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Autopilots - how tight a course can they hold at low speed?

Old 11-06-2015, 03:53 PM
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Default Autopilots - how tight a course can they hold at low speed?

I'm thinking of an autopilot for a 22 foot boat with 150 hp outboard. One purpose would be navigating through bays and creeks at low speed to the dock. The autopilot would have to hold a pretty tight course at low speed, maybe plus or minus 10 feet on either side to avoid hazards. Is that doable, or expecting too much? Any suggestions on the better units for that purpose?
Old 11-06-2015, 04:39 PM
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I troll at 2 mph with a Garmin reactor ap with a smart pump and turn 180 degrees regularly. You'll be fine with any quality ap.
Old 11-06-2015, 04:43 PM
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You will want Simrad or Garmin for sure. Both have proven that they can hold course and slow troll very well.
Old 11-06-2015, 05:23 PM
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If you're at risk of running aground, I personally wouldn't rely on autopilot. There are too many potential points if failure in the system. They might be very reliable but it only takes one error to result in major damage.

If you're planning on keeping a very close eye on the AP to counteract this, you may as well have the wheel under your control.
Old 11-06-2015, 06:39 PM
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Originally Posted by OZFish View Post
If you're at risk of running aground, I personally wouldn't rely on autopilot. There are too many potential points if failure in the system. They might be very reliable but it only takes one error to result in major damage.

If you're planning on keeping a very close eye on the AP to counteract this, you may as well have the wheel under your control.
This. Most modern APs will usually keep you in a 5 foot circle. If you have a good GPs fix. If the current isn't too bad. If the wind isn't too bad.

So, if you're floating along alone in this narrow space you're probably OK.

Now, lets take Fred's third cousin twice removed and three sheets to the wind. Also trying mightily to navigate in that same space. No AP in the world can deal with him.

TL;DR - No, this is not a reasonable expectation of any AP system. It is not a safe idea. This type of maneuvering requires a human being at present. Boat APs are way less sophisticated that the ones coming down on automobiles. They are also one hell of a lot cheaper.
Old 11-06-2015, 08:20 PM
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If nav (waypoint) steering...which is moving GPS antenna from point A to point B not good.

If 'auto' using compass, good.
Old 11-06-2015, 11:18 PM
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With the right settings on the right boat and good GPS and compass inputs, a very high degree of accuracy is possible even with some wind and current. However, as noted by others, relying on an autopilot to steer in a channel with boat traffic or obstructions is a really bad, unsafe practice unless you are 100% on watch and ready to take control manually. A good example of where being able to do something does not necessarily mean it is something you should do.
Old 11-07-2015, 12:37 AM
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Originally Posted by dove72 View Post
I'm thinking of an autopilot for a 22 foot boat with 150 hp outboard. One purpose would be navigating through bays and creeks at low speed to the dock. The autopilot would have to hold a pretty tight course at low speed, maybe plus or minus 10 feet on either side to avoid hazards. Is that doable, or expecting too much? Any suggestions on the better units for that purpose?
That's expecting too much. In good conditions, your GPS accuracy is about +/-10 feet by itself. For your mission, that leaves no margin for error in the autopilot, and that's not feasible with any AP.

Even in ideal conditions, a perfectly tuned AP will have some cross-track error (XTE). Throw in variable wind angle and currents, and even a perfect AP could easily have >10 ft XTE on top of the potential 10 ft GPS error.

I agree with ColdWetDog. Many years from now, autonomous car technology (car APs) will filter down to pleasure boat APs. With multiple vision systems and sensors up the wazoo, marine APs will have access to enough technology to navigate a channel right to the dock. But right now, and for the next 10-20 years at least, human navigation will be the best way to do what you need.
Old 11-07-2015, 01:01 AM
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For such an application I would recommend a satelitte compass. It will both provide increased GPS accuracy and improved heading.

http://www.navico-commercial.com/en-...ass-en-us.aspx

Simrad HS70 has better than 3' position accuracy 95% of the time and 0.75% heading accuracy.

Other advantages with satellite compasses are insensitivity to magnetic disturbances and no calibration. I added the smaller HS60 a few weeks ago and it looks very good.
Old 11-07-2015, 09:16 AM
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With a heading sensor gremlin that nobody can help you with, you can expect errors of up to 400ft in my experience.

Get your gremlins sorted out and you can expect an accuracy of just a few feet.

I use the Lowrance AutoPilot.

http://www.lowrance.com/en-US/Produc...ack-en-us.aspx

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