Monday, February 14, 2011

Self-tacker track construction

In order to fit a self-tacking jib to the boat i needed a method of curving a track for the jib car to run on. On other boats i have seen this done in a couple of ways:

  1. A ronstan I-beam (series 19) track is ordered from ronstan to be bent to a given radius, which can then just be mounted to a support at each end and a support in the middle to prevent the track from twisting or warping off its supports. (e.g. 3 mounting points that are not co-linear are the minimum required to make the mounting stable). When i investigated this the track was roughly $300, with a lead time of 3 weeks and the car cost around $170.
  2. A thinner track like a ronstan series 14 BB track or a Harken Micro CB track is flexed to a rigid backing or a series of brackets attached to the deck.
  3. The track as above is directly attached to a concave deck that matches the curvature of the track.
I chose to use a rigid backing support that i could then force the bend in the track as it was bolted to the support.

To make the support i screwed two pieces of 18mm form ply together giving a piece of timber 36mm thick. Next the required radius was traced using a pencil and piece of string. This was then cut out carefully with a jigsaw and neatened up with a belt sander. To give a radius on the edge a quarter round radius bit was used in a router (with a ball bearing).

This mould was then covered in packaging tape as a mould release and two strips of 10mm foam where cut to cover the mould.

The laminate is described in the image below. First the two layers of glass are laminated onto the mould surface with a +-45 degree orientation to the length of the mould. Next two layers of 300gsm UD carbon that is only wide enough to cover the flat surface are laminated. If the strips are wider they will not stick to the sides of the mould.

There are several reasons for laminating the glass on the -+45 deg (or carbon if you wish, i would still use two layers ontop and atleast 1 on the bottom).
  1. When the fabric is on the -+45 deg it wraps very easily around the corners of the mould and also traces the curved radius of the track well. Otherwise you need to place cuts all the way along the edge if the fabric is on the 0/90 deg
  2. When this beam is in bending the one outside skin will be in tension while the other is in compression. If there were no skins on the side the foam core would need to carry a shear load between the two outside skins. Since foam has a low shear modulus (stiffness) the will deflect significantly. Since woven fabrics have there highest shear modulus and strength when the fibres are on a -+45 deg it is a good idea to place these skins on the side as it will give you a stiffer and stronger beam.
  3. Unidirectional composites are generally stronger in tension than in compression because when the fibres are under compression the individual fibres tend to buckle. In order to improve this compressive strength a laminate on the -+45 deg to the unidirectional fibres should be used. This is because the cloth layer on top allows the load to be passed from one fibre the neighbouring fibre through a shear stress which is taken by the woven fabric. I'm fairly sure that i read somewhere this can improve the compressive strength by 10-15%.

Next spread a runny bog of Q-cells and epoxy resin onto all surfaces of the foam and place them on top of the laminate. Then continue to laminate the rest. Once the peel ply is placed over the top carefully place strips of packaging tape over the laminate starting in the centre and working towards the ends.

Once cured it can be easily trimmed to length and the track is bolted to the support. Care needs to be taken when clamping the track to the support. If you place a clamp in the centre and tighten it up you will buckle the track and place a bump in the track, which can stop the car from moving.

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