Mathematic approach to anchoring scope

[Dockhead]

Originally Posted by MathiasW

. . . And finally, if we model swell as an energy burst impacting the vessel, then again, more chain has more room to store this energy, which is again pointing at going to deeper water. This argument assumes that the swell is the same in shallow water and in deep water, which often it is not. But the SAME swell is easier to deal with in deeper water.

Using good long snubbers or bridles will help you to stay in more shallow water, but at least the chain wants to go to deeper water... (I mean, not only gravity-wise... )

All this, I am sure, is obvious to the experienced sailor and is preaching to the choir, but for me it took me a little while to appreciate the nuances and implications...

A lot of experienced sailors don't really understand the "nuances and implications" of this stuff -- take my Dad. Nor would I claim to fully understand it myself. So these conversations are always worthwhile.


Concerning swell -- yes, this is the "energy burst" which we have to deal with. The shape and not just magnitude of it. Swell tends to be steeper in shallower water, and steep means the energy is transferred over a shorter time period, so more peak force, which is precisely our problem -- dealing with these peak forces so that they don't smash anything.



So deep water is doubly beneficial, in my opinion. If I'm expecting bad weather, I try to avoid anything shallower than 10m or 12m.

[MicHughV]

yes, but when you add a prefix to the word " rope....ie, boltrope, then the word " rope" is no longer a plain "rope"...it is now a "boltrope" a " rope" with a specific function, even though the word " rope" is still in the mix.

....I don't think this came about to be fancy or otherwise a scheme to impress a landlubber, but more a throwback to old time sailing ships, so you would not hear the command " throw me that rope" and get confused looks....."ropes" on a boat have a specific purpose to identify a specific task...this is why they have specific names...pretty simply really....

[Dockhead]

Originally Posted by MicHughV

yes, but when you add a prefix to the word " rope....ie, boltrope, then the word " rope" is no longer a plain "rope"...it is now a "boltrope" a " rope" with a specific function, even though the word " rope" is still in the mix.

....I don't think this came about to be fancy or otherwise a scheme to impress a landlubber, but more a throwback to old time sailing ships, so you would not hear the command " throw me that rope" and get confused looks....."ropes" on a boat have a specific purpose to identify a specific task...this is why they have specific names...pretty simply really....

Well, I think it IS a scheme to impress landlubbers. A scheme by and for landlubbers. You never hear really experienced people criticizing someone for using the word "rope". It's kind of hazing done by people who've had the first three sailing lessons, to those who are about to have the first one.

"Rope" is like the word "animal". It's a generic term for a certain type of cordage or for pieces of that cordage. Would you ever hear a farmer tell a city person, "we don't have animals on the farm -- once they get on the farm, they can only be cows, pigs, dogs, etc." And sneer at the city guy for using the word "animal", as if that's a sign of ignorance. No, you don't, because there's nothing wrong with having a generic word for different kinds of animals. If you need specificity you use the specific term. If you don't, you don't. Farmers do use the word "animal", and real sailors do use the word "rope" -- when it's appropriate, when a more specific term is not needed.

They say "learn the ropes", after all.

[MicHughV]

you said it yourself..." learn the ropes"...can't add much more to this than that expression !

[Dockhead]

Originally Posted by MicHughV

you said it yourself..." learn the ropes"...can't add much more to this than that expression !

Well, sure. Certainly, you need to know what sheets and halyards are, if you want to sail, or even not be in the way, on a sailing vessel.



But knowing what sheets and halyards are no more banishes the word "rope" from your vocabulary, than knowing the words "sheep and goats" banishes the word "animal".

[MicHughV]

ok...you win !

[StuM]

Originally Posted by MicHughV

you said it yourself..." learn the ropes"...can't add much more to this than that expression !

If there were no ropes on a boat, you couldn't learn them.

It's those who haven't "learnt the ropes" who perpetuate the "no ropes on a boat" false trope.

[MicHughV]

ok, you win as well.

[MathiasW]

In case somebody noticed and wondered why I am posting in the middle of my night when I should be asleep...

In the late evening I had noticed a new small vessel anchoring some 200 m away from us - well that was where they had dropped the anchor at least... During the night I watched it slowly drag past us... So I was on anchor watch for quite some time...

