Who is Richard Macfarlane? Here is his anchoring advice...

[MathiasW]

Originally Posted by aqfishing

So our new Moorings Leopard 40 has been sitting around waiting to be used for almost a year. It finally arrived in the Seychelles first week Jan. So we have a new yacht we have not yet seen as we cannot get to Seychelles.

This discussion prompted me to take a look at what anchors it was supplied with.

I was pleased to see it came with a 20kg Stainless steel DTX. Looking at the Lewmar website, I see it is one size over spec’d. Well done to Leopard and Moorings. Now I just need to get there and ensure that what was on the contract is what was supplied!

20 kg for your cat sounds sufficient, but it is not excessive. I have 35 kg on my 51' tri, which is very comparable to your 20 kg after scaling our different vessel sizes. But in hindsight, I would wish for more... like 40 kg.

[MathiasW]

For the sake of comparison, I have attached screenshots of my app for two different scenarios:

- shallow water (5 metres), massive swell, no snubber at all, chain limited to 50 metres
- somewhat deeper water (9 metres), massive swell, no snubber at all, chain limited to 50 metres

When you compare the anchor loads, you can see a massive difference: In the first case it is 1322 daN, whilst in deeper water it is "only" 480 daN, which is a little over a third of the first case. An anchor load of 1322 daN will most likely lead to dragging or the anchor breaking out altogether.

The swell is exactly the same in both scenarios, only the anchor depth differs. This only shows how deadly swell in shallow water is. The only remedy is to use excellent snubbers/bridles. The results then go down to a fraction of these anchor loads... And you even need less chain then...

Cheers, Mathias

PS: The swell energy is related to the vessel velocity at anchor. In this case I chose 0.6 kn, which is rather a lot...

[aqfishing]

Originally Posted by MathiasW

20 kg for your cat sounds sufficient, but it is not excessive. I have 35 kg on my 51' tri, which is very comparable to your 20 kg after scaling our different vessel sizes. But in hindsight, I would wish for more... like 40 kg.

Cool. Luckily there is mostly good holding ground in Seychelles. When we do take the boat to go cruising in a few years that will by my small spare anchor! I like peaceful sleep! Just need to be sure the windlass can handle a bigger one!!

[Tonali99]

Reality check...
Does one expect ANY anchor to work in those conditions?
Just off the beach in heavy breaking seas?

[Happ]

I’ve always used 5:1 in calm protected water. 7:1 more open bays. 10:1 for more open areas (surge) or breezy conditions. I ALWAYS use the added length from the water to the bow and a bridle with the connection point between the Bridle and the Rhode under water.

[MathiasW]

Originally Posted by Happ

I’ve always used 5:1 in calm protected water. 7:1 more open bays. 10:1 for more open areas (surge) or breezy conditions. I ALWAYS use the added length from the water to the bow and a bridle with the connection point between the Bridle and the Rhode under water.

In very shallow water, this may still not be enough (see my example above), but in deep water it may be absolute overkill. The point, really, is that this scope formula is so dependent on water depth and swell that it is hardly a reliable metrics.

[john manning]

I have had a lot of experience anchoring in bad conditions. I live in Wellington NZ so anchor in the Cook Strait and Marlborough Sounds. Neither area being friendly to anchored yachts.

My conclusion after 40 years is that there is no formula that works. The most significant factor I have found is the type of seabed the anchor is in.

So the "formula" I use is drop anchor, mark position on GPS, feed out at least 4xdepth of rode and mark position again. Stay on anchor watch for 30 - 60 min depending on wind strength. Change settings if not holding. If wind strength is above 40 kts then anchor watch is continuous.

[Moontide]

With all chain I use my own rule of thumb.



100' plus water depth. 10' water, 110 all chain. 100' water, 200' chain (although I actually put out more like 250 in 100' of water.


Also have a modern anchor (Mantus) which performs much better than any "plow" type anchore (I still don't understand why all the charter boats seem to use Deltas. A plow plows.

[redneckrob]

Originally Posted by MathiasW

Thank you Jon!

One of the highlights of my work is a better understanding of an anchor chain's elasticity.

By this I mean the following: If wind or swell pull (horizontally) a little more on the chain at the bow, say with 1 kp, how much *more* energy can the chain absorb? Obviously, you would want this value to be high. The enclosed graph is the elasticity of an anchor chain. Now, what does it mean?

Let's take a chain that is long enough so that there is still some chain on the seabed in front of the anchor. Within the framework of my model, you can then calculate the elasticity of the chain - see the attached diagram. On the left is calm, on the right is storm. The horizontal axis is scope, i.e. the ratio of chain length to water depth, whereby only the chain counts that is no longer in contact with the seabed. The vertical axis is elasticity divided by its peak value (which in turn is proportional to water depth).

So, there is an optimal working point for the chain! On either side of it its performance will degrade!

Perhaps this thought experiment will help you understand the graph better: I anchor at 5 metres, measured from the bow roller. There are 100 metres of chain paid out, but they are not being used yet, because there is almost no wind. I am now on the far left of the diagram. The elasticity of the chain is poor, but that doesn't matter because there is hardly any wind. Now the wind starts blowing stronger and stronger and we slowly move to the right in the diagram. When 1.4 x 5 = 7 metres of chain have been lifted from the bottom of the seabed, we are already at the maximum elasticity of the chain. Of course, this is still not a lot of wind - just 7 metres of chain are needed so that it still pulls horizontally at the anchor at a depth of 5 metres. So, now it's blowing more and more and we are moving further to the right, away from the maximum. When the 100 metres chain is completely off the ground, I have reached a scope 100 : 5 and thus the right edge of the graph. So what happens is that although we pay out more and more chain, the chain becomes less and less elastic. In other words, it is less and less able to absorb strong gusts or swells the higher the base wind is. Sure, it can absorb large static forces (provided the anchor still holds), but gusts and swell, no way. Consequently, you need a rather good snubber or bridle to absorb these peak loads. This effect is much more pronounced in shallow water than in deep water, and it is the reason why I tend to anchor in somewhat deeper water than others.

Am I misunderstanding you when I think you're saying the only elasticity the chain provides is when it lifts up from and falls back to the sea floor? Meaning once the last link is off the sea floor you're calculating that it has no remaining elasticity?

If that is what you're saying, isn't there still considerable energy required to straighten the chain, representing additional elasticity?

[MathiasW]

Originally Posted by redneckrob

Am I misunderstanding you when I think you're saying the only elasticity the chain provides is when it lifts up from and falls back to the sea floor? Meaning once the last link is off the sea floor you're calculating that it has no remaining elasticity?

If that is what you're saying, isn't there still considerable energy required to straighten the chain, representing additional elasticity?

No, I am not saying that once the last link is off the seabed, there is no further energy gain possible. As the links get pushed higher up in the water when applying more force at the vessel's bow, energy will increase. This effect is fully accounted for. It is just that once the is already a huge tension on the chain and it has all come off the seabed, the additional gain for storing more energy becomes smaller and smaller.

Of course, there is also elasticity in that each chain link can stretch elastically a tiny bit. I mean the metal itself. This effect I am not including, but Bjarne does in his model.