|Virtual Javelin - Part III|
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Javelin Design Innovation - Part III
In the third article in a series describing the building of a Javelin we get to see some physical progress and some dirty hands.
Construction started in early May on the foredeck and sidedeck moulds. Stringers were machined down to 15mm square out 6 x 2 lengths of Redwood. We found this to be the cheapest option for a reasonably straight grained timber. Number 1 grade Redwood was used instead of knot free clears, but with careful hand selection you can get minimal wastage.
The frame templates were printed onto sheets of A3 paper and then transferred onto 16mm chipboard and cut out with a jigsaw. To save fairing work the stringers were dry screwed and packed to remove any low spots. The stringers were then epoxy glued and screwed to the frames and the high spots planed fair when the glue had dried.
The mould was sheaved in a 3.6mm Meranti 3-Ply. The process was pretty simple, all the stringers were coated in epoxy glue. the ply was held horizontal and stapled, and screwed down the centreline. Working from the centreline out, the ply was bent into shape and then screwed through each stringer and frame. The screws needed to be around 2 inches long, at about 150-200mm centres to resist the tensions in the ply until the epoxy sets. It is important to orientate the ply so that it bends as easily as possibly over the stringers and in hindsight we didn't think about this as carefully as we should have.
The ply was then scuffed up in preparation for a layer of chopped strand matt. Before the chopped strand went on a primer coat of polyester resin combined with a cobalt/styrene mixture was applied to the ply to help adhesion to the mould. This was then left to cure before the chop strand / resin was applied. Once this had cured a polyester fairing compound was smeared onto the mould, left to cure, and then sanded back with a longboard to get a fair surface.
The final stage in preparing the mould involves getting a high quality, gloss surface. An quick and easy way to do this is to use a high build primer. We used a polyester primer specially for mould surfaces called Duratec. We applied this by brush, although spraying would be much easier, with a lot less brush marks to sand out later. After sanding back, the surface is cut and polished to get a gloss surface. The mould surface must be coated in multiple layers of release wax before it can be used.
A sidetank mould has also being constructed in a similar fashion to the foredeck mould. This mould is made completely out of chipboard, because the surfaces are flat, stringers are not required. A broken mast section has been used as the inside curve of the mould to give a better shape to hike against.
Half a Javelin
The interest generated from the boat building program has generated a lot of interest around the country with the redesign of old hulls and new hulls being planned. Probably the most radical hull modification is being undertaken by Peter Precey at the Navy Sailing Club in Devonport. Peter's boat, www.javelins.org, was a Spencer Mk IV, the last Javelin designed by the late John Spencer in 1988. Modifications started radically with the hull chopped in ½ with a Skill Saw at the front edge of the centrecase.
Using the original Spencer plans a CAD model of the original hull was developed and then modified to produce new aft sections. The modified design has a straighter aft run with considerable more curvature producing a flat 'u' section stern. The maximum chine beam is 50mm narrower than the Spencer, with more section curvature to give greater buoyancy. The rocker aft of the centrecase has been decreased by 6mm in an effort to increase planning performance.
The new stern sections are being constructed over a male stringer mould in 8 mm foam, carbon and epoxy. This will be faired back into the original balsa front sections in a feat that will make Young America proud.
As well as the articles in Sailing New Zealand, this project is being followed on the Internet as the "Virtual Javelin" project. As the boats are built the Javelin Association Web Site, www.javelins.org is being updated with digital pictures, technical details, costs, and a description of progress on a daily basis.
Our aim is to get some interest generated in New Zealand for people to build their own boats. This seems to be happening with a number of new people interested in building hulls, and very strong interest for the Wellington South Pacific's in December.