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Why autohelms don't like following or quartering seas, plus fluxgate compass location advice.

Collated by: Stu Jackson Contributors: major input from John Nixon who provided great technical answers to the basic question of why autopilots can't anticipate certain sea states, and why you shouldn't expect them to do so. Others include The Jacksons on #1406 True Love, Stu Jackson (no relation!) #224 Aquavite in San Francisco, Charlie Pearsall, Mark Mitchell, Dave on #713 Nonsense, Barry Gordon, Mitch Brown, Gary Wiseman, Donna Oakley and Dan Brail.

A number of respondents wrote in about their difficulties in having their Autohelm 4000 units steer properly in higher seas, especially in following or quartering seas.

A few samples of the types of questions:

  • On our recent trip to Boston going from Pt. Judith to Padenaram we experienced 20-25 knot winds on a broad reach along with a 4-6 foot beam sea . Had our brains beat out for three hours. We learned that the Autohelm 4000 just doesn't hold steadily. We also felt it was much safer in the rougher seas to hold on to the wheel ourselves.
  • I am glad I'm not the only one that is experiencing this problem with the Autohelm 4000. It seems to only happen when I am sailing off the wind with a trailing sea. The autopilot sometimes overcorrects as much as 35-40 degrees! I have tried to adjust the sensitivity but it doesn't seem to have helped much. I have recalibrated the fluxgate compass but that hasn't done much to alleviate the problem either. Does any one have any suggestions?

Others wrote in questioning the location of their fluxgate compasses for their Autohelm 4000s:

  • Could you explain why the Navico which is totally self contained and is mounted (compass and all) right on the binnacle guard/wheel doesn't seem to have these problems, assuming you haven't attached your cockpit table to the binnacle with big magnets anyway! I'm not knocking Autohelms, it just seems that what seems to be a serious issue with Autohelms is a non-issue with Navicos .

The system aboard our 1997 MK11 Hull # 1361, seems to be factory installed and the fluxgate compass is located aft of the rudder post. The fitting instructions recommend it be located in the middle 1/3rd of the vessel both fore/aft & athwartships. No one seems to be quite sure where our fluxgate compass is located (also factory installed). We're still looking! Our broker suggested that ANY metal near the fluxgate compass can cause exaggeration of compensation which sounds like the 'wacky' behavior we reported a few months ago. The problem did not arise following seas, and seems better now. We may have stowed something metal near it which was later moved.

Following on from the thread "Wacky Autohelm 4000", the discussion referred to the Autohelm's quality or lack of in quartering seas. We too have experienced this and have been wondering, since checking the Autohelm Manual, whether we should reposition the fluxgate compass to improve things. The system aboard our 1997 MK11 Hull # 1361, seems to be factory installed and the fluxgate compass is located aft of the rudder post. The fitting instructions recommend it be located in the middle 1/3rd of the vessel both fore/aft & athwartships.

The initial responses included the following:

