Self  Steering

(Posted 10/28/2016)

(See bottom of page for post-voyage updates)

Our DIY self steering system is based on the rudder head mount (RHM) pendulum self steering system developed by Jan Alkema.  If it works, it will save us countless hours of hand steering. 

Part of our desire to build the self steering system stemmed from my preference for an outboard rudder (transom hung), as they are easier to fix if a problem develops.  Minimus has an inboard rudder, so the addition of an outboard auxiliary rudder was appealing.  Under sail, our plan is to adjust the sails and main rudder so Minimus is balanced, then lash the tiller.  At that point, relatively little stress will be on the self steering system, which will enhance its ability to steer the boat. 

If the system doesn't work, I suspect it will be because the axis of rotation of the pendulum oar is not parallel to the axis of rotation of the auxiliary rudder.  Jan Alkema's boat had a vertical transom, so the two axis were parallel.  Minimus has an angled transom, which causes a difference of about 30 degrees between the two axis.  This difference introduces a slight rearward vector of motion in the oar and oar carrier as the oar swings out.  If the restraint lines are tight, as they ideally should be, the oar carrier consequently binds when the oar swings out.  So, the restraint line will have to be left slightly loose, which will cause a bit of hesitation before the lines come tight and the oar causes the auxiliary rudder to correct.  How this will work in practice remains to be seen. 

As with many other aspects of our trip, we hope to be posting updates here as we gain experience with the system.

Rudder head mounted self steering system, showing wind vane,
auxiliary rudder, pendulum oar carrier, pendulum oar and restraint line.

When boat gets off course, wind vane cants over, causing pendulum oar to swing out,
forcing auxiliary rudder to opposite side, thus steering boat back on course.

Showing wind vane canted to one side, pendulum swung out and auxiliary rudder
forced to opposite side.

Another angle showing pendulum oar swung to one side,
forcing auxiliary rudder to opposite side.

Wind vane assembly

Side view of wind vane assembly.

Close up of wind vane assembly.  Donut-shaped disk is rotated to face vane in
line with wind, then red clamp holds it in place.  When boat gets off course,
vane cants over, thus moving cords through pulleys which cause pendulum to swing
to one side, in turn causing auxiliary rudder to swing to opposite side,
steering boat back on course.

Top view of RHM self steering system, showing cords through pulleys that transmit
motion of wind vane to pendulum oar. 
Post-voyage comments (updated 10-14-2017):

On the shakedown trip from Santa Barbara to San Diego, our variation on the Alkema RMS (rudder head steering) self steering didn't work.  When we got to San Diego and I had a chance to review the design I discovered that since Jan Alkema had used an upside down wind vane and ours was the more conventional rightside up design, I had crossed the control lines between the vane and the pendulum oar incorrectly.  Once that was straightened out, it worked well and did nearly all the steering on the passage to French Polynesia. 

Although it did almost all the steering, it often required considerable tinkering to get it dialed in with changes in wind direction or velocity.  It just wasn't able to generate the corrective power it should have had.  The reason was exactly as I'd predicted in the description above. 

I eventually came up with a solution, but by the time we were in a position to carry it out the voyage was nearly over.  In the event that anyone else wants to build a RMS self steering system for a boat with an angled rudder like on Minimus, I've included a couple photos below describing the fix.  My only regret is that we never had the chance to make the fix and then sail with it, as the improvement would no doubt have been huge.     

The fix for a RHM self steering system on a boat with an angled transom like that on Minimus is to mount a sturdy support in the position shown above.  

(Top view) 
Once the support was in place, the oar carrier restraint line would go from the oar carrier directly to an anchor point at the aft end of the support.  The restraint line would then be led forward to a quick release cleat on the stern.  The same would be done on the other side of the transom.  This modification would prevent the oar carrier from binding as it pivoted from side to side in response to corrections in the pendulum oar.   
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