Design of No Anchor area

[Minty]

Hello!

I am an engineer designing a "no anchoring" area for some submerged erosion control gabion baskets. These are essentially large wire mesh baskets filled with rocks, and the holes in the mesh are large enough to catch anchor hooks. I have read a little about anchor dragging, and am wondering what would be a "safe" offset from the edge of the baskets to place our "no anchor" zone. Bear in mind that the boats may not be using the best quality anchors or proper technique. The boats are small sailing boats or similar sized craft.

I haven't been able to find good technical guidance or regulations to support this design, so any thoughts on this would be appreciated!

[Jammer]

How deep is it? How close to shore? You may not need to do anything.

[Minty]

The baskets are 6 to 8 m deep (20-26 ft), extending from the shoreline out to 165 m (540 ft) from shore. There is a sailing club and public boat launch, with boats commonly anchored nearby. We need to define a "no anchoring" area as part of our government approval conditions, so the only missing piece is how far of an offset to make it from the edge of the baskets. I want to minimize risk of anchor fouling on the baskets, but I don't want to take up an excessive amount of space and cause undue inconvenience to other water users.

I am actually not 100% sure if there even is a risk of anchors catching on these. The mesh looks similar to chicken wire, except the holes are bigger (64 mm / 2.5").

There is a pipeline running along the middle though, so even if there is no risk from the baskets, the no anchor zone would still need offset from the pipe alignment.

[Kello]

Maritime law is a completely different jurisdiction than "dirt" law. Even if the gabions are on private property they may be subject to maritime law, find out. Contact the Coast Guard and / or a maritime atty.

I know what a gabion is, and my boat has dragged anchor in storms several times. FYI, if a boat is dragging anchor it would be kind of convenient to hook onto a gabion to slow it down.

[Foswick]

This is going to really invite a bunch of opinions. I'll give you some information.

Boaters are advised to use a rope/chain that has a length that is some multiple of the depth. This multiple varies according to conditions and the individual, but people rarely use more than a 7-1 ratio, and virtually never over 10-1. So, if anyone drops an anchor at 8 meters, at most, they would swing around a circle with a radius of just under 80 meters.

Now, swinging over these baskets isn't a problem, so you don't need such a wide buffer. The danger is in dropping onto the baskets or dropping near them and dragging into them.

This is where the opinions come in. How much drag is "reasonable" to allow for? I would say a buffer of something like 10 meters would be perfectly adequate. Someone would have to drop an anchor right at the edge of that and drift quite a bit in the wrong direction. And, it is fair to say that they would have been adequately warned.

[Foswick]

Originally Posted by Kello

FYI, if a boat is dragging anchor it would be kind of convenient to hook onto a gabion to slow it down.

You know, that is actually a very valid point. At that point, the concern is damage to the baskets themselves because you will have done the boater a favor.

[hpeer]

When all else fails make your best guess decision. Someone is bound to say you are wrong. Then tell them to show the proof. That is how to get free and easy research!

Joking aside, what size boats in the sailing club? How popular is the ramp and what kind of boats?

If the sailing club is just small day sailors and the ramp boats are small bass boats and such then the risk of anchoring is small. Especially if there are better areas to anchor nearby. 20 to 26 feet is not prime anchoring for small boats and then only in good weather.

As I recall pipe lines and power lines typically do not define an off set, they just indicate the path. It is up to the skipper to assess the situation and anchor accordingly. I would be interested in hearing alternative examples.

I have no special knowledge but were it me, I would provide a pretty minimum space, outline the gabion field.

[John_Trusty]

Interesting question, and it depends on what is anchored there. Somewhat 3 dimensional considering that the distance above the sea floor may also snag the anchor line. Based on assumption that “small” boat is under 10 meters, an anchor will definitely snag your 6 cm mesh, and small boats use rope line, not chain, which means it will extend farther without as much catenary drag on the bottom. Anchor drag is not predictable, but this is more about human reactions. If people are on the boat, it would be detectable in a few boat lengths, if not it may never be. Suggest asking locals how anchorage is used - long term, unattended or just lunch and fishing. Also ask about the bottom, as some material is much better holding.

