Mathematic approach to anchoring scope

[MathiasW]

Great question!

There are, in fact, two ways to account for gusts in the anchor chain calculator app. One is by using the wind strength input field, the other is by using the velocity at anchor input field. Both are marked with a red circle in the screenshot attached.

Now, if the duration of the gust is extremely short, then the most accurate way is to key in the base wind in the wind input field and use the maximal reverse velocity at anchor as measured on your chart plotter in the velocity at anchor field. (Only the component of the velocity vector that is pointing away from the anchor.)

You may ask what extremely short means... well, the vessel must still be moving well backwards under the influence of the gust when the gust stops. That is short. So, it is like getting hit by the gust for a split second.

On the other hand, a katabatic wind is usually a longer lasting gust that pushes the boat for quite a while, way beyond the time it has reached its maximum distance from the anchor. In this case, it is more appropriate to use the value of the gust as input for the wind strength and then use again the maximal reverse velocity at anchor for the second input field.

To be on the safe side, I usually always use the latter approach.

Cheers, Mathias

Anchor Chain Calculator

[Johan Leopard51]

I came here looking for answers but remain full of questions.

We are mostly in tropical waters (precious coral). The winds don’t normally, but can vary and related to this is current around islands that vary. So even in light wind, people that put out a lot of chain should really do themselves a favor and go snorkel to see what that chain did to the coral :/ (we are trying to get a thing going with government to put out proper buoys and implement massive penalties for not using them where provided)

On our last trip we twice dragged anchor. Had about 4x out in moderate about 9 knot wind and think it was under 10m water. The bugger was current. Cat, on bridle, would often sit side on to wind because of current, probably presenting a larger “sail” area to wind so that caused drag?

My main question : how does one figure out the “best” anchor for your vessel? I have a few because have recovered anchors from reefs where I must assume somebody panicked or got it all horribly wrong - chain still attached. We try and always anchor in a large sandy patch mainly because there are very large coral growths in Seychelles that you may not spot easily and that would be sad to damage or sad if they damaged us. Virtually or actually NO MUD AT ALL.

[MathiasW]

I appreciate and share your concern for corals!

Finding a sandy spot is one thing, the 2nd task is to pay out enough chain - but not more than that - to keep the swinging circle as little as possible. The anchor chain calculator can help you with figuring out what this optimum is.

The beauty is that one can split the problem at hand in two. The first problem is how much load does the anchor need to absorb. That's what I have been addressing in all my work. The 2nd problem is what anchor can achieve that, and this is an entirely different question.

In my view, this 2nd question is best answered by real tests of a wide selection of anchors. Have a look at the top of this forum section. The latest generations of anchors perform consistently much better than the old ones that have been around already for decades.

Coming back to your original concern. An approach to protect corals is to use a series of incompressible floating buoys attached at regular intervals along the chain. They need to be strong enough to lift the chain off the seabed in calm weather. How it works physics-wise is described here (rather towards the bottom of that page):

https://trimaran-san.de/en/die-kette...atiker-ankert/

Despite the German in the link, it is actually in English, well, my English...

Cheers, Mathias

Anchor Chain Calculator