NavList:

David Pike, you wrote:
"It just has moveable hands and no mechanism, as you might buy for a child to learn to tell the time.  I intend mounting it on the forward bulkhead under the real clock where I can see it through the cabin entrance.  Her previous owner tells me it’s a ‘shooting clock’ from a trawler.  It was to remind the skipper or oncoming watch what time he shot his trawl.  I’m not sure whether to believe him or not."

Hmm. Sort of a 'deadline indicator' then. It's possible, I suppose, but the story from the previous owner sounds like a simple legend. Someone told him the story, and he re-tells it. While it's possible that this thing was created or marketed at some point in the role described, I think your first theory is the most likely: it was made to teach analog time-keeping. The phrase "shooting clock" turns up nothing useful that I could find in Google and Hathitrust searches. The similar "shot clock" is common in sports, but that's usually for a short-duration countdown, and it may only require a running "seconds hand" if there's an analog display at all. There's a new clock in action this year in American sports. In this article (based on reporting in this paywalled NYT article which points out that the new pitch clock has "turned the clock back" in baseball by 30 years), there's an image of a pitch clock running down, probably captured from game video. It's a digital countdown. But then they could probably make it an analog countdown with a click of a mouse since the clock itself is probably virtually painted into the video of the game. The digital clock is a digital illusion...

For a purpose like the one described, why not just right it down? In the 21st century, working vessels often (sometimes? ...just a few I've been aboard) have a prominent whiteboard, usually used for dirty jokes, but when the work starts, it's important. You need to take note of something like the time when the trawl was put out, you just write it on the board in giant digits, put a big circle around it, and label it "trawl out" or something. Everyone can read, right? Even more likely on a 21st century vessel, you set an alarm, either on the ship's main system or on your phone. 

Can everyone read? The story of a clock like this reminds me of the era when literacy could not be taken for granted. Even in the 21st century there are illiterate people, at least in English, either elderly or from a very different language background, or from some unusual cognitive issue, but it's nothing like a century ago. My own grandmother, who spent her life working a sewing machine in a textile mill, was 100% illiterate until very late in life, and she had to develop various tricks for dealing with daily tasks. How do you find your favorite soup at the store? Easy. You look at the artwork on the label. How do you find your favorite cold medicine? Not so easy, but maybe by the color of the logo for the brand. And if you were working on a fishing boat, a simple analog clock-like sign could be set to display a target time or deadline. Even for literate crew, it's something that can be seen and read from a considerable distance (not necessarily your example, which appears to be difficult to read from a distance) so the idea is at least plausible. 

You wrote:
"I set HW on it to remind me when it’s time to turn and head back for Brough Haven.  We get about 1.5 hours either side of HW. "

All this tide-planning work still surprises me when I hear about it from boaters around Britain. It's an aspect of navigation that is far more important around the UK and in northwest European waters than it is for us Americans. There are a few areas near New England with extreme tides and tidal currents, like from Cape Cod and on north to the Bay of Fundy and Nova Scotia in Canada, but in many areas on US coasts, tides are not much more than a nuisance or a curiosity.

Frank Reed