  • Autohelms are like that, and the problem is perhaps not specific to the 4000. As the literature says, pilots won't steer when things are really gnarly (unless you're dealing with a super sized pilot on an ocean going boat). Especially in following, and even worse with quartering seas, very few pilots will be able to handle the constant change in motion. If, for instance, your sailing takes you motoring out through a long channel or the ICW, every time a big powerboat comes by, the waves'll knock you on your ear unless you change course either into the waves or parallel to them. That's the same situation your autopilot is trying to correct in quartering or following seas. It just can't keep up with constant changes not because of the sensitivity settings, but because of the hardover time. Suggest you check the West Marine catalog or their website Advisor on autopilots which gives a pretty good rundown on this situation. Don't throw away the pilot!
  • Any automatic helm system will have its limitations. Our simple systems all react to deviations in heading with some very basic feedback "learning" capability. None of them have any way of anticipating sea conditions or upcoming deviations. They are particularly overwhelmed by violent or dramatic changes in heading. Beam, quartering, stern and confused seas will tax even a good helmsperson in daylight. Try it at night or with your eyes closed to approximate the conditions in which you are asking a simple auto helm system to work. Very sophisticated systems with multiple feedback programs and high rate ram systems can do better. For $1000 or less you get what you pay for. In rough weather be prepared to stand watches on the helm and enjoy the times when the electro/mechanical wizards can do what they can to help.
  • As others have mentioned the wheel pilots just aren't designed for tough conditions otherwise they would cost a lot more. As also mentioned. when the seas are tough for you they are even more tough for your autopilot. One suggestion not yet mentioned: Make sure your boat is well balanced to the helm. It makes it much easier on the autopilot. Other choices, modify your course for easier sea action, or steer by hand!!!!
  • The Autohelm 4000 is the best thing since sliced bread as far as I am concerned. If the oversteer is on a down wind course it will oversteer, that is normal, otherwise the rudder control knob should be set to the sea conditions. Don't use a VHF radio in close proximity to the control head, that will make it go looney. Make sure the is no excessive play in the steering cable, you should be able to move the wheel only about 1" +/- before the rudder moves, not too tight, not too loose. Sail trim will also have an effect on the oversteer. If excessive weather helm trim up. Other than the above the unit should keep you right on course.
  • Doesn't seem fair that two foot seas should be enough to confuse the Autohelm, regardless of heading. Before you stick pins in the voodoo doll of your broker it might be worth a quick ferric inventory. Could there be other factors you have overlooked? Make sure there isn't a toolbox, spare anchor chain or other magnetically attractive loose object within four or five feet of your fluxgate compass. We have friends that stored canned goods under the v-berth for a long trip and caused temporary insanity for their A4000. Spent a few days on Lake Ontario a week ago with friends aboard a C36 and in 15 to 18 knots of wind the A4000 kept panicking and needing to be reset when running slightly off wind. I think the boat was yawing enough in three to four foot seas that the unit detected it as a major heading change.

John Nixon contributed a series of more technical and detailed responses that are organized here:
In general, the Autohelm 4000, and any of the other equivalent systems that I know of, do not have the speed, power, or steering sophistication to handle difficult steering situations such as quartering or following seas of any significant height. Any situation that you would have to work at to steer manually is not a good candidate for steering by a smaller AP. They definitely have their place and can be a valuable tool, but only within their limitations. Even larger cruising boats with long full keels and substantial below deck AP systems have to work hard in quartering following seas, and to do a good job often need additional sensors (such as a yaw rate gyro) beyond the simple flux gate compass sensor. After almost 30 years as an AP designer, I can tell you that we humans make it look too easy to pilot a plane or steer a boat well in a wide variety of conditions. The human servo loop is pretty damn good, but it gets tired, scared, or bored easily. The electronic AP system is much more focused, but not as good overall as the human.

While I haven't used a Navico 300 on a C34-class of boat, I have used various versions of the Autohelm 4000 on several C34-class boats, and all the things posted by various folks on the Autohelm 4000 series AP sounds very familiar to me.

Why would the Navico 300 seem to handle following seas better than the ST4000? Let's look at the numbers. For strictly comparison purposes, I'll use the specs from the Autopilot Comparison Chart in the latest West Marine catalog, and I'll assume that your Navico 300 is basically the same as the 300CX.

The thrust and speed for the ST4000 is listed as 37.5 foot-pounds and 5.5 rpm. The Navico 300CX is 60 foot-pounds and 7.8 rpm. The first thing to notice is that the 300CX is about 42% faster than the ST4000, and secondly, has about 60% more available torque. Ignoring for the moment that manufacturers play all sorts of "numbers games" in their specs, this means that the 300CX should be able to turn the wheel faster at the same load, and potentially may be able to keep turning the wheel some after the ST4000 would have already given up. In this situation ( i.e. - quartering, following seas) a 40% faster wheel response is significant, and the greater torque is also a plus. The 40% higher speed could mean the difference between generally keeping up with the boat and not being able to keep up. You indicated that your 300 "sometimes struggles" to keep up, so I would speculate that the extra wheel speed from the 300 is just enough more to allow the AP to generally keep up, even if you are at the ragged edge. Note, too, that the power of the 300 ( torque x speed ) is more than twice that of the Autohelm: nothing to sneeze at.

