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F-39 Building project


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The construction of the F-39 is a sandwich of foam and reinforced epoxy. The reinforcement consists mainly of a stitched glass fabric. In places where great forces occurs, like in the beams, the glass is replaced by carbon. These modern materials as carbon and Kevlar makes it also possible to replace the traditional used materials like aluminum and stainless steel for rigging and hardware by these modern and stronger substitutions.

Sandwich Construction has been well established in boat building for over 30 years. Like the civil engineer who use I-beams in the construction, the yacht designer specify the sandwich construction for much the same reason: to increase the stiffness while at the same time reducing weight. The engineering theory shows that the flexural stiffness of any panel is proportional to the cube of its thickness. The laminate skins act as the I-beam flange and the foam core act as the beam's shear web. While the laminate is put into compression or tension, the foam core is put into shear. It therefore follows that one of the most important properties of the foam is its shear strength and stiffness. 

Core materials are cedar (nature's honeycomb but heavy), end-grain balsa (water resistance ?), honeycombs (expensive, amateur-friendly ?), foam and prefabricated combinations of these. Balsa, foam and honeycombs are mostly used in rigid moulds by professional builders. Cedar and foam makes it possible to "construct" a hull with simple male or female frames and without the need of full moulds. With foam I don't mean the well-known foams used for thermal insulation applications (polyurethane, polystyrene), who are completely unsuitable for structural applications in a marine environment, but the modified (cross-linked PVC) or unmodified (linear PVC) structural PVC foam. 

Of course the designers preferences and specifications are of vital importance and the core materials must suit the calculations and worked out building method.

Volvo60s ASSA ABLOYI choose for Core-Cell foam from former Canada's ATC-Chemicals but nowadays SP-Systems, to be the structural foam. More than  the cross-linked PVC-foam (like Divinycell) but less than the linear foam (like Airex) this SAN-based foam is more flexible to suit the hull lines and curves necessary for Farriers vertical foam stripping method. The handling characteristics are better and thus easier to work with. In fact Core-Cell combines the best of two worlds, the strength and toughness of the cross-linked types and the good impact strength and flexibility of the linear types. Impact tests shows that a sandwich with Core-Cell offers more resistance against impact forces than cross linked pvc. Core-cell is accepted by Lloyd's Register of Shipping. Another issue is that with relatively thin skins, PVC foam outgassing has been an issue, which does not happen with the Core-Cell. Unfortunately, the main disadvantage here in the Netherlands is the price, about 50% more than the other foams. Another disadvantage is the combustibility. It burns like hell (Divinycell doesn't). But the resin isn't any better. The prevention of fire on board will be (as always) an important safety issue. In fact, as I see it, fire is the one and only danger whereby this trimaran can sink.

A500 80 kg/m3 is used for the hulls and beam bulkheads and A1200 200 kg/m3 is used as high density inserts for the mounting of hardware. T400 is used for the interior panels. The core thickness is 15 mm.

A fabric is defined as a manufactured assembly of long fibers of glass, carbon, aramid or a combination of these, to produce a flat sheet of one or more layers of fibers. Most fabrics in the F-39 are Biaxial 0/90 and 45/45. Where concentrated forces are to be expected, the fabric is of the Unidirectional type. The majority of these UD-fibers run in one direction only.

Both fibers type are non woven. Because of the interlacing of warp (0°) fibers and weft (90°) fibers the woven fabric will lose a certain degree of strength due to crimp and the wrinkles and kinks. The fabric I use is made by a stitching process, which effectively combines two layers of unidirectional fibers into one fabric. Because of the fibers in these layers are kept straight and thus eliminating the crimp and shear factors that generally affect the strength and stiffness values of a standard woven fabric, there is a significant increase in mechanical performance. Another advantage of this type of fabric is the higher glass/epoxy ratio by the increase packing of the fiber which reduces the quantity of resin required. These knitted cloths have just less interstitial space for the resin.

Epoxy or polyesterPolyester or Vinylester are suitable resins and relative cheap. In fact, the design is engineered with polyester in mind. But epoxy resins have far more better mechanical and physical properties and is much more forgiving. The balance of properties epoxy can offer is hard to match. Higher compressive properties combined with higher toughness, higher adhesion with lower shrinkage, outstanding fatigue performance, low moisture absorption and (by virtue of their different curing chemistries) a complete absence of potential to suffer osmotic blistering. Last but no least, epoxy doesn't have the nasty styreen smell. So the choice between epoxy or polyester/vinylester for the "high-tech" F-39 is no question for me. The difficult part here is to choose the brand. There is quite a price difference between known brand names and unknown names but what is the difference? For my skills and the building method, specially the vacuum technique, the epoxy system must be flexible according to the curing schedule. There must be enough (gel) time for doing the job. Health and safety is another important issue (with epoxy even more than polyester). I finally decided for Bakelite AG, a German company and the trailblazer in production of epoxy resins. This decision was also made because of the knowledge in the field of resin infusion and the technical support I can get from the supplier. For easier mixing I made a Resin Chart. An article very well worth reading about epoxy vs. ester resins is on Kurt Hughes website, follow this link.

float_construction.jpg (93057 bytes)I would never undertake such a huge project without the super detailed building plans of world class designer Ian Farrier. Every part of construction is very well worked out in detail. Just follow the indications exact as composite building is something you do (still) without any feelings in respect of strengths. Working with wood or steel give you direct indications of strength and stiffness. Composite building is much different. Create the best working conditions, follow the recipe, wait for the hardening and trust the result. In my professional job we do it with sand, gravel, steel, cement and water (reinforced concrete). For this project it is foam, fabrics and epoxy. It is the designers recipe that guarantees the strength and stiffness. A lightweight and strong result is the most important thing so one has to ignore one's natural pressure (as most amateur-builders have) to make things stronger and heavier. Throughout the building process a checklist instruction system is used that keeps the work in order and ensures each job is done correctly and in the most efficient way.

