You can get your boatbuilding project off to a great start by using a pre-cut steel or aluminum kit for any metal sailboat or powerboat. Modern kits contain accurate pre-cut parts that you can easily assemble into a complete hull, deck, and superstructure. The latest computer software allows the designer to model the boat so that extremely accurate computerized files can be prepared to direct the cutting machines. These files contain all the information to facilitate computer-controlled cutting of all the metal parts for your boat. It may not interest the average builder, but a huge amount of work is required to turn a boat plan into a cut-to-size boat kit. Every part has to match that of its neighbour exactly, the slots need to be in the correct locations, and everything must fit together perfectly. All this is necessary to enable you to complete the assembly of the hull, deck, and superstructure with the minimum of problems. We’re always amused when we receive a request from an uninformed customer that goes something like this: “By the way, now that I have the plans for your design, just send me the cutting files.”
DESIGNING AND CUTTING PRECUT METAL BOAT KITS
Many of you may be surprised that it’s not possible to take a regular boat plan—even one that is already prepared using the latest computer-aided design techniques—and use it for automatic computer-controlled cutting. There are many steps between creating the original design and having the boat cut out on a computerized plasma-oxygen cutter. If a particular design is to be sold as a pre-cut steel or aluminum hull, deck, and superstructure package, then this should be decided at an early design stage. Some designs can be converted, but it is preferable to start with automatic cutting in mind.
The main steps in preparing a new design for a boat that is destined to be cut out by a computerized plasma-oxygen cutter is as follows. It is usually the customer who gets the process started by contacting the designer with a brief outline of what they have in mind. Further correspondence quickly establishes the client’s wish list, which usually includes things such as type and style of boat, intended usage, and overall length and beam. Draft limitations should be specified at this stage.
Accommodation requirements, including the number of regular crew versus occasional guests, should be defined. Speed requirements are important, as are the client’s attitude to fuel costs. This list may need some refining since some elements may conflict with one another. The communication ensures the client ends up with a boat that meets most if not all his or her desires and overall requirements. So far the process is very similar to what would be followed no matter which material or building method was used to construct the vessel.
The client and designer then enter into what can be a simple agreement where the designer agrees to prepare preliminary plans for the proposed vessel for a reasonable (a relative term!) fee. In our office the preliminary plan includes lines plan, general arrangement drawings (consisting of exterior profile, deck plan, accommodation profile, and plan views), plus sufficient calculations to ensure that the final design can meet the client’s requirements.
Before a preliminary plan is produced, the designer produces a 3-D computer-generated model of at least the hull of the vessel. Once the preliminary plans are completed and both the designer and the client are satisfied with the overall concept and layout of the vessel, complete plans for the vessel are prepared.
Next, a comprehensive 3-D computer model is completed that includes all parts of the hull (including transom, keel, and rudder), all decks, cockpits, a complete superstructure, main interior bulkheads, and any other features such as a flybridge, radar arch, and exhaust stack. Special items such as transom steps and other similar features are included in this model. Depending on the complexity of the design, this process can take between 80 and 200 hours.
From this model, all of the salient hydrostatics—such as detailed weight calculations to enable material requirements and final displacement—are calculated. Stability calculations are also made at this time. During this process, fine-tuning of the model can be undertaken to make sure that the finished vessel will meet all the design requirements.
When the comprehensive 3-D model is completed and checked, copies are provided to a team of specialized designers who prepare the final model, which includes all the scantlings (such as transverse and longditunal framing, sole bearers, deck beams, and engine beds). This team separates out all the parts for the frames, stringers, engine beds, bulkheads, hull, deck and superstructure plating, etc., and add notches to the frames and bulkheads before nesting the parts on plates.
The design team numbers each item and draws reference lines on each part to represent frame locations, etc. (the numbers help builders identify each part, and the lines are used during the assembly process to locate frames and other structural members). The designer then works out a path for the computerized plasma-oxygen cutting machine. The path is the point at which the cutter enters the plate and starts cutting the parts. It must make sure the parts are cut in the correct order. For instance, if a window has to be cut from a cabin side, then the window aperture must be cut before the larger cabin-side part is cut; otherwise any movement in the cabin side after cutting could cause the window to be cut in an incorrect location.
Several sheets of assembly drawings are now prepared. For instance, each frame is shown separately with all parts clearly numbered, and measurements are given to assist in welding up the frames. Other drawings show how to set up the building jig supplied with the kit. The location of every part that forms the completed hull, deck, and superstructure is shown in the various assembly drawings supplied with the kit.
Finally, all the parts are listed in a spreadsheet program and checked against the drawings and cutting files. Another designer is simultaneously working on the engineering drawings for the engine room layout. Battery placement, drive train and bearing location and sizes, exhaust system, fuel tank sizes and placement are shown in these drawings.
Of course, all of the above steps have to be carefully checked and the whole design package coordinated before the cutting files are released to the client (to have the kit cut locally) or sent to the cutting shop that produces our kits. In terms of investment we figure that each set of cutting files and associated plans for a boat of between 36 and 65 feet costs the originating design office between $35,000 and $50,000. Because this figure is too large for an individual customer, we try to group orders for similar kits as well as treat a large part of the cost as investment against future kit orders.
The size of readily available plate varies from country to country, so sometimes it is necessary to renest the cutting files so they fit the available plate stock. Re-nesting may be also required if the size of locally available cutting tables is less than that of the equipment used to cut the first kit. Fortunately it only costs a fraction of the original expenditure to renest the plates to any convenient size. As you can see from the above, the amount of careful and intense work required to turn a existing or new plan into a set of cutting files far exceeds the expense in creating the original design. It’s only possible to justify these costs if a firm can expect to market several kits of similar design. Often, cutting files for a particular design can be made in such a way as to give several customers the custom items they desire. Some custom items are relatively easy to incorporate in the cutting files, while other more-complex changes require redesigning the basic boat and remaking all the cutting files.
The metal-cutting shop uses the Numerical code cutting files to produce your kit. The kits are cut from pre-shot-blasted and primed steel (or aluminum) and are delivered ready for easy assembly by any competent welder. The primer used on the steel kits is especially formulated so that it doesn’t give off harmful fumes as you weld the kit together. This primer doesn’t burn off on the reverse side of the metal in welded areas. It’s truly a remarkable coating used to protect the steel until additional paint is applied.
The parts are all nested, including all of the hull, deck, and cabin plating. You can easily assemble the hull, deck, and superstructure. All you have to do is to match each part to the special assembly plans you receive with the kit. Whether you decide to purchase cutting files and have the kit cut locally will depend on your location. For instance, due to the availability of excellent cutting facilities in the Netherlands, most customers in Europe opt to order a pre-cut kit as opposed to cutting files. Conversely in countries with high steel import duties, such as Brazil, then cutting files and plans can be purchased on a CD. The kit is then cut locally.
In our own case we have exported complete cut kits to the United States, Canada, Philippines, Russia, and many other countries, including almost all of Europe.
The tack-and-weld method described below is in many ways similar to the stitch-and-glue procedure used with plywood. It’s a practical and economical way to get your boatbuilding project off to a great start. You can achieve a professional result, especially if you already have some welding experience. If you lack welding experience, then any local person with suitable welding knowledge can help you assemble your kit. Of course, many thousands of boats have been built from a set of plans and frame patterns, so if there isn’t a kit that meets your requirements, building from plans is the way to go. Nonetheless, if you can afford a kit, you’ll have a hull in the least time and this alone may justify the modest additional expense. The resale value of your boat will be enhanced if you can show that the hull was built from pre-shotblasted, primed, and computer-controlled pre-cut metal parts.
STEEL KITS
In high-quality kits, all steel plates are shot-blasted and primed with a zinc-rich primer before cutting. Cutting of plates is carried out with computer-aided lofted surfaces on an NC-driven plasma-oxygen cutting machine with a maximum plate size of 82 by 10 feet (14 by 3 m). The best material is Lloyd’s-approved, “A” grade, “shipbuilding quality,” or the equivalent. The kit includes a setting-up jig as well as detailed assembly plans. All required steel profiles are also shot-blasted and primed with a zinc-rich primer. Kits are constructed from the steel product specifications mentioned above.
Normally, all the plate material is supplied as a flat pack with marking lines engraved in the plate surface (a zinc line) and part numbers painted on the surface. The maximum size is usually 19 feet 6 inches, by 6 feet 6 inches (6 by 2 m). All steel profiles are supplied in sufficient length to ensure the minimum number of joints in the plating. All parts that require forming or bending are supplied already formed to the correct shape.
Those who prefer aluminum as their basic building material will be pleased to learn that kits are available pre-cut from marine-grade materials.
