Sunday, 7 February 2010

Magic boat

Look, no hands! A headless Mark Schroeder watches as Impulsive steers herself to windward.
We started sailing up Lyttelton Harbour around 11am on Saturday, in the last of the morning cloud cover. That had burned off by the time I'd rigged up a sheet-to-tiller self steering system, using some spare bungy cord and a line with a pulley. The idea is that a boat will sail upwind well balanced in light winds, but as wind increases the boat heels, and the hull shape causes it to round up. This system uses the increased wind force to pull the tiller to windward, as a human helmsman would, keeping the boat on course.
The system has two main parts. A bungy cord comes from the left to the end of the tiller, and the black cord runs from the blue mainsheet at the bottom to the tiller, via a pulley at the stern. The mainsheet cord is set up with some tension on the sheet; when the sheet pulls harder as wind increases, it pulls the tiller up, counteracting the rounding-up effect. Here is a YouTube clip showing a more sophisticated version of this system, controlling a J-80 on a reach.

We modified the system after these photos were taken, as the original control line wasn't pulling hard enough on the tiller. We figured that the 4:1 mainsheet gearing meant that the sheet tension wasn't "high geared" enough, so we ran a cord from the end of the boom to the tiller, giving us a direct linkage to wind speed. That worked better, and as we used a lighter grade cord the rolling hitch adjuster didn't slip, which had been a minor nuisance. The next step is to work out a way of adjusting the tension on the bungy. (Update Sunday 7th - the bungy is now 4 shorter lengths looped, free to adjust under load, with a length of cord incorporating a rolling hitch adjustment.)

Of course we have to unhook the lines and swap the two controls after tacking. We don't plan to use this very often, but on long trips it's nice to be able to move about (though on a light boat that can induce variations in trim, sending the boat in unintended directions) and to attend to other matters like eating and drinking, using the (bucket) toilet, or rigging spinnaker gear for the downwind run.

In the gentle 10 knot breeze with a negligible swell, we sailed out past Adderley Head, and round into Port Levy, but we didn't stay long as it was nearing 2pm. Once back in Lyttelton Harbour, we hoisted the spinnaker and had a great high speed run down toward Purau. As we passed Purau, we saw Chris Hutching in Henry Salad motoring into Pile Bay, so we called him on the radio and said we'd join him. We did a tidy job of downing the spinnaker, then rounded up and sailed over to the bay, dropping the sails and motoring in with our keel wound half up. We tied to his stern, then sat and chatted for a while, before raising our mainsail and sailing back to the club, in a much stronger breeze which had built during the sunny afternoon. We both slept very well last night!

Sailing past Adderley Head into Port Levy.

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