Stratification : The birth of an AMEL 50’s Hull
Table of content:
Preparing infusion
Lay-up
Infusion
It is armed with the theoretical notions mentioned in the articles “Sandwich or monolithic structure ?” and “Stratification: contact or infusion?” I propose you to move on to the describtion of the manufacturing process of an AMEL 50 hull. This case is particularly interesting since we will exploit the infusion, the “contact” stratification, the sandwich and the monolithic construction.
We therefore first start our stratification with an empty hull mold. This mold was designed using 3D drawing tools and then cosntructed by a specialized company. A mold of this size represents a significant investment. The first mission of a laminator is to ensure that its mold remains in good condition over the hulls it is used to build. The mold is first coated with a release wax that will allow the future layers not to adhere to it and make the demolding operation easier.
Then come the first layers of gelcoat, applied with a sprayer. In addition to its decorative function, the gelcoat protects the fiberglass, highly sensitive to UV. On the an AMEL 50 hull, two colors of gelcoat are used: the beige of the floating line, which is applied first (visible on the picture above) and then the white of the hull.
An anti-osmosis protection layer is then applied. This is a resin reinforced with fiberglass flake. A second anti-osmosis protection will be spread on the bottom before the application of the antifouling.
The first layers of fiber glass are then placed manually, according to the so-called “contact” stratification technique. This remaining manual step is necessary to create a protective layer between the gelcoat and the layers that will be plated on the mold during the vacuum setting. Without these first protection layers of fiber glass, the latter would mark the gelcoat that would then not be plain once demolded. The surface is first moistened with sprayed polyester resin, then a layer of fiber glass previously cut to the right size is put in place. The laminator then impregnates the second face of the fiber glass with polyester resin and then manually penetrates it with a roller. Some know-how is necessary because the operation must be carried out quickly. The right amount of resin must be applied otherwise the long layers of fiberglass do not hold in place. The operation is then repeated by overlaying the fiberglass fabric strips about one-third of their width until the entire mold is covered.
Lay-up
The “lay-up” can now begin. This involves putting in place the fiber glass tissue layers as well as the PVC foam panels that will then be infused. The laminators therefore install previously cut pieces of fabric, which they temporarily glue to make sure the whole thing does not move.
Monolitic construction for the bottom
The bottom therefore host a stack of woven fiber glass fabric sheets to form a monolithic structure. This type of structure offers rock solidity and will be able to withstand impacts against underwater obstacles. Different frames of tissue are used. The first one, for example, is in “mat” that is to say, not rasterized, it looks more like a cluster of compressed fiber and allows a better adhesion of the following layers thanks to the asperities it offers. The following ones will be woven and will thus offer an excellent structural solidity. The total thickness of the finished hull oscillates between 25 and 35mm. Several types of weft and fabric weights are used.
A PVC foam / Fiberglass Sandwich for the upperworks
The bottom are structured in fiberglass / PVC foam sandwich to ensure both good thermal insulation, and lightness necessary to make a performant hull. The PVC foam panels are cut and chamfered at the factory beforehand to save time. They are glued on the upperworks and then covered with sheets of fiberglass which come to close the sandwich. This step represents about a week of work. At this stage, no liquid resin is used.
Vaccum preparation
A first network of resin inlet pipes is put in place on the last layer of fiber. The type of pipe used is similar to the open spiral sheaths used in office automation to channel computer cables. These pipes will allow the distribution of resin over the entire length of the hull from the resin inlet pipes, despite the fact that the plastic coating that allows the vacuum will be plated and will compresse the different layers to infuse. In other words, without these pipes the resin would arrive on the hull at the junction point between their inlet pipes and the waterproofing membrane, but would have difficulty spreading uniformly in the hull.
We can observe the network of black pipes in the axis of the mold on the photo above.
A second pipe network is installed all around the top of the hull. Once connected to a vacuum pump, the latter makes it possible to generate a suction from the bottom to the top of the mold. In the same way as the pipe network of the liquid resin, the suction pipes have a spiral structure that distributes the suction homogeneously over the entire surface of the hull despite the plating of the membrane on the mold, once the vacuum is switched on.
A felt membrane covers this set. It guarantees that the suction pipes, which are slightly aggressive due to their spiral structure, do not damage the waterproofing membrane which ensures the vacuuming. The latter will indeed be plated on the mold by the vacuum generated by the vacuum pump.
As illustrated on the photo above, the green plastic membrane used for sealing is delicate. Shoes forbidden!
The vacuum preparation stage is the installation of the waterproofing membrane. This plastic membrane consists of a single piece. A seal, which is in the form of an adhesive, makes it possible to stick the membrane on the top of the mold and to make it hermetic. The difficulty of handling is to ensure a total hermeticity of the device without which it will be impossible to distribute the liquid resin. The smallest pinhole in the membrane gives rise to a loss that will jeopardize the next step of infusion.
The photo above is evocative, it is an overhead view from the top of the mold to the bottom. We clearly see the gray seal at the bottom of the picture. There is also the air suction pipe in white, covered with its protective felt.
The last step is to install and connect the resin inlet pipes. 2 resin finish points are provided for an AMEL 50 hull. This is a large container that operators will fill along the way of the infusion. A whole network of pipes starts from these receptacles to connect to the network of pipes trapped under the waterproofing membrane. They are spread over 4 levels from the bottom to the top of the mold. Each level of piping will be opened in successive stages during the infusion. Again, the tightness of the whole must be absolute.
Infusion
The infusion itself takes about 3 hours for an AMEL 50 hull. Once the entire device is in place, the vacuum pump is started and vacuum is created. The laminators can then begin to fill up the resin reservoirs. Once these tanks are full, the infusion begins. The laminators will open the taps of the pipes at the bottom of the mold. This way the resin will start to infuse the keel and then the bottom of the hull. When the resin has finished saturating the bottom of the hull, it begins to rise upwards, sucked up by the suction hoses. Once halfway between two levels of line pipe, the laminators cut off the lowest level, and open the next level.
The video above shows the resin pipes at the beginning of the infusion. Only the pipe feeding the bottom of the hull is open, the others remain closed until the resin is mounted to their level in the hull.
In this way, they manage to regularly raise the levels of resin on the entire surface of the hull. The infusion accelerates once the waterline is reached because the resin must penetrate the PVC foam, much more permeable than the layers of tissue at the bottom of the hull.
The level reached by the resin in the PVC foam is visible on the picture above.
About 700kgs of resin will be used during the infusion.
Once the infusion is complete, a drying time of about 4 hours is respected, then the laminators can begin to “unpack” the hull by removing the consumables such as the waterproofing membrane, the protective felt and the pipes. The first joinery can then be installed to stiffen the assembly and allow demolding.
