Foam Sheets

Hull Bottom for a Barefoot 40 being planked in transverse PVC foam strips.

Thanks to Julian Griffiths for the Photos.

3.7 Rudders and Steering Systems

3.7.(i) Rudder Systems from Jefa



Jefa manufactures and sells a broad range of rudders and steering systems including systems designed specifically for catamarans with the option of wire steering, push-pull rod and transmission steering. 

They provide very comprehensive specifications and pricing on their two web sites:


Jefa Steering Systems: www.jefa.com/steering/steering.htm


Jefa Rudders www.jefa.com/rudder

3.7.(ii) Hydraulic Systems from Hydrive

Many of our boats (probably hundreds) have been fitted with hydraulic steering systems from HyDrive based in South Australia and with a world wide dealer network.

Check the HyDrive web site for details. hydrive.com.au

3.7.(iii) Build you own Rudders

This is the rudder blade and stock for a Barefoot 40 catamaran built by Arnie Duckworth. The fibreglass plate on the blade CL (rudder planform) and the profile shapes that define the foil were provide with the plans by Grainger Designs. 

The rectangular stock was made in carbon fibre and then a female socket was moulded around the stock so the rudder blade could be fitted to the stock without having to lift the boat or digging a hole in the factory floor as is often done to allow the rudder to be fitted inside the factory.


The spaces between the profiles are filled with PVC foam and then carved to shape and given an external glass laminate.