Autopilots
 

On Tue, 29 Apr 2008 17:44:52 -0400, Larry wrote:

Goofball_star_dot_etal wrote:
On Tue, 29 Apr 2008 10:53:50 -0400, Larry wrote:

Goofball_star_dot_etal wrote:

Give it a rudder so it can become the happy little second-order system
it was born to be.
Well, you can't add a rudder to a jet boat.

Try to remember that you sold that boat last year and want to control
a boat with a 15 hp Yamaha outboard at "trolling speeds (about 1 knot
or 1.5 mph)"!! with an autopilot which expects a certain (range of)
yaw rate proportional to the control input, not a pig dominated by
small forces and relatively large moment of inertia.

Yes, I understand, but how would you add a rudder to an outboard? Or do
you mean use the steering mechanism as a rudder? (actually, the big
engine is a Yamaha, the 15 hp is a Suzuki)

I can see two options.
The first is to attach a foil to the 'small' outboard
eg. http://www.freepatentsonline.com/3991700.html
This would be steered through your existing arrangement with or
without an autopilot. I assume that this small outboard can be raised
when not in use, removing the rudder from the water, but if it is
robust enough could be left in the water at speed.. The disadvantage
of this scheme is the large load on the autopilot of having to control
both motors though the steering mechanism.
The second option would be an auxiliary rudder similar to a typical
sailing dinghy rudder with a lifting blade to be used only when
trolling. This could be large enough to do the required job but quite
light and weak. If the rudder/boat response is good enough the
cheapest autopilot could be attached and used just for low speeds,
with a remote control if required. This arrangement would be a
nuisance and likely be damaged at speed so needs to be retracted then.

I would assume that any autopilot would net out yaw, but I don't know
and don't want to pay all outdoors to find out. Are you saying it's a
large factor?

No, I mean that most boats at normal speed turn more or less like a
car, with a turning circle radius that reduces with helm movement. The
assumption is that autopilots are designed for typical boats behaving
in this way. Observing autopilot behaviour, they seem to move the helm
in proportion to the error, although there is also a dead-band, some
integration of error and some damping involved. It seems they would
assume a rate of turn proportional to the error signal and actuator
movement whereas a jet or outboard at low speed produces a very small
transverse thrust in response to the angular error signal, causing
angular acceleration that increases the rate of turn with time.
It seems a better plan to try to fix the boat's low speed steering
than to expect a control system to cope with this. You need the gain
of the factors that relate angular heading error to the boat's rate of
turn (yaw rate) to be more or less constant and within the bounds of
the autopilot design, then all should be well. You don't want the
error integating up because nothing has happened yet!


I fish northern New England lakes - yaw isn't a big factor. Also, I am
not a commercial fisherman - I fish for fun. If the weather goes south,
I generally head in.

I don't bother to go out sailing unless there is some decent wind and
waves..

Autopilots
 

On 2008-04-27 20:46:09 -0400, Larry said:

The autopilot I used had no sensor for speed. Does any? It would seem
to me that there needs to be one because response to the wheel is
proportional to your speed underway.

Haven't been paying attention to this thread, but my tiller AP will
"read" some NMEA speed sentence and the swing can be adjusted.

--
Jere Lull
Xan-à-Deux -- Tanzer 28 #4 out of Tolchester, MD
Xan's pages: http://web.mac.com/jerelull/iWeb/Xan/
Our BVI trips & tips: http://homepage.mac.com/jerelull/BVI/

Autopilots
 

Bigfoote wrote:

Raymarine Sportpilot. It is a wheel mount unit and may not fit but I have
had one on an old Slickcraft for 13 years and it is still doing the job
trolling for salmon on lake Michigan about 200 hours a year. Did have to
replace the electronic compass module once but at a very reasonable price.
Even accepts waypoint data from my old Garmin GPS/Plotter. Biggest problem
if it fits the wheel is where to mount the compass at lest 3 feet from any
large metal mass (i.e. engine). Ideal is low and to the rear to minimize
roll and pitch effects on compass but not easy in a small boat.

Would you please describe your Slickcraft? I think I had a Raymarine
Sportpilot, it may have changed, but perhaps it can be made to work.

Thanks!
--

Larry R
rapp at lmr dot com

Autopilots
 

Goofball_star_dot_etal wrote:

I can see two options.
The first is to attach a foil to the 'small' outboard
eg. http://www.freepatentsonline.com/3991700.html
This would be steered through your existing arrangement with or
without an autopilot. I assume that this small outboard can be raised
when not in use, removing the rudder from the water, but if it is
robust enough could be left in the water at speed.. The disadvantage
of this scheme is the large load on the autopilot of having to control
both motors though the steering mechanism.

The steering is hydraulic. I haven't done any testing, but it doesn't
seem like much of a load. I can disconnect the arm which connects the
two motors - the force required to turn the wheel doesn't change very
much.

