Wow this video has well over 2 million views and, IMHO, the conclusion is all wrong. What do you think?
It’s pretty bad. Not only do they perpetuate the myth that friction of the chain against the seabed provides most of the holding power, but the following discussion of “catenery” fails to explain how it contributes to holding power. The illustration of seaweed also shows that they have never experienced this in practice.
The fact that they do not provide a definitive answer to what would happen if an Iowa class battleship dropped anchor at full speed, but instead cop out with a “probably” and fail to address what would happen to the windlass, amply demonstrates the level of substance.
What they do get right are some of the mundane details, which makes me wonder who this is i for. At first glance you’d think it’s aimed at children, fascinated that the big boats go “toot.” However, what do they care about such mundanery as paint markings and arcane distance units?
To answer your question, I think the video delivers exactly what you could expect from a six minute presentation titled “The truth about anchoring huge ships.” I’ve already had to tell Youtube in no uncertain terms that I’m uninterested in this channel, and now that I watched one of their videos to the end, I’ll probably have to go through it again :-/
I have dived in about 100’ on a few cruise ship anchors on sand when its blowing 10-15kts, in every case the anchor was standing up on the bottom with a huge pile of chain over it.
Ships were 100% holding on chain friction on the bottom some distance from the anchor pile.
If you get to needing the anchor you are on the edge of holding
I have indeed observed my own chain pile. In my experience, the chain pulls taught at a small fraction of the wind speed required to drag on sand, say 1/4 or so. On clay the holding contribution of the anchor is much greater. Granted, my experience is with much smaller ground tackle than that of “huge ships”, typically 5/4" chain and 250kg JIS anchor. Also, anchoring at greater depths changes the relationship, since you get more chain on the bottom at a given scope. Still, I believe that the anchor itself makes a non-trivial contribution to holding power, even with large vessels.
I seem to remember reading an interesting research paper on the matter, where various anchors were dragged by a winch, resulting in a distance / force plot that would have been very useful in this discussion. It’s the one that documents JIS anchor inversion. Anyhow, I can’t find it off-hand; does anyone know which paper I’m talking about?
Yes sure the anchor can add to holding but if your relying on the anchor the chain story is fading away.
Thinking about the times its hard to get the anchor up is in clay etc.
I have dived on lots of megayachts back in the day 40-60m and same story as the cruise ship.
Lots of trenches in the sand as the flukes dont bite in but just drag till there is enough chain out to stop the vessel.
Rigs dont anchor with ship designed anchors for a reason…
Good paper here:
The video had good production values but weak on the technical side… It does a good job conveying the role of catenary in absorbing energy, otherwise not so good.
Our AC 14 anchor was 8.4 tons, the 81 mm chain 4 tons / shot (shackle)
Admiratly stockless dont work in hard sand…everyone just gets away with it 99% of the time.
The other important factor is the environmental forces involved. Many people including mariners have exponential growth bias. Wind force increases exponentially with speed.
Spent a fair amount of time anchoring in marginal conditions. In my experience these numbers / graphs hold up good in practice.
This is all assuming the ship doesn’t start jacking around in the wind, turning beam to the wind/seas.
Another problem is changes to the superstructure. A lot of older offshore vessels have modules welded on the superstructures or stack more boxes on deck than the ship was designed to carry. But the anchor and chain is rarely ever upgraded.
Even a lot of new drillships and such have anchors sized for the hull form only and don’t take into consideration the Derrick.
Although there are some mistakes in it the video was decent effort for a layman who had no prior knowledge on the subject, if he had used that paper as a reference he could have made it a lot more scientific and accurate.
The problem with videos like that is that cadets will watch it and think it is true and start repeating the inaccuracies.
Thanks. If I read that right, in your case the friction of the anchor chain would give equal holding power in mud to that of the anchor when you have about 20 shots on the bottom. In my case, that equilibrium would be reached at only one and a half shots. This does not rhyme with my observations, but I trust your source more than my gut feeling.
Don’t know for a fact but I believe that smaller vessels have more holding power in the anchor gear in proportion to the forces created compared to a larger vessel.
Yes more holding power particularly with the new claw anchors but also, unless they are using all chain (which a small minority do) a lot more elasticity in the system.
I would agree for our WAFI case (48’ frp sloop) - in diving on our anchor in various places, I’ve noted that even in mild (10-12 knots) conditions the chain is free of the bottom (actually sliding back & forth atop the sand) clear back to the dug-in anchor. We generally set to about 7:1 scope and we’re all-chain, though we use a 20’ nylon snubber to take off some of the shock loads.
Edit to add - in my experience, experienced cruisers are almost always using all-chain, while weekenders and fishermen use nylon (or worse/cheaper!).
I anchor a 65’ T-Boat 2 or 3 times a week using an undersized (it’s the “lunch hook”) plow anchor with 8’ of chain and a nylon rode (75’ total). We’ve anchored in shallow and clear enough water to see that the anchor and most of the 8’ of chain is buried in a sand bottom (8’ water depth). It wont hold in anything over 15 knots of wind due to the large sail area.
On a calm day, say 5 knots or less, even with the nylon rode, there is a substantial catenary.
On another note, I used to own a small lobster boat and used a Lewmar “Claw” ( Bruce knock-off). Best small boat anchor I’ve ever used. Always found it and at least a couple feet of chain buried.
Sounds about right.
This is what I heard as well. It does tally with the fact that larger vessels typically anchor in deeper water, and thus have a greater length of chain on the seabed. However, it does not tally with the numbers I’ve plugged into the formula so far.
There is clearly something wrong with my interpretation of the model, though. I saw it as two stacking values, one for the pull-out force of the anchor, and one for chain friction. A closer examination reveals that this is clearly not the case. The model assumes that the holding power of the anchor is constant, regardless of the weight of the chain, and thus catenary, which we all know to be incorrect. Furthermore, it calls for the total amount of chain paid out, and not the length of chain on the bottom, which would be the relevant measure.
I must conclude that the model seeks to incorporate catenary action, and thus assigns an importance to the weight of the chain that goes far beyond its friction on the seabed. This makes perfect sense, since it was developed to aid mariners in determining the allowable anchoring envelope, not to settle academic disputes about the relative roles of various friction components.
I’ll have to do some digging to get to the heart of this. It’s an interesting subject, especially since there is a widespread belief in conflict with my empirical observations.
Edit: This was in response to @Kennebec_Captain, I have no idea why I replied to myself.
The “Anchor Bible” for Offshore Marine Operations:
Here is the 2015 edition in PDF format:
Definetly from the history section in the library.
Yes, the goal of the exercise is to come up with a “critical wind speed” below which the risk of dragging is considered low. Having a specific number is often useful for operations.
The formula uses the amount of chain laid on the bottom “L” with “Lc” being the amount of chain veered out.
That said it would be interesting to know how much chain is equivalent to an anchor.
An 8 ton AC14 anchor in sand would have 8 tons * 7.0 = 56 tons
Chain at 4 tons / shackle , holding factor between 0.75 and 1.0 to get 56 tons of holding from chain alone would require between 14 and 19 shackles of chain laying on the bottom.
Assuming I didn’t make a dumb error somewhere. Seem right to you?