I was thinking about auto-pilots and their inputs. My gut tells me they must have an actual heading input: sat-compass, or gyro, or magnetic (electronic) xxx.x input. Some situations you also get a derived COG, course over ground, and that also translates to a xxx.x number. If you’re bopping around near a dock, that number will fluctuate wildly, but you’d never be using autopilot tied to a dock. The more I thought about it, using COG for auto-pilot should work. Does that make sense? Bad idea? Why?
Oooo yup. You’re exactly right. With any swell, the COG would shift pretty quick. Of course, the actual HDG would also shift… but it would act very differently. My gut tells me this is a horrible idea - but it the idea really got me thinking.
Ended up fixing the sat-compass, and using standard HDG for the auto-pilot. Thanks all! Exercise in thought I suppose
It would depend on the autopilot and the inputs that it recognizes.
Whether it would or could be a bad idea would depend on the vessel you’re operating and where you are. If you were on a small vessel just crossing the Gulf of Alaska in nice weather and no traffic, no problem.
If your COG was likely to shift rapidly due to weather or other influences and you were in traffic or large seas the resulting rudder could be catastrophic.
Oooo yup. You’re exactly right. With any swell, the COG would shift pretty quick. Of course, the actual HDG would also shift… but it would act very differently. My gut tells me this is a horrible idea - but wanted some other opinions.
Isn’t it the case that NMEA heading outputs come from heading devices, but downstream devices don’t “make” HDM and HDG output sentences, just repeat them? Just a thought…
I can’t think of a reason it wouldn’t be preferable in most cases to use COG for the autopilot instead of HDG. I constantly wish my autopilot worked that way.
It does, if it has a Nav mode. The plotter/gps will compensate for set and drift and maintain navigation as COG. It compares to heading to calculate course-to-steer to reduce cross-track-error.
I guess my question now is, how many boats out there have an autopilot that runs off the magnetic compass, so that if everything else fails you’re good to go?
(The second question is, how many vessels removed their magnetic compass and converted it into a lamp or ashtray or something).
There is a rule for my vessels to carry a 12" bell. Presumably for signalling in fog. Obviously never used. But every two years the captain roots around for the cardboard box the bell is in, stuck in some locker, to show the bell to the inspector, who solemnly crosses it off his list.
Sometimes I wonder if this isn’t case with magnetic compasses on some boats nowadays.
On my recreational boat I have a solid state compass feeding the autopilot directly. I also have a digital magnetic compass mounted and ready to plug in if the solid state fails. I’ve tested both and the magnetic works fairly well in stable seas but can get stuck sometimes.
A magnetic compass that outputs digital nmea is called a fluxgate compass.
I’ve seen a few boats with two sat compasses with a switch selector as one input and the mag compass as the backup input.
There are places on the Alaska peninsula/Aleutians where you can run into some pretty severe variation and running on autopilot with a mag compass as input wouldn’t be wise.
I don’t believe I’ve seen a vessel with the mag compass removed, but I’ve seen more than a few with correction cards dated 1997 or so.
There is a rule for my vessels to carry a 12" bell. Presumably for signalling in fog. Obviously never used. But every two years the captain roots around for the cardboard box the bell is in, stuck in some locker, to show the bell to the inspector, who solemnly crosses it off his list.