By David VanDenburgh Sr.
We were a week out of Hawaii heading for San Francisco and both our autopilot and our backup autopilot had packed it in. We tried a fix but upon opening the case found the gears ground to powder. It looked like someone would need to steer all the way home (about three or four weeks). I figured this would be the perfect time to see if I could make Pygmalion (a Westsail 32) steer herself.
When the wind is forward of the beam, self-steering is not too hard to achieve (at least not in a full-keel cruising boat – those of you with fin-keeled flat-bottom modified racers are on your own!). To accomplish it, you have to understand the forces involved.
First, put the boat on course and hold her there. Then tinker with sheets until the helm is well balanced – just a touch of weather helm. The principle here is to move the center of effort and the center of resistance very close to one another, with the center of effort just slightly aft of the center of resistance. The center of effort is moved aft by easing headsails and hardening the mainsail (on a sloop or cutter) and/or the mizzen (on a ketch or yawl). Conversely, the center of effort is moved forward by hardening the headsails and easing the main (and/or mizzen). (There’s not much you can do to move the center of resistance unless you want to move stores around and rebalance your boat.)
If the wind were steady in force and direction, all you would need is a light bungee cord to hold the tiller or wheel against the slight tendency of the boat to head up, but of course the wind is not steady. Puffs will cause the boat to heel which will cause her to want to head up (stretching the bungee cord). Lulls will let her stand up which will cause her to want to head down (as the bungee cord pulls the tiller up). Fortunately you can take advantage of your boat’s reaction to heeling.
What you want to do is translate the extra tension on a sail created by a puff into a force that pulls the tiller/wheel in a direction that will head the boat downwind a little to resist the tendency to round up. Conversely, you want to translate the reduced tension on a sail during a lull into a force that turns the tiller/wheel in a direction that will head the boat upwind a little to resist the tendency to head off.
There are a number of ways to do this, but the easiest way is to tie a line into a headsail sheet with a rolling hitch (or taut line hitch) and bring it to the tiller (through a block or two) so that increased tension in that line will pull the tiller up and resist the tendency of the boat to head up. This force will probably need to be balanced with a bungee cord pulling in the opposite direction. In a puff, the boat heels and wants to round up into the wind, but the increased pressure on the sail pulls harder on the sheet and therefore on the self-steerer line tied into the sheet, which pulls the tiller up and resists the tendency to round up into the wind. When the puff eases, the bungee cord pulls the tiller down, heading the boat a bit off the wind. It takes tinkering to get it right.
Using this rig, we were able to get Pygmalion to steer herself for hundreds of miles at a time, setting us free from the tiller and reducing fatigue.