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  #1  
Old 08-20-2012
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Hurricane Anchoring - Why not this way?

Another long tread, but the devil is in the details.

When you ask about how to place anchors, you generally get the same answers in the text books or from most people. But I believe that anchoring for hurricanes, with combination chain/nylon rodes in relatively shallow water with boats having fin keels (or worse, wing or bulb keels) and with the high wind loads on the anchors, most of the tried and "true" methods will come up short. Hang with me through the detail and I'll try to show you why and present an alternative that I think will work better. But maybe I have overlooked or underestimated something, so I welcome your comments positive or negative.

What I will be describing is my Plan B. Plan A for my boat is to stay at the marina. I have done so successfully for many hurricanes and pretty much know what to do. My marina is somewhat sheltered in most directions (the most sheltered place that I have access to) and I use a spiderweb of lines to multiple tie points, all redundant. I am convinced that, in my circumstances and situation, it is much better than anchoring out. But just in case that, in certain circumstances, I have to leave the slip for a hurricane, this is my plan and I have the equipment on hand just in case.

The scene: 32 ft. sailboat, 3 ft free board, 10 ft normal water depth, 9 ft. storm surge, open river water 1-3 miles across. Boat has a wing keel. Will use multiple anchors with chain/nylon rodes 200 ft. long. Will set for 10:1 scope when water is at high point. Anticipate winds of 70-90 kts. A hurricane hole is not available, so will be in the open. Will deploy all available anchors.

Major concerns: anchors and rodes holding, the need to avoid any slack lines in the water which could result in a keel wrap (causes boat to be held beam to storm overwhelming anchors), other boats dragging down on me, or anchoring too close so boats can get together when they swing as wind shifts. Also, snagging a rode on some underwater obstruction (partially buried crab pots, sunken trees or other debris from previous storms, etc.), the large area needed for unobstructed swinging (~600 ft. diameter circle when line stretch under load is figured in.)

Hurricanes are different from most storms because extremely gusty winds that will swing through at least 180 degrees in the course of the storm. A hurricane rotates counterclockwise. This means as the storm moves up the eastern seaboard, high winds will first be out of the east, and after storm passes through, will be out of west on the backside. If the eye passes to the east, the wind direction will be from E, NE,N, NW, W; and if eye passes to the west, the wind direction will be from E, SE, S, SW, W. Usually, one has a fair guess as to which side storm will pass, but not always since the track is always a bit eratic, so anchoring pattern must accommodate a 360 degree swing. ABYC formulas for a 33 ft. boat indicate that winds of 90kts. will apply 7,523 lbs. pressure on the boat. If we are lucky and get only 70 kts., force will be 4551 lbs. We will have access to four anchors, which we believe will hold 1,000 lbs., 2,500 lbs., 3,000 lbs., and 1,200 lbs. for a total combined maximum possible holding of 7,700 lbs Three of the rodes will be 3 strand 1/2" nylon, 200 ft. with 20 ft. of 5/16" chain. One rode will be 3 strand 5/8" nylon, 200 ft. with 20 ft. of 5/16" chain. Maximum strength of 1/2" nylon is assumed 6,000 lbs. and 5/8" is 11,000 lbs., for a total rode capacity of 29,000 lbs. But lines should be loaded in range of 15-20%. Assume we can go to 30% for the storm. This gives a holding capacity of ~8,700 lbs. Three strand nylon stretches ~16% at 20% maximum breaking strength load. A 16% stretch on 200 ft. lines will then be about 32 ft., and at 30% loading (needed for storm), stretch will be in range of 45 ft. (nylon stretches to about 40% before it breaks). How to place the anchors so we get the combined holding capacity that we need?

Generally, when discussing anchoring patterns, possibilities are 1) a single anchor (none will hold the load) 2) 2 anchors set at 45% either side of bow (no combination holds the load, when 180 or 360 swing occurs, boat will be held by one anchor and second one will have slack rode in turbulent water, which could result in a keel wrap), 3) Bahama mooring with 2 anchors 180 degrees apart..same problems as #2, 4) three or more anchors spaced about equidistant in a circle with boat at center...same problems as #2, and 5) 2 anchors in tandem...will not hold the load with available anchors, plus, assuming you can get both anchors set, when the boat swings significantly in any direction, only one anchor is holding because the other is at some angle other than direction of pull.

But if we place the anchors in a narrow spread and play some games with placement and line lengths, we can get all the anchors to share in the load. To do this, we will place the anchors on N-S line, 20 ft. apart (do this by placing some marker buoys first, place first anchor from boat, set when at 5:1, then place remaining anchors one at a time with dinghy. Use dingy as tug to maneuver boat so there is no chance of getting into the rodes...boat engine will only be used to set the anchors after being positioned by dinghy). You can check this with paper and a compass, but if we position the boat 30 degrees off the E-W line and adjust the length out to full length of rodes (i.e. just take all the slack out in the rodes - in this process rodes will vary somewhat in length by design), we can create it so that the outside rode swing circle to inside rode swing circle distance differentials range from about 0 to about 21 ft., which falls well within the stretching under load that will occur and no anchor will be overloaded. (To do this, we will need to make a good guess as to which side eye will pass on...if we guess wrong, we would probably be ok still, but there will be more stretching of the lines and possibly slightly higher loading on one of the anchors.)

