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hi Roger. My first foray on the BCC chatshow! I have a beautiful wooden 29'10" lyle Hess cutter,(Taleisin style), here in New Zealand. She is very well balanced and sails perfectly using the backstay windvane / trim tab set-up; actually, I think Larry Pardey's Mk IV vane. However, I have not yet had any success in using my small Simrad tiller pilot, to act on the trim tab "tiller". Briefly, the tillerpilot is set up on the port side, immediately aft of the taffrail and attaches to the trimtab tiller directly below the normal clutch position.
To allow for it being port-hand mounted rather then stb, if it were direct on to the normal tiller, we have recalibrated the unit. The problem is that the tillerpilot constantly over-reacts and pushes/pulls the trim-tab tiller too far, regardless of how much we try to tune it, resulting in violent course swings and eventual "lock-over" at full extension. It can of course be "set" to compensate for prop. torque or slight weather helm, but it will not follow a compass course. I am thinking now of fitting a slave tiller to the trim-tab tiller (!) and re-mounting the tiller pilot back inboard on the stb side. The "slave tiller" would pivot on the taffrail to give a ratio of approx 4:1; i.e a lever effect which would allow the tiller pliot to work through its normal arc, yet result in a reduced arc for the trimtab tiller.
This all sounds highly complicated, and probably is ! Do you have any helpful suggestions?
Fenton Hamlin
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Fenton:
John Letcher, Self-Steering for Sailing Craft, discusses positive and negative damping feedback in the self-steering system. If the system has negative feedback, i.e. the trim tab angle increases as the rudder is turned, the boat will oscillate in ever widening turns - see diagram. Most self-steering systems build-in a positive feedback linkage which causes the trim tab to decrease its angle of attack as the rudder is turned more and more off-center - positive damping. To determine if you have positive or negative damping in the system, manual offset the trim tab via the auto-pilot linkage and turn the rudder in the direction the trim tab would move the rudder. If the trim tab angle decreases, then the system has positive damping. If the trim tab angle increases the system has negative damping and the boat will continue to increase its oscillation amplitude, i.e. it will not come back to the correct course.
I am reading Letcher's book now. Letcher has a Ph.D. in aerodynamics and hydrodynamics and has sailed offshore for over 25,000 miles. He works as a naval architect in Southwest Harbor, Maine, USA. Unfortunately, I believe the book is out of print.
Rod
S/V IDUNA
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Hi Fenton,
I would like to trade places with you right now...I loved NZ and hope to return someday...anyway..
I had the same problem with my Navico which is now Simrad. No matter how I adjusted it, it would over steer. Totally frustrated, I called on a friend and fellow BCC owner, Roy Myers who used the same unit with great success. He told me to set the Sea State on 2 and the.....power (?) or other on 1. I admit, I have not ever changed the setting after it begin to steer a nearly perfect course so these number are from memory from a couple of years ago, they may be reversed (Roy, if you are there, correct my comments if I am wrong).
The autopilot must attach to the trim tab arm as near as possible to the axis of the rudder's pintles. When you take the boat out everything must be in the correct position and steering a straight course before it is engaged. Also, keep the speed down to about 4 knots for this break-in period to prevent it from over steering too much. The Autopilot has a memory and will begin going off course slightly but will settle back to nearly a straight course. It will improve after it is used a few times. I know what you are thinking...I did this already....so did I but for some reason I must not have had the boat steering a straight course, going too fast or not set correctly before engaging....
Now, If this does not work the autopilot could be defective. I have two aboard and had to send one back. They never found the problem so sent me a new one and it now works perfectly like the other.
I would like to know if this solves your problem..
Roger Olson
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Lads,
The largest single input that can cause your boat to wander is the attachment point at the tiller arm. This is the feedback loop being discussed by Letcher, which, with the vane's sail can be used to advantage. With the tillerpilot, the feedback loop is much more critical -that it not be there in the smallest way. In essence, the head of the little 1/4 inch dia. pin that the tillerpilot connects to must be not less than exactly on the centerline of the rudder's gudgeon and pintle axis. Any deviation here creates an input that the autopilot doesn't "know" exists. The AP makes a course correction based on how far the boat has wandered; the fore and aft placement of the head of that pin can add or subtract a steering moment which the autopilot is not able to compensate for. The adjustment is so critical because the trimtab requires so little input to steer the boat. The best way to do this is with a long slot in the tiller arm, though some I've seen had only holes. This way, the pin may slide on a bronze block, so infinite adjustment is easy.
After that, setting the Simrad up is a matter of choosing the very lowest settings, 1 is what I began experimenting with, for both gain and seastate. I believe Roger's memory is correct, in any case, mine is no better; one is set to 1 and the other is set to 2. I'll check the next time I have it out. Ok, Ok, I'll do it now. -- Nice night out- Gain is set to 2, seastate to 1.
There you have it. -- S/V MINX
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