Typical scope (line out to water depth ratio) is 7 or 10:1, so for a minimum first take the height of your gabions over sea bottom and multiply by 10 to make sure the line rises sufficiently if the boat drifts over the structure. As you noted, more is better, but you do not want to block everything.

[wholybee]

Foswick's reasoning is sound. How is this area going to be marked? Is it only to be marked on charts, or will there be buoys placed to mark the boundaries? If you are marking the boundaries with buoys, then you really don't need much buffer at all, because in order for the boat on the surface to stay out of the area, the anchor already needs to be as far away as the scope of the chain. If it is only going to be marked on a chart, then I would suggest a considerable buffer zone, as people's ability to read a chart and follow it varies considerably.

[Minty]

Thanks everyone for your replies! This gives some great context.

The current plan is to have the following warnings in place:

• A sign on the shoreline that indicates No Anchoring as well as the hazard type (Pipeline), with text legible from suitably far away
• A warning buoy anchored above the end of the pipeline

My questions are:

1. How far from the buoy would a boater keep clear when anchoring?
2. Would a boater see both the buoy and the sign on shore and infer to keep away from the line between them? Or would they just keep clear of the buoy? The sign on shore and the buoy would be 115 m (380 ft) apart, which seems like quite a long distance.

[Foswick]

Originally Posted by Minty

My questions are:

1. How far from the buoy would a boater keep clear when anchoring?
2. Would a boater see both the buoy and the sign on shore and infer to keep away from the line between them? Or would they just keep clear of the buoy? The sign on shore and the buoy would be 115 m (380 ft) apart, which seems like quite a long distance.

Well, I can't answer what a boater "would" do, but I can suggest what a sensible boater would probably do.

1) No one would want to swing into that buoy. So, it is reasonable to assume no one would drop an anchor within about 30m of it in any direction.
2) I would certainly stay out of that line. And, I think most would. But, daysailers in small boats often do things that most on this forum would not. If you wanted to be really clear, a buoy every 30m along the structure seems abundant warning.

[Kelkara]

1. In 10m of water, I would drop my anchor a minimum of 40m from the buoy in a crowded anchorage, and 50+m if there were space to.

2. If the buoy were clearly marked "no anchoring, pipeline" I would assume that the pipe were running to shore, In daytime I would then look for your shore sign, but at that distance in the dark I won't see it unless it is made of retroreflective material. However in the dark I probably wouldn't approach the buoy close enough to read it, so I'd not be aware of the pipe.

If the pipe isn't on charts the boat you have to worry about is the boat coming in cold and fatigued in the dark, desperate for the shelter of an anchorage because adverse winds have made him much later than anticipated ... he sees a clear spot on radar where no one else is anchored between the buoy and shore with room to swing, heads there and drops the anchor happy that a long hard day is finally over but not thinking about uncharted pipelines.

[wholybee]

If a boater was interested in going ashore with a dinghy, you can expect that they will drop anchor some distance from the buoy, and then let out line and back down so that they are almost touching the buoy, or are right on the edge of the no anchor area. The anchor would be well clear of the area, the boat, not so much.

Are these baskets just in a line from the shore, or a larger area? I have seen no anchor areas marked with 4 buoys, or 2 buoys and marks on shore. I have also seen the opposite, instead of marking where you can't anchor, mark where you can. It is pretty common for pipelines to be marked on a chart, with no other markings.

[deblen]

What you are describing is known as Foul Ground.
Suggest you investigate Foul Ground as applicable to USA.
IMHO-Foul Ground marked on a chart is the actual area fouled.
It is up to individual captains to decide how far to avoid it.
Suggest marking area with buoys may be all that is required,until it is charted by NOAA.
Cheers/Len
https://www.nauticalcharts.noaa.gov/...1/ChartNo1.pdf

Australian : https://iho.int/uploads/user/Service...%20grounds.pdf

[Minty]

Thanks all for your advice on this project. I think we have a path forward now. Rest assured we do have the appropriate approvals in place, including instructions on the required warning markers and signage. I just wanted to check in with everyone here to see if there was anything further we could do to make this infrastructure less burdensome to the local boaters and I think I've got what I need to do that now!

Thanks again!