gdy musimy bawić się „w aptekarza”.

Trochę OT, bo bez odniesienia do AC i "aptekarstwa",

So the math major in me wants to bring up another topic. It's a detailed
one that nearly everyone ignores. You see, so many of the responses
involved explanations of how the navigation product or app used for
anchor alarms draws a circle. Other people watch the track their boat
creates and the circle drawn as they swing with wind and tide.

Circles, circles, circles.

And yet, none of them should look like circles.

So I checked a couple of anchor alarm apps to see what they do. And sure
enough, some draw perfect circles based on your alarm distance. In all
cases, the circles are drawn over Google Maps, satellite imagery, or
nautical charts. And all of them are drawing the swing area incorrectly.

By now, some of you are saying that I've lost my mind. Of course it's
a circle with the anchor in the middle and the radius equal to the alarm
distance. The thing being forgotten is that the Earth is not flat. It's
a sphere. And nearly all the map data, satellite imagery, and nautical
charts are drawn with a Mercator projection.

The Mercator projection of the Earth produces some exceptional features
that are perfect for navigation. The main one is that bearing lines
appear as straight lines. In nearly all other projections, maintaining a
constant bearing will create a curved line. Not so with Mercator.

The disadvantage of the Mercator projection is the distortion that
happens as you move away from the equator. This is what makes Greenland
look huge.

So the problem with many existing anchor alarms is that the alarm circle
itself needs to be projected through the Mercator projection too. But
doing that involves a lot of trigonometry and many of the developers
writing these apps don't have a good understanding of it. The reality
is, a proper anchor alarm circle projected through the Mercator
projection should look squashed and not perfectly round as long as
you're not near the equator. As soon as you start to get 30 degrees of
latitude away from the equator, the circle will not look like a perfect
circle.

So go check your anchor circles drawn by alarms. Are they squashed?
If not, perhaps you should be using a different anchor alarm.

A second fun exercise is to plot your track through tide cycles as you
swing around your anchor. That won't ever be a circle either - if you
look carefully, it will also be a little squashed.

The effect of tide upon anchor swing is another one of those surprising
results that many people don't think about. Let's assume the tide swing
is 10 feet. If you anchor at dead low tide, it sure feels like you should
move 10 feet closer to your anchor at high tide, right? OK, maybe 8 feet
instead?

Those feelings aren't even close. Tide has surprisingly little effect on
your swing. It's easy to show using real numbers and the Pythagoean
theorem to measure the changes to the triangle at low and high tide.

Using some real numbers, let's say we're anchoring in 10 feet of water
at low tide and your rode is attached to the boat 5 feet off the water
line. You put out 75 feet of rode. That makes a triangle with a side of
15 feet and a hypotenuse of 75 feet. Solving for the distance along the
sea floor from the anchor to your bow gives 73.48 feet. That's how far
your bow is from the anchor when fully pulled back at low tide.

Fast forward to high tide and you're now in 25 feet of water. Solving
for the distance along the sea floor gives 71.71 feet.

So the difference with 10 feet of tide is a whopping 1.77 feet. That
feels all wrong but it's right. Tide has very little effect on changes
to swing distance.

But...

Tide can have a big effect on scope. And scope provides confidence in
the anchor set and the amount of force you can withstand before
dragging.

At low tide in this situation, your scope would be 75:15 or 5:1 which is
a pretty good scope for the night.

Now switch to high tide and you're at 75:25 or 3:1 which is
significantly less and might not hold if the wind comes up.

So when anchoring, ignore the swing effect of tide but always consider
the reduced scope you'll experience at high tide.

Czy do opisania alarmu kotwicznego w OpenCPN wykorzystaliście „perfect circles” czy może „squashed circles”?

Oby więcej takich OT,

gdy zamierzamy ustawić na odbiorniku GNSS alarm kotwiczny to powinniśmy pamiętać nie tylko o długości wydanej liny/łańcucha kotwicznego ale także o długości naszego jachtu.

Często tak sam sobie słodzisz?

W uzupełnieniu głównego tematu: traktując alarm kotwiczny jako "ostateczną ostateczność" sporo informacji o zachowaniu się jachtu na kotwicy można uzyskać z precyzyjnego zapisu śladu. W załączonym przykładzie zmieniałem kolor śladu co (chyba) 12 godzin i ew. popełznięcie kotwicy byłoby szybko zauważone...

But the reality is that cruising puts your anchor into mud at
times. Thick, sticky, dirty mud. The kind of mud that coats your line or
chain. The kind of mud you have to remove before storing your rode back
within the bow of your boat.

(...)
Natomiast w trzecim artykule poruszona jest kwestie cumowania na wodach pływowych, niestety skupiono się na „skoku pływu” a moim zdaniem bardziej ciekawe(ważniejsze/niebezpieczne?) jest zjawisko prądu pływowego.

Tide swing I tide range to dwie różne rzeczy... I artykuł, który uważasz, że mówi o skoku pływu, mówi o efektach prądu pływowego i dlaczego nie jest on tak istotny jak Ci się wydaje Chyba, że załączyłeś coś innego niż zapowiadałeś...

koleżanko, w moim rankingu zaczynasz rywalizować z kolegą „bratanek65”, tak samo łechcesz moja próżność.

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