[Dockhead]

Originally Posted by MicHughV

ok, you win as well.

Don't be offended, please

This is a conversation which gets repeated on here from time to time. Stu and I are notorious pedants; don't take us too seriously

[MicHughV]

ok...since you have offered me a beer...all is forgiven

[Dockhead]

Originally Posted by MicHughV

ok...since you have offered me a beer...all is forgiven

[Lodesman]

Originally Posted by Dockhead

Most of the places I know where people anchor chock a block are coves with good shelter, shelter which is not available in the open roadstead, so maybe not such a "sheeple" phenomenon as that.

That may be true of some places, but I'm sure it's not wise to be chock-a-block in a big blow, regardless of what shelter's on offer. IME, even when there's a vast expanse of similar anchoring ground, the punters will flock together in the spot where there's an anchor sign on the chart; and when there isn't they cozy up to the only other boat that's anchored there.

You don't "sail around wildly" but nevertheless it takes a lot of room to deploy 135m of chain, which creates a swinging circle of almost 1.5 cables, which doesn't fit in a big majority of really sheltered places you might want to be anchored in in a big blow. That gigantic swinging circle also covers a lot of seabed and a lot of potential snags. A circle with radius of 135m covers more than 57,000m2 -- almost 6 hectares! -- .

In an assumption that the wind will shift gradually through 360º, all while maintaining hurricane force, you might have to worry about sweeping that real estate. In real conditions, you'll move the same length of chain, and sweep an identical amount of seabed, regardless of how much undisturbed chain lays resting on the seabed.

[Rothblum]

"Have you ever done scale modeling? I was referring to your simplistic analogy that implies a 30ft cruising boat and a 1000ft oil tanker react in the same way."

I spent 31 years testing ship models at the US Navy's David Taylor Ship Model Basin, and the answer is yes, you can scale the reaction of a 30 ft model to that of a gigantic oil tanker. The trick is to express the significant variables as dimensionless numbers. Then, if the large scale version of the model has same dimensionless parameters as the model, then the behavior will be the same. The problem is that not all significant parameters can be duplicated simultaneously, or that some variables might be significant at model scale and not at full scale -- surface tension, for example.

[Dockhead]

Originally Posted by Lodesman

That may be true of some places, but I'm sure it's not wise to be chock-a-block in a big blow, regardless of what shelter's on offer. IME, even when there's a vast expanse of similar anchoring ground, the punters will flock together in the spot where there's an anchor sign on the chart; and when there isn't they cozy up to the only other boat that's anchored there.

Well, it would be a bad choice to have to make, between worse shelter but no crowding, on the one hand, and crowding on the other hand. It's not an issue I'm familiar with. Not many "punters", where I sail, and no anchor signs on the charts. Sometimes no charts.

Originally Posted by Lodesman

In an assumption that the wind will shift gradually through 360º, all while maintaining hurricane force, you might have to worry about sweeping that real estate. In real conditions, you'll move the same length of chain, and sweep an identical amount of seabed, regardless of how much undisturbed chain lays resting on the seabed.

In my experience, or at least where I sail, really bad weather is usually accompanied by a significant wind shift, which can turn good shelter into a lee shore, which is really dangerous. And can do so in a way not predicted by the forecasts. That's why for really bad weather, we normally try to get into a cove with shelter from all or most sides. Swing room is often, maybe usually, limited in such places. Even if you have the swing room, you would be loath to be dragging the whole bottom of the whole cove with 130m of chain out, as the wind shifts. Even 100m is too much in many such places, which is why I'm often on short scope.



That is why the idea of just finding a wide open space and putting out a massive amount of chain, doesn't work for me. I guess maybe storms might work differently in different latitudes, without such wind shifts as we get here, so YMMV, perhaps.


In really high latitudes, shore ties are kind of SOP for bad weather. That is a technique I have used in the Aegean, but didn't use in the Arctic. It's very awkward to do, if you don't have drums on deck with very long lines. The problem with shore ties is they prevent you from swinging, so the anchor may be pulled out if you end up abeam to the wind. The answer is shore ties from more than one direction, and to do that takes a massive amount of rope, hence the drums you see on the decks of true high latitude boats.