In the case above, if I read the link correctly, the "rudder" is nothing
but a flat plate behind the propellor. And, yes, the motor can be
raised. I don't understand how the "rudder" adds anything as steering
is still accomplished by moving the motor. The only thing added here is
direct feedback from motor pointing (via steering). That must be
important for the system to function as it was suggested by the
Sportpilot (if that's what it was). I don't see what it adds as all it
would do is provide an indication of potential slack or "play" in
steerage.


The second option would be an auxiliary rudder similar to a typical
sailing dinghy rudder with a lifting blade to be used only when
trolling. This could be large enough to do the required job but quite
light and weak. If the rudder/boat response is good enough the
cheapest autopilot could be attached and used just for low speeds,
with a remote control if required. This arrangement would be a
nuisance and likely be damaged at speed so needs to be retracted then.
I would assume that any autopilot would net out yaw, but I don't know
and don't want to pay all outdoors to find out. Are you saying it's a
large factor?

No, I mean that most boats at normal speed turn more or less like a
car, with a turning circle radius that reduces with helm movement. The
assumption is that autopilots are designed for typical boats behaving
in this way. Observing autopilot behaviour, they seem to move the helm
in proportion to the error, although there is also a dead-band, some
integration of error and some damping involved. It seems they would
assume a rate of turn proportional to the error signal and actuator
movement whereas a jet or outboard at low speed produces a very small
transverse thrust in response to the angular error signal, causing
angular acceleration that increases the rate of turn with time.
It seems a better plan to try to fix the boat's low speed steering
than to expect a control system to cope with this. You need the gain
of the factors that relate angular heading error to the boat's rate of
turn (yaw rate) to be more or less constant and within the bounds of
the autopilot design, then all should be well. You don't want the
error integating up because nothing has happened yet!

My understanding of yaw differs from yours. You're probably correct.
My understanding of yaw is coupled with pitch and nets out but does
provide considerable movement. I _thought_ a fluxgate compass enclosed
a gimballed affair with sufficient time constants to do that (net out
pitch and yaw).

With the Sportpilot, it seemed as though if the time constant of the
response curve could have been lengthened, the system would have worked.
What I perhaps should have done was to measure the amount of time the
boat took to go from one compass point to another (say 45 degrees apart)
and compare that with what the current boat does, both at the same
speed. Actually, that sounds like a plan and I can still do that as I
will be seeing the owner of the Utopia in early July and will have my
boat with me. At trolling speeds, if the boat turns substantially
faster, then the old system might work - without a rudder!

Your explanation makes sense to me, but unless the control electronics
are very "smart" that gain has to be variable and it wasn't. Well,
there were, as I recall, three steps, but the longest response time
wasn't long enough and horrible overcorrection occurred at low speeds.
In fact, the longest response was what I had to use at higher speeds.

"Bigfoote" seems to indicate that a Sportpilot would work and, as I
recall, that was nowhere near as expensive as other units. Of course,
you get what you pay for, yada, yada, yada...

Thanks!
--

Larry R
rapp at lmr dot com

Autopilots
 

Jere Lull wrote:
On 2008-04-27 20:46:09 -0400, Larry said:

The autopilot I used had no sensor for speed. Does any? It would
seem to me that there needs to be one because response to the wheel is
proportional to your speed underway.

Haven't been paying attention to this thread, but my tiller AP will
"read" some NMEA speed sentence and the swing can be adjusted.

Do you know if reading the NMEA sentences replaces the fluxgate compass
or is used for longer range navigation (plotting a course with
waypoints). Thanks
--

Larry R
rapp at lmr dot com

Autopilots
 

On 2008-04-30 10:30:07 -0400, Larry said:

Jere Lull wrote:
On 2008-04-27 20:46:09 -0400, Larry said:

The autopilot I used had no sensor for speed. Does any? It would seem
to me that there needs to be one because response to the wheel is
proportional to your speed underway.

Haven't been paying attention to this thread, but my tiller AP will
"read" some NMEA speed sentence and the swing can be adjusted.

Do you know if reading the NMEA sentences replaces the fluxgate compass
or is used for longer range navigation (plotting a course with
waypoints). Thanks

I think I should have said "phrase" instead of sentence.

I'm a little confused as the speed phrase I'm talking about from the
knotlog wouldn't be affected by the direction the boat was pointing.
The AP can adjust rudder sensitivity and calculate current set from
that speed phrase, as I recall.

The input from the fluxgate would have a different phrase (If our AP
can take an external one).

Our AP calculates a specific-width "lane" between two waypoints,
adjusting its heading (against internal or external fluxgate) vs GPS
heading depending partially with heading to the next mark, but it's
more concerned with getting back into the lane.

--
Jere Lull
Xan-à-Deux -- Tanzer 28 #4 out of Tolchester, MD
Xan's pages: http://web.mac.com/jerelull/iWeb/Xan/
Our BVI trips & tips: http://homepage.mac.com/jerelull/BVI/