Substantial reef damage from boat anchor... again

[letterboy]

@Wookie how does it work as it seems they were told where to anchor and the Anchorage is within a protected area? It is ultimately the captain of the vessels call on where to anchor, correct?

 

[Insta-Gator]

If the scope is too large the anchor will bite or, set into position but the vessel will most likely surge and drift as forces act on it. In this case we will use the same water depth of 60 feet (18 M) but increase the length of the rode to 600 feet (180 M) this gives us a scope of 10:1 which is not inappropriate if winds or currents are very strong but is not the best ratio for general anchoring.

The scope that is best for keeping the anchor set and keeping tension on the anchor rode is around 7:1. If we plug our numbers into the formula a water depth of 60 feet (18 M) will require a rode of 420 feet (126 M).

A scope of 7:1 will not pull the anchor free but it will maintain tension for a safe and comfortable stay in the anchorage.

 

[Jax]

Think about it. 14,000 square feet is an area 140' x 100'. On any day I anchor in good holding ground, I will lay out 3-400' of rode in 50 feet of water. Spree is 100 feet long. Tattoosh is 300 and something feet long, so they probably lay out an equivalent rode in chain . 3/4" stud link, maybe 1" stud link. The day after they anchored, a norther came through, which would have shifted Tattoosh on her anchor and spun her around. I can easily see this kind of damage. I think 14,000 square feet is even a small estimate.

How do boat anchors work? Well, they work by adding tons of weight to the bottom, and making it difficult for the wind to push against the hull. The Spree is pretty light, 70 or so tons displacement. We do not use a chain rode, we use rope for ease of handling, and because chain isn't necessary. As light as we are, we get pushed against the rope rode and it stretches, and we bounce back. A big steel hulled vessel displacing many hundreds of tons will get inertia up as it swings on the anchor, and it takes a lot of weight to slow down and stop the swing. The chain lays on the bottom and provides that weight. The more the chain is tangled up on the bottom (plowing through silt, mud, sand, and yes, coral), the less chain is needed to stop the swing. The anchor's job in this case, is to keep the chain from slithering off in the sand. The anchor "anchors" the end of the chain. It's the chain that does all of the work.

If the scope is too large the anchor will bite or, set into position but the vessel will most likely surge and drift as forces act on it. In this case we will use the same water depth of 60 feet (18 M) but increase the length of the rode to 600 feet (180 M) this gives us a scope of 10:1 which is not inappropriate if winds or currents are very strong but is not the best ratio for general anchoring.

The scope that is best for keeping the anchor set and keeping tension on the anchor rode is around 7:1. If we plug our numbers into the formula a water depth of 60 feet (18 M) will require a rode of 420 feet (126 M).

A scope of 7:1 will not pull the anchor free but it will maintain tension for a safe and comfortable stay in the anchorage.

 

[kelemvor]

Wow, who knew? I guess you guys did. Thanks for the lesson!

 

[Littlerayray]

Contrary to what many would think, it's the weight and length of the chain laying across the bottom that keeps a vessel in place, not the actual anchor itself.

If a vessel didn't have a chain on the anchor line every time the vessel was raised by the crest of a wave the anchor would be picked up and moved.

People are often shocked when they buy a boat and find they need an anchor chain 2, 3, 4 times the length of their vessel, depending on the depths they intend to anchor at.

Voreection you need a 7 to 1 ratio for anchoring that means for every foot of water you anchor in you need 7 feet of rode in the water to be effective when I took my seamanship course this is what we were taught when anchoring this gives the proper angle for the anchor to grab this being said in 10 ft of water in rough current or seas you would have 70 feet of rode laid out now that being said you can give or take 20 feet depending on conditions the calmer the seas the less rode you need

 

[Littlerayray]

Sorry Jax we responded at the same time

 

[Wookie]

@Wookie how does it work as it seems they were told where to anchor and the Anchorage is within a protected area? It is ultimately the captain of the vessels call on where to anchor, correct?

No, they (the port authority in Georgetown) are very specific where they tell you to anchor. Most mornings there are a number of boats in to clear customs which is (was) done pierside, so they anchor you up to get up to the pier in order. They have to keep an egress and ingress clear, so it isn't all willy nilly anchor where you want. Most boats (including mine) will have the sounder on to tell what the substrate is (on my sounder the sand is yellow, mud is thicker yellow, and rock/hard bottom is red), but the sounder isn't under the anchor, you only know what is going on under the sounder, not under the bow.

In the accounts I've read, it was the crew of the Tattoosh who told the PA that they had damaged the coral. As anyone would.

Almost every port in the world has a designated anchorage. Key West has a massive one, even Dry Tortugas National Park has a designated anchorage. Sometimes there are coral heads in that designated zone. Those anchorages are historical, some going back hundreds of years. The anchorages were designated before anyone cared about coral or fish. My point is that the corals in these designated zones is sacrificial. Or, if it isn't, change the laws.

 

[The Chairman]

I understand the wiping action @Wookie referred to and that makes sense. Thanks dude. However, where is the 'white'? Every time I've seen damage to a reef, I see lots and lots of white on each and every broken part. Exposed rock gets smaltz growing all over it. Corals have color too. Break rock or coral and the white calcium innards stand out dramatically against the smaltz and the other colors. I'm just not seeing that in the video. Also, where are all the fish? They love structure and I'm seeing all sorts of structure here. I always see fish in and around damaged coral.

Don't take this as criticism: I'm trying to understand the process and what the damage is I'm seeing.

Also, while it's cool to bash the rich guy, he didn't set the anchor(s). I've been on boats that have set both bow and stern anchors when the wind was going to push them around. When a hurricane was passing within a hundred miles, one liveaboard I was on put out two bow anchors and a stern anchor to boot. I slept like a baby that night.

 

[Kharon]

Quite honestly, the video doesn't look different from what I saw diving Devils Grotto and Eden Rock (Sept 2014?) - bare rock, very little coral, and very few fish.

Disclaimer: I'm told that there are much better sites anywhere but on the west.

 

[Wookie]

So, here is a chart of Georgetown harbor. Notice that the "protected area" is well south of the tank farm, and the "no anchoring zone" is well to the north of the harbor. I have no idea where Tattoosh anchored, maybe someone else can tell us? Note that the entire area between the "protected area" and the designated "no anchoring zone" is a designated "anchoring zone, some for large ships, some for others.