Self Stering (Technical Post)
24 October 2014
Richard
Since leaving San Diego, we have been doing a lot of sheet-to-tiller self-steering.
At first, it was just for something interesting to do. At other times, it was because the tailwind had lightened enough that sheet-to-tiller steers better than the windvane does (because of friction in the windvane and because the windvane paddle (sensor) is much smaller than the sensor (a sail) used for sheet-to-tiller.
Sheet-to-tiller self-steering is worth at least practicing on any voyaging boat in case the windvane ever fails, and for downwind in very light conditions when the windvane can't handle it.
In theory, sheet-to-tiller steering is simple:
1. When the boat heads up, apparent wind increases, which increases the force on the sheets. If the sheet is indirectly connected to the tiller via a turning block on the other side of the boat, it can turn the boat away from the wind when the boat heads up (due to the resultant apparent wind increase).
2. A bungee cord or similar pulling the tiller in the opposite direction will turn the boat towards the wind. When apparent wind decreases (which decreases force on the tiller, allowing bungee cord to exert more control)
Note that the sheet does not directly connect to the tiller. The sheet is made down as usual. A control line is tied to the sheet in such a way that it deflects the sheet, and this deflection of the sheet is what goes through the turning block to the tiller.
In practice, there is some trial and error involved (much more at first, until one learns the system). It will take some messing about to get the right positioning of the control line and the right amount of elasticity for the wind speed and point of sail that you are on.
Variables:
--What sheet to use? Any that one can get a good lead on the control line from. I usually use the main staysail, but any sheet can work.
--How many bungee cords? I usually use one, sometimes two, and aim to get the bungee close to neutral (no pull) when the tiller is at or near midships.
--Where on the sheet to attach? Depends on how much deflection you want--I aim for a deflection of about 15cm/6".
Limitations:
After learning how to do it, it takes a few minutes of adjusting to get sheet-to-tiller setup for a particular course.
Sheet-to-tiller demands a better balanced boat than a windvane does. Either will handle a slightly unbalanced boat, but, for instance, Issuma's windvane can usually steer on a broad reach with or without poling out the jib. Using sheet-to-tiller on a broad reach, Issuma's jib needs to be poled out. This also means that setting/dousing/reefing sails is likely to require an adjustment of sheet-to-tiller steering.
So, while I really like Issuma's windvane, its great to be able to use sheet-to-tiller as well.
EDIT: 20171115, Added picture