Re: NMEA 2000 to NMEA 0183


Brent Cameron
 

Paul, it sounds like your autopilot gain and response settings are off.  I’m not familiar with your exact unit but most autopilots have settings for Wind Trim, Rudder Gain, Counter Rudder and Response.  They are basically just little programable logic controllers (PLC’s) that act to minimimize the difference between the input value (course, heading, wind angle, etc) and the set point. To do this, they NEED small pertubutions and then based on the size of the deviation, they apply a correction. For us, those perturbations are usually waves!  

Anyway, that correction takes three main forms. The most oblivious is the Rudder Gain. I’m really going to dumb it down (there are text books written on control algorithms) but you can think of gain like boosting the effect of the correction (i.e. how much rudder to use to get it back on course after the pertubution). Too little and it won’t work. Too much and you over correct and that can set up a nasty cycle of the boat wandering all over the place and never getting back on course. This isn’t exactly right but for your purposes, the analogy works. The second main variable is the Counter Rudder which you can think of as how big an opposite movement of the helm is required to stop the turning motion.  Too big, and the boat overshoots and wanders all over the place.  The balance between the Rudder Gain and Counter Rudder is crucial to goood course keeping. The two settings should be pretty close to each other and certainly no more than two settings apart.  If you were helming the boat, you wouldn’t wind the wheel from crank to crank to adjust for small deviations... you’d use the smallest adjustment you could, so again, the goal is to find the minimum setting that will get the boat back on course.  But it is also tied into the Response. 

For the Response, you can think of that as how many corrections per minute it uses to get the boat back on course. Think of it as if you were making a correction how often you’d want to wait to try the next one. The higher the Response the faster it will act but again it can be set too high so that the rudder is moving all the time which of course really runs down your battery but also causes the boat to hunt. If you watch a good pilot or helmsman, he/she won’t slavishly turn the wheel on every wave but rather averages out the boat’s course to minimize the steering but maintain heading. You want your autopilot to do that. In light conditions, you should knock the response way down to conserve power and reduce hunting but in frisky conditions and especially when steering on wind angle, you’ll need to run it up. A lot of people set it and leave it alone but unless your autopilot has auto response (I’ve seen that the B&G ones change the response based on boat speed which can be a crude approximation), you should adjust it for the conditions.  (The rudder gain and counter rudder are usually autoset during provisioning and then not played with much after that as they are really tied into how well the boat handles excepting if it changes as it gets more frisky). 

The wind trim function allows you to set the sensitivity when acting in wind mode. This is often over-looked but if you are struggling to keep your wind angle, try this rather than the response.  A high value results in the boat following every gust and shift which might be wanted during light conditions with flat water upwind or on downwind on spinnaker but you want a low value when the winds and waves come up on a reach. Most people just leave them at default but that doesn’t optimize your boat’s or your autopilot’s performance. 

Finally, a pet peeve of mine, is to really watch your boat’s Rudder Angle when on autopilot. These autopilots all have Rudder Angle displays but you can also see it by marking your wheel at top dead centre and noting when it is off. I used to be a commercial pilot and it drove me absolutely bonkers to watch other pilots flop on the autopilot and then sit there obliviously when the autopilot keeps adding trim as the plane changed speeds and conditions. It’s the same in our boats. When the wind changes, the sails become out of balance and if you’ve got much rudder on at all, it basically means you’re dragging a big brake in the water. It is literally screaming at you to adjust the sails!  A wee bit of rudder helps us keep the boat stable but that’s really a WEE bit. I hated flying with guys who gripped the wheel like a baseball bat because they couldn’t feel the airplane and they’d constantly overcontrol it and we’d be flinging ourselves across the skies. As a former instructor, I used to teach that you only needed to hold it by the tips of your fingers and feel the pressures. You can’t do that if the plane or boat is out of trim. Same for your autopilot. Get the boat in trim, and your rudder actions (and battery usage) will go WAY down. 

So, to net it out. Make a note of any settings before you change them so you can easily get back to them if needed. Next time there is no wind and you are motoring on a long passage with not much wind,  try playing with the Rudder Gain, Response and Counter Rudder settings (one at a time) while watching your wake. That will give you a really good idea of the effect of each. 

When setting Response, take it off autopilot and hand steer it. Note how often you are correcting it and then set the Response to match that (assuming that you aren’t one of those people who steers the boat from lock to lock on every wave). 

Finally, and most importantly, remember that the autopilot can only respond to HISTORY. It can’t predict what is going to happen, it just responds to what did happen. That’s YOUR job. Knowing how they work and when to mess with the settings is important to optimize your boat’s (and your autopilot’s) performance. 

Again, people that understand control theory are probably screaming under their breath about calculus and integrals but for the layperson, these analogies work. 


Brent Cameron, Future SM2K owner

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Brent Cameron

Future Super Maramu 2000 Owner & Amel Owner Registry Moderator

Oro-Medonte, Ontario, Canada

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