With regard to your question about the "all in the same place" packaging of the Navico versus the distributed location of the ST4000 components, Navico did their homework. Right at the binnacle is a pretty good place to locate a compass (note the presence of your Ritchie C5 ......) In general, fixed location (relative to the compass) magnetic disturbances can be compensated out as long as the disturbance is not too severe. The first design factor is that the technology to produce magnetic devices with controlled and limited magnetic disturbances is readily available as evidenced by the current crop of "low magnetic disturbance" cockpit speakers. In the case of the Navico unit, the magnetic disturbance ( i.e. - the permanent magnet dc servo motor) is not only fixed in its location relative to the compass sensor, but the characteristics of the disturbance are known and repeatable within a reasonable tolerance. What this means is that it would be possible to pre-compensate most of the magnetic disturbance (assuming that it wasn't "shielded" completely away) created by the servo motor out of the magnetic heading sensor in the autopilot's microprocessor software, and then fine tune the compensation to consider local magnetic declination angle and the magnetic field differences between motors during the "auto-compensate" function provided by most all current AP products in this class. Pretty cool, huh?
Maybe Navico has borrowed a concept from the old Avis commercial: "When you're number 2, you have to try harder." That said, I should note that I selected and installed an Autohelm ST7000+ for GEMINI DREAM.
While I would agree that behind the rudder post is a pretty poor location for the fluxgate compass, repositioning the fluxgate compass sensor won't do too much to improve the ultimate performance of the ST4000 in quartering or following seas. The only reason to try to have the fluxgate compass sensor in the middle of the center third of the waterline is to try to keep the sensor as near the center of gravity of the boat as possible. The fluxgate sensor is pendulously suspended (but with no damping to speak of) in its case to allow the sensing axis of the sensor to remain nominally "level" with the world in roll and pitch, but that also means it is sensitive to fore and aft as well as lateral accelerations. ("Pendulous" just means a rock hanging on a string..... it tries to align itself with the earth's gravity by pointing "down".) The further it sits away from the center of gravity of the boat, which also is pretty close to its center of motion in a seaway, the more disturbing accelerations the unit sees as the boat pitches and yaws around. (In the "rock on a string" example, move your hand around while you hold the string and see what happens to the direction the rock and string think is "down".) This extra "swinging around" of the sensor creates errors in the sensed magnetic heading, and while it will in general average out over a long enough period of time, in the short term it can make it more difficult for the AP to figure out what it should be doing.
That said, the real problem for an AP such as the ST4000 in a fin or winged keel boat like the C34 (or anything else without a really long, full keel) is the inability of the AP servo to generate large enough and fast enough corrections at the rudder to keep up with the typical yaw disturbances created by quartering and following seas, or really any large and fast disturbances to the magnetic heading of the boat. As I have said many times, any situation that makes it hard for you as the helmsman to keep the boat going where you want it will be even harder for an AP to keep the boat going where you want it.
To make it a little more graphic, imagine yourself on the boat and cut off from any and all sensory perceptions except the compass card, knowing nothing about what is going on around you but what you can learn from looking at the compass on the binnacle, then insert quartering and following seas. To make it more interesting, you can only move the wheel a maximum of about 20 to 30 degrees per second. How well would you be able to steer the boat??

The Autohelm has two power consumption settings: low and high power. The default is set for low power consumption. In a quartering sea, was the Autohelm set for high power? If so, was there any improvement in Autohelm's response to a quartering sea?

If you set the Autohelm to block the "automatic sea state" adaptation, it will increase both the average power consumption and the responsiveness of the system. It will do some better dynamically in this setting, but won't be a cure-all since it still can't increase the maximum wheel speed. If you get a chance to give it a try I would be interested in hearing what level of improvement you notice, if any.

Thanks to Stu Jackson "Aquavite" for compiling this FAQ.

Important: The opinions expressed here are those of the individual contributors to this page, and not those of the Catalina 34 National Association or Catalina Yachts, Inc. Additionally, this material has not been reviewed by Catalina Yachts, Inc. for technical accuracy. This page's maintainer cannot guarantee the accuracy of this information or the desirability of suggested modifications or upgrades. Please obtain assistance from a competent marine mechanic or boatyard prior to making any significant modifications to your vessel.   .. 


Last modified by Phil Imhof, Wednesday, August 11, 2004 . Copyright 2001 by Catalina 34 International Association.  All rights reserved.