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..