Tuesday, April 20, 2021

OpenMarine and Signal-K

 I heard about these less than a week ago.

This is very interesting. Very interesting.

I have a partially complete IoT anchor alarm.

The idea of leveraging the boat's other devices through a Signal-K interface is appealing.

The problem is, I don't want the power draw.

I think I want to continue on the path of a small, thrifty, stand-alone device that can be used when the rest of the boat's systems are mostly shut down.

However. Seeing these projects causes me to rethink my use of Arduino and C++. While the Arduino is the thriftiest possible device -- the power consumption is negligible -- I think that a small upgrade to a Python-based device might make the software a tiny bit simpler. 

An Arduino Uno, specifically, is just barely capable of the UX I was hoping to build. The two-line LCD with a "mark" push-button, an "anchor circle" knob, and a "display page" button is right at the limit; I'm using analog inputs instead of digital for the buttons.

A Raspberry Pi can support more sophisticated displays, at some cost in power consumption. An e-ink display might be a better choice than the two-line LCD because -- well -- anchoring details change slowly.  Once you've drifted too far (or have a consistent COG away from the marked point with a steadily growing distance) then the alarm sounds and the display is more-or-less irrelevant. You're going to get up, and eyeball the situation to see what's going on. Wether or not the display updates doesn't matter much. 

We haven't drifted very often, so I don't have too much data.

  1. Once we slid to a new position. It was a stormy, blowy day. We eased out more scope. And we never moved again. We stood on deck, taking visual bearings. An e-ink display of the details wasn't what we depended on.
  2. We used to rely on an iPhone app to monitor our position. (We've switched to using SafeAnchor, we used to use an older app, no longer available.) We were moving slowly, but steadily. It was during a hurricane, we weren't surprised. We started the engine, raised the anchor and motored to a new place to reset. Again, we weren't using the display on the phone, we were looking at Pungo creek.
  3. And once we were not on the boat when she moved. That would have been awkward for our neighbors. So. We'd need to have a "reset the anchor alarm" switch in the cockpit. This would mark a new position. Fatty Goodlander's advice is to leave a big sign with a string showing them where it is.

A full 1.0 knot of speed is 1.7 feet per second. We often have 50 feet of line, meaning any movement under 100' is likely ordinary boat motion. That means 30 seconds until we're suspicious of a problem, and a full minute before we must sound the alarm. (In the middle, a constant COG and increasing distance is a leading indicator of trouble; alarm chirps might be helpful.)

As intermediate data gathering format, the Signal-K data stream is appealing. It steps away from the NMEA GPS talker messages. It's heavy-going for an Arduino Uno. But. Might work out well on something a little bigger.

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