Tuesday, February 15, 2011

Self-tacker design

To work out the radius of the self-tacker track i started with working out what i wanted from it as follows:

  1. The leech tension to remain the same on all positions of the track: This was so that i could adjust the car position without having to adjust the jib sheet as well, so that simple tweaks could be made.
  2. The foot remains that same tension on all positions of the track: This was to keep the foot tension on the jib an independent variable from the outward position of the car. I adjust the foot tension by the position of the jib sheet on the clew board.
Some people like to set the boat up so that the leech tension decreases as the car is eased or have the foot tension increase as the car goes out. I'm personally not a fan of this as you are effecting two parameters with one variable and i feel you could end up in a position that you need to constantly tweak two controls just to setup the one parameter you want to vary.

If order for these criteria to be meet the jib car will need to follow the jib clew as it moves exactly. To do this it means the distance from the head to the jib car must remain the same as it travels. Secondly the distance from the tack to the car must also remain the same. This gives us the simple problem where we have a triangle with constant length sides that pivots about the forestay.

Therefore it can be found quite easily. Either by the geometry of the jib and then find the line length that runs through the clew and is perpendicular to the forestay. Or once the boat is rigged up you could just use a tape measure and builders square to find the distance perpendicular to the forestay.

The only other things you need to consider is to account for the fact you self tacker track will sit something like 30mm above the deck at the centreline and to allow for pulley's etc.

Also if the track has a slightly smaller radius than needed the sheet tension will increase slightly as it passes the centreline during a tack. If the track is on a slighlty larger radius then the sheet will loosen slightly during the tack. I personally favour the track to be on a slightly larger radius as i don't want the sheet to tighten up and jam during a tack.

From memory my current track is a 1580mm radius

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