|Virtual Javelin - Part II|
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Javelin Design Innovation - Part II Construction Ideas
In this, the second article in the series looking at development of a new Javelin from design to the water, we will be dealing with the planing of the project. Having come up with a design the next stage is to work out how the boat is to be constructed.
The Javelin has no restrictions on construction materials, so a look at the various materials available is worth while. Plywood has been by far the most popular building material producing hulls that last many years when looked after. A good example of a Javelin built from plywood is Blunderbus built in 1983 and finishing 2nd in both this years Nationals and Sanders Cup. The next most popular material is foam sandwich with this years National and Sanders Cup winner Freckle Deckle being a good example. Another material used is strip plank cedar or balsa with www.javelins.org being a good example finishing 3rd in both this years Nationals and Sanders cup.
Each of these materials has their inherent advantages and disadvantages. Plywood is cheap and produces a robust hull that is easily repaired. A reasonable skill level is required to produce a light and strong ply hull. Construction is labour intensive and not suitable for "batch" construction. Foam sandwich produces a light and stiff hull that can be produced quickly, and easily once a mould is built. Foam hulls can be dented easily and repairs can add significant weight. Strip plank boats are very strong and robust but are very labour intensive to produce.
With the intention of producing a number of hulls foam sandwich is the most suitable building material. Foam sandwich is made up of layers of fibre reinforced plastic sandwiching a lightweight core. The most commonly used core materials in boat building are end grain balsa, or a PVC foam like Airex, Divinycell or Klegecell. The core adds little or no strength by itself, relying on the separation of the fibre skins to give the laminate strength.
In this project we have chosen to use 8mm Divinycell with a density of 80 kg/m3, as it is easy to work with and relatively cheap. The fibre reinforced plastic layers are made up of fibres (e-glass, s-glass, kevlar or carbon) suspended a plastic resin (polyester, vinylester or epoxy resin). Of the fibres commonly used e-glass, your typical run of the mill fibreglass, is the cheapest $6/m and is easy to work with but has the lowest strength. S-glass is similar to e-glass but has better properties and is more expensive. Kevlar has excellent impact properties (its used for bullet proof vests) and good tensional properties but is expensive $40/m, performs poorly under compression and can be difficult to work with. Kevlar is particularly tricky to repair because of its tendency to "fluff up" when sanded or cut. Carbon Fibre has excellent properties and easy to use but can be fragile and is expensive $50/m. Of the resins, epoxy is the best performing and is almost odourless in comparison to both polyester and vinylester resin. This is a definite advantage when building the boat at home. One disadvantage of epoxy resin is the longer curing times compared to Polyester which can result in a bit of sitting around waiting for the resin to "go off".
The lay-up chosen for the hull is two layers of 200g e-glass on the outside with a single layer of 200g e-glass on the inside with unidirectional carbon reinforcement in the high stress areas all laminated with epoxy resin. This lay-up should give a hull weight of approximately 2.5 kg/m2.
Foam sandwich construction is usually performed in three ways either inside a full female mould, over a full male mould or over a male stringer mould. Once again these methods have both inherent advantages and disadvantages.
Female Mould - In a female mould, the hull is built completely inside the mould. The inside surface of the mould produces the outside surface of the hull. The main advantage of a female mould is that the hull will be perfectly fair when taken off the mould. The disadvantage is obvious, a perfectly fair hull (or male mould) must be built first before the female mould can be made. The sequence of construction is laminating the outside fibreglass skin, foam core and then the inside skin. The internal framing, and the false floor can be added whilst the boat remains inside the mould ensuring an accurate hull shape is maintained. No outside hull fairing is required.
Male Mould - In a male mould, the hull is built "on top of" the mould. The mould is made smaller than the desired final shape of the hull. An allowance is made for the thickness of the foam and fibreglass laminates. The outside of the mould is made relatively fair, as this will be the inside skin of the completed hull. The sequence of construction is laying the inside skin directly against the mould, then adding the foam core, and finally adding the outside skin. The outer skin is then manually faired. The hull is then taken off the mould for the addition of the internal frames and false floor.
Stringer Mould - A stringer mould consists of a number of frames, and fore and aft stringers giving an approximation of the final hull shape. The stringer mould is made smaller than the completed hull shape to allow for the thickness of the foam and fibreglass laminates. A stringer mould is the cheapest to construct, and is faster to make because time does not have to be spent making a mould surface fair. Building on a stringer mould is quite a bit slower because the foam core needs to be attached to the mould. The common methods are tying the foam using wire or screwing/stapling the foam directly to the mould. The sequence of construction is attaching the foam core to the mould, then laminating, and fairing the outside skin. The hull is then removed from the mould for the inner skin, frames and floor to be added.
We will be using full male moulds to produce the hull shell, side decks and foredeck and a stringer male mould to produce the false floor. The mould is to be built out of MDF with interlocking longitudinal and transverse frames. Stringers are then laid over the frames, which are in turn covered with thin plywood. The mould is faired and sheathed a layer of fibreglass to protect the surface of the mould.
An important factor to consider when planing the construction is the configuration of the finished boat as the bulkheads need to be designed for the configuration. Again, this is pretty open in the Javelin, the only real restriction on internal layout is that a foredeck must be fitted. Standard configurations would be bow tank and aft side tanks, false floor and single bow tank or full-length false floor. The configuration we will use is a full-length false floor, as it makes fitting a retractable gennaker prod easier and the boat floats more level after the inevitable capsize.
The first consideration is a space frame to take the rig loads that can be significant in a Javelin. This is usually made from foam sandwich and consists of a single longitudinal bulkhead running under the mast from bow to stern with two angled bulkheads running from under the mast to each chainplate. The next consideration is in the area of the centrecase. The centrecase takes considerable loads during sailing and the surrounding area must be reinforced to take the loads. This can be done with extra reinforcement where the centrecase penetrates the hull and false floor, but more on that in the next article