ANCHOR SWING AND THE INFLUENCE OF TIDE
Reproduced from Active Captain:
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 Pythagorean 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.
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 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.