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. .. |
