JEC COMPOSITES MAGAZINE - Issue #55 - February/March 2010 - 54

KNOW-HOW Design & Manufacturing g…/… Advanced composite racing yacht construction size of the tables depends greatly on the flat stock that needs to be built. Because the tables need to be heated for curing, a variety of solutions have been employed. From electrically-heated tables with an aluminium or composite surface to tables built inside ovens, there is no ideal answer to their construction. One thing is sure, such large flat surfaces do not exist off-the-shelf anywhere, so they have to be made. Processing elements Vacuum vs. autoclave Autoclave curing is now common for all the highly-loaded components such as spars (masts, booms, poles) and appendages (rudder stocks, dagger boards, foils) as it yields significantly higher mechanical properties especially in solid non-cored composites. Unfortunately for hull and deck shells, we are limited to vacuum pressure not only because of cost and lack of equipment availability, but also because it is specifically forbidden by the Rules in many racing classes. Time constraints Most one-off racing boat projects have a build window of 5 to 9 months depending on the size and complexity of the project, as well as the type of boat. High-performance sailing cruisers of 2025 m LOA usually take around 18 months to complete, but more than half of that time is consumed by installations and interior fit-out and finish. There is a universe of difference between this state of affairs and the approach taken in production boat building, where the actual build time and total hours are proportionally much shorter. When transferring methods and technology from advanced composites to production, it is essential to reengineer the processes so they fit the economics of series production. This is relevant even though there is a reduction in time of up to 40% just by repeating a one-off process. Debulking practices Trimarine’s standard practice is to debulk prepregs one layer at a time, especially when laminating unidirectional fibres. When using carbon UD of 200 g/m2 and above, the permeability of the prepreg film is very poor if not completely airtight. This makes any trapped air difficult to escape when debulking more than one layer. Warm debulks do improve the compaction and air elimination but it is a blind process, defeating visual inspection and practically impossible to check before the component is fully cured. It can also affect the flow characteristics of the resin when curing certain low-temperature systems with shorter working times. Perforating pin rollers can be used to puncture the film and release the trapped air but Trimarine considers this practice somewhat traumatic to the fibres and prefers to avoid it. Time/cost constraints In one-off projects, using advanced materials and techniques is always a race against the clock. The first thing fixed in the contract is the delivery date, and although this may be true of any contract, the race start date does not usually get postponed because a participant is late out of the shed. If you are building specifically for one race, you better be on time. Shorter build times mean higher costs, and with this maxim come a few other disadvantages: - Last-minute planning equals higher costs - Last minute ordering equals higher costs -Last minute design equals wasted time, reprogramming of the schedule and inevitably more hours, which translate into higher costs The ingenuity of builders working against the clock does produce some technical shortcuts of merit every now and then. However, more often than not, shortcutting a process already at its limits of execution results in failures that can put at risk or completely forfeit the whole campaign. Such issues should not transfer into production boat building where process specifications should leave little to the imagination of the operator and building schedules have to be adhered to very strictly for economic and competitive reasons. Prepreg debulking – 1x layer at a time vs. multiple layers As mentioned above, it is advisable that all collimated-fibre unidirectional reinforcements be debulked one layer at a time. Most other weave styles and multiaxial materials have sufficient porosity to allow a good air bleed even when debulking multiple layers. This is one of the fundamental issues of prepreg processing as it weighs heavily on the labour cost but is absolutely critical to ensure a top-quality product, especially if the process is limited to vacuum pressure. There are a number of prepreg type materials on the market, layering dry fabrics with epoxy resin films, specifically designed to overcome these air entrapment problems that allow for the complete elimination of the debulking process. Being easier to use and much cheaper to process, they are ideal for high-end series production. Scaffolding solutions – Access to work surfaces Access to the work surface is critical for operators to perform quickly and efficiently. Many solutions can be used for scaffolding but Trimarine recommends a system that can be rapidly adjusted and removed so that time wasted on such adjustments is minimised. jec composites magazine / No55 February - March 2010

JEC COMPOSITES MAGAZINE - Issue #55 - February/March 2010

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #55 - February/March 2010