The key factor in Farriers building method is building half hulls, who are later joined at the centerline. The center hull, the cabin sides, roof and most of the deck can be combined and all made at the same time. Many of the interior panels can be added before the hull is joined. An overlap join is required down the center, but this is exactly the place where any extra glass-reinforcement should be, for stiffness, and abrasion resistance. This method is much more easier and faster than the traditional hull-with-deck joining.

The construction process starts with the set up of the temporary female form frames (18mm. MDF). The cutting of the hull line is made larger by the thickness of the fore and aft battens. The frames are spaced by 635 mm. The fore and aft battens are spaced every 150 mm. or so, even closer in tighter corners. This wooden carcass, being a female mould, is planked with 100 to 500 mm. wide foam strips with joins at right angles to the battens. Self tapping screws, screwed from the outside, hold the foam down and to the battens. The joins are glued with thickened epoxy. Once glue has set, there is a half foam hull, ready to receive the inner glass laminate. vertical_foam_stripping.jpg (43917 bytes)

Laminating is almost the same as wallpaper. At least, it is the same mess! All layers and extra reinforcements are laid in the same laminating process, wet in wet (wet-out and lay-up method). The last one layer is peel ply, a nylon release fabric, to which the epoxy resin will not adhere. It shows proper wet out and improves the surface finish by filling the weave and holding the glass flatter and it reduces the need for sanding. Once the resin has cured, the peel ply can be pulled off to remove amine blush and expose fresh, uncontaminated resin surface that will require little if any sanding. Sanding the bare epoxy is a tricky business. It is very hard and unfriendly for yourself and for the sanding paper. But worse is the fact that sanding can damage the glass fabric. So, peel ply is the way to go and I use it on everything that I glass.

The resin is poured and spread over the glass fabric and excess resin as well as air bubbles are removed with a squeegee. When the laminate is getting clear the fabric is satisfied. White spots are to dry and need to be wetted again. This hand-lay-up method is in principle sufficient when done right. However, layers are very thin and so there is little margin. Hand laid glass/foam boats often have bonding problems between the core and skin, as air is trapped in the open cells on the face of the core. That's not what I want!

Vacuum baggingWell let's face it, building this trimaran is a lot closer to building an aircraft than it is to traditional boatbuilding. So, one step further in achieving an absolute premium quality regarding to lightness, durability and strength is "vacuum bagging". The laminate is wrapped in a plastic bag while a vacuum pump creates a vacuum in the bag. In this way all trapped air bubbles are soaked up and the whole laminate is under pressure for an better bonding with the foam surface without trapped air in the bonding area. A breather fabric allows the air to escape from all over the completed part and absorbs excess resin. The difficult part here is to airtight the bag without (to much) leakage.  This vacuum bagging is an aerospace technique from origin, but is relatively simple to do once the principles are understood.

So, while in the process of preparing my skills, I decided that vacuum bagging is the way to go. The next problem is the fact that I am working on my own in a rather small workshop. Vacuum bagging requires a lot of helping hands as one is working against the resin clock. It also requires preparation in cutting the fabrics, mark and store them in a way they are immediately usable for the job. There had to be a better way, .............. and there is. I found the solution in Vacuum Resin Infusing, a real big step forwards and the most ultimate laminating method within my range.

infusion.jpg (36411 bytes)The above schedule is almost the same, except that the bleeder fabric is replaced by a distribution fabric. The vacuum pressure is now used to drive the resin into the dry laminate instead. With this technique there is no messy hand-lay-up anymore. All fabrics are dry placed and after sealing the bag injected with epoxy, with the vacuum as the driving force. The progress of the resin through the layup can be monitored visually, and when complete, the inlets are clamped-off. With this technique I can proceed piecemal with the layup, working alone in a spare hour here or there, putting the vacuum assembly together at a leisurely rate, and only when ready pulling a vacuum on the hull (or part), mixing the resin, and infusing. Because the laminate is compressed prior to introducing epoxy, a more optimal ratio of resin to fabrics is obtained. 3D diagramAnd, not the least advantage, human exposure to potentially harmful epoxy vapors is minimized by reducing the time one actually works with the epoxy and the difficulty of what one has to do with it. It is merely mixing the epoxy and pour it into a bucket, from whence it is sucked in fully automated fashion into the laminate. Most important for me is a  reassurance that this technique provides a very high quality, both structurally and cosmetically, and that this is not dependent anymore on the human variable (my skill). Once the process has been designed, quality will be consistent.

Interior panels and bulkheads are also made with vacuum infusing. Here I use the double layer infusing system, where both laminates on each side of the foam core are infused simultaneously. For this purpose I made a flat table, covered with Formica  to be perfectly flat. To allow air and resin to escape from the bottom layer, the foam core has perforations every 25 mm.

Here you can read more about Controlled Vacuum Infusion, with which I am building all the hulls and other parts. 

Once internal glassing (with stringers included) is complete, all internal stiffeners and bulkheads are applied. The bulkheads sit on a wet resin putty filled bed and are then glass taped to the hull, wet in wet for good bonding and without the ugly sanding. The hull is then removed from the mould and turn over to laminate the outer skin. The procedure is the same as with the internal glassing. For the second hull half the form frames are reversed and the same operation starts again. After finishing the hull halves are joined together with epoxy glue and glass tape inside the center line. Outside is done during the infusing of the outside laminate. Controlled Vacuum Infusion made it possible to do the outside glassing of the floats in one shot!

What follows is the building of all other structures, the interior, the beams, the daggerboard, the rudder, hardware, engine etc. and by far the worst job: fairing-sanding-painting. 


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