Someone will say that the anchors are too close and will contact each other if one breaks out. But all the lines that are holding are stretched under high load while the one that breaks out will have its stretch disapear since there is no load. It will move in the direction of the boat at that time, away from the other anchors. Because with stretching, the boat has moved further away from anchors, the loose anchor will be pulled away from those anchors that are holding by the amount (distance) of stretch in the loaded anchors, thus moving it away from the loaded anchors. When the boat eventually swings past the anchor line (i.e. N-S line where they are aligned, each anchor rode will be ~2ft. (make a plot and you will see this) above the nearest adjacent anchor. Thus, they should not hang up. Also, since all anchors are pulling in essentially the same direction, the rodes should not interfere with each other. (Generally, the case for an anchor dragging is same for all the other cases above...at some point in that 360 degree swing, one anchor rode is going to cross over the anchor that broke out....so the key is to don't have any anchors dragging, which is most likely achieved when all anchors are sharing the load (as in the narrow spread option).

What, if anything, have I overlooked or underestimated?

If you ask me why then do some boats survive with only one or two anchors, I will suggest to you that they didn't really get the full force of the storm. The high winds that are used to characterise hurricanes are really located in just a small area, often the NE quadrant, near the eye wall of the storm. So you can still get hit pretty hard, but most often, you won't see the maximum winds unless you just happen to be in one particular spot. But I would like to hold in that spot too if I can, hence the narrow spred plan.
sww914 and HDChopper like this.

Last edited by NCC320; 08-21-2012 at 10:14 AM.
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Old 08-21-2012
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Re: Hurricane Anchoring - Why not this way?

There was a study done some time ago about hurricane anchoring: mushroom vs 3 anchors @ 120* Not sure, but I think that I read it in Chapmans.
I believe your anchors are too close, and I would like to suggest some refinements to the bottom arrangement.
What I have to offer comes from Chapman, and experience.
Please consider 3 anchors @ 120* apart separated by 75 feet of chain, each converging in to a center ring. Now from this ring, go up with swivels and chain to your mooring buoy. Another swivel and then a pair of lines to your boat completes the set up.
Now your only problem would be cast off boats drifting down on you.

Last edited by Flybyknight; 08-21-2012 at 06:39 PM.
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Old 08-21-2012
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Re: Hurricane Anchoring - Why not this way?

"Please consider 3 anchors @ 180* apart"
Should that be 120 degrees apart?
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Old 08-21-2012
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Re: Hurricane Anchoring - Why not this way?

Quote:
Originally Posted by Flybyknight View Post
There was a study done some time ago about hurricane anchoring: mushroom vs 3 anchors @ 180* Not sure, but I think that I read it in Chapmans.
I believe your anchors are too close, and I would like to suggest some refinements to the bottom arrangement.
What I have to offer comes from Chapman, and experience.
Please consider 3 anchors @ 180* apart separated by 75 feet of chain, each converging in to a center ring. Now from this ring, go up with swivels and chain to your mooring buoy. Another swivel and then a pair of lines to your boat completes the set up.
Now your only problem would be cast off boats drifting down on you.
While this would handle some of the concerns, the issue is still around the holding power of the anchors involved. If one could place really big anchors and get them set deep, or smaller anchors set even deeper, then the suggestion of three anchors on chains, 120 degrees apart, attached to a mooring ball would be a really good solution. Not sure how I could do that because this is really a mooring, which requires special permits with surrounding property owners permits (even if I were to remove it later). And I have to do this....put it down, take it up at every storm threat...we have one or two every year... even if I ignore the fines/permits. With the anchors I have, we are trying to hold 7,500 lbs. pull at 90 kts. with anchors that range in holding ability of 1,000-3,000 lbs. each. Of course, no one really knows the holding power in the particular bottom that we will be encountering...sand/mud. Unfortunately, Chapmans does not deal with holding of the anchors vs. the pull of the storm. They do give some recommendations of mushroom anchor size for moorings vs. boat size...not sure that this allows for hurricane winds....they make no statement regarding either bottom type or wind capability if I remember correctly.

Specifically, why are the anchors too close together? The further apart they are the less the amount of load holding they can do (force component/resistance directly to/from the boat), and the greater the forces on the anchors to move closer together (force component/resistance directly towards other anchor (s). With a narrow spread, long rode, the component of forces trying to move the anchors together is minimized while the component of forces holding the boat is maximized. In any case, if an anchor drags or breaks out, it is going to first move in direction of boat by the amount of stretch in the rode (we are under high wind when the anchor breaks out), and then as the boat swings with the veering wind, that loose anchor should be pull around on a different arc than that of the holding anchors, so the loose anchor should always be below the holding anchor rodes and off to one side as the boat responds to the winds going from E to W over time.

Last edited by NCC320; 08-21-2012 at 11:30 AM.
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Old 08-21-2012
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Re: Hurricane Anchoring - Why not this way?

Here is the story of someone who was at anchor in Simpsons Bay in St Marten when Hugo hit.

Hurricane Hugo in the Caribbean.

Sobering reading for anyone who thinks their boat might survive at anchor sourrounded by other anchored boats.
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Old 08-21-2012
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Re: Hurricane Anchoring - Why not this way?

Quote:
Originally Posted by TQA View Post
Sobering reading for anyone who thinks their boat might survive at anchor surrounded by other anchored boats.
What I clearly get from that most useful tale is that if there are other boats in the same anchorage, like a chain you are only as strong as your weakest link and any one of them could come loose and waste all your effort.

Sounds like if you really want to save your boat you have to be on it, with a good working motor (of adequate power) that has a ton of fuel, and enough anchors and lines to secure your boat twice over with only half out at any one time.

Also sounds like there are times when you will have to cut your own tackle free to get out of the way of dragging boats, and possibly drive ashore on a sandy beach or even scuttle as a last resort since a boat is actually fairly safe sitting on the bottom out of the waves as opposed to being torn to bits on a rock or concrete shore.
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Re: Hurricane Anchoring - Why not this way?

Protecting your boat during a hurricane is not going to require Mooring permits.
Information given by various experts often come from anecdotal evidence because they did something and survived. Common sense will tell you to get to as protected water that u can Protected marina...(if other boats near you are well rigged). Hurricane Hole , Mangroves etc. Put down as many anchors (big) as you can get (star pattern). Protect from chafe , Reduce windage , Stow your gear and wait to see what happens.
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Re: Hurricane Anchoring - Why not this way?

Quote:
Originally Posted by TQA View Post
Here is the story of someone who was at anchor in Simpsons Bay in St Marten when Hugo hit.

Hurricane Hugo in the Caribbean.

Sobering reading for anyone who thinks their boat might survive at anchor sourrounded by other anchored boats.
I agree...that's why my Plan A is to stay at the pier, and fall back (Plan B) is anchoring out. Fortunately, I don't have to contend with rocks, and boats that drag go ashore and into woods. One of the boats at our marina suffered $70K damage when another one dragging hit that boat and stayed there for the storm. Most of those, who anchor out, do so in a bay on the river or just outside the marinas (where the bottom doesn't hold well in these storms). After one storm that beat things up pretty well, I talked with an old guy who anchored out with no problem....his reply was that he anchored out in the river proper where most people avoid and took his chance with the rougher water and bigger waves there in the more exposed location. Better that than becoming a "boat sandwich" was his response. If I have to anchor out, that's what I'm going to do also.

I disagree strongly with staying on the boat whether at pier or anchored. It is only a boat, not worth dying for. Do what you can before the storm, wish it well, and seek safety ashore on high ground.
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Re: Hurricane Anchoring - Why not this way?

If I lived on my boat and wasn't rich enough to keep it insured or afford another then it would be worth dying for. For a rich persons plaything obviously its not worth it.
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Re: Hurricane Anchoring - Why not this way?

As suggested above, my story is anecdotal, I am in NC (just outside Oriental) and my boat (35' sail boat) survived Irene anchored in a creek. It was my first hurricane with a boat. We had recorded wind bursts of 90 mph in town, I doubt I was exposed to that as the creek I was in is sheltered from the river and tree lined. My boat location is shown below as B, location A was my first choice but another boat beat me to it.



There were 3 other boats anchored in our creek, well separated; one parted its anchorage and ended up 35 miles up the river mostly unharmed. Fortunately, it did not hit any of the other boats on its way out. I had out 2 anchors both 10:1 for high water (9 foot rise), a Rocna 15 with 60' chain and 5/8" rode and a 25 lb Danforth with about the same. I also tied to two trees on shore with heavier line. I tied the boat off fore and aft, so she could not swing and this lead one of the anchor lines to chafe thru (the water went out worse than it came in). The boat was held by the other 3 lines, neither of the anchors moved as best I could tell.

I would likely put out the same ground tackle again, however I would like to put the boat in the middle of the 4 holdings and allow it to swing. But I don't trust using one swivel and bringing the lines up thru both side chocks seems like asking for twisted line trouble. I think the idea of bringing the 4 lines up thru a ring and then thru the bow roller might work but I would worry about how to protect the lines thru the roller. I have a 3' piece of 4" fire hose they could pass thru, it is rubber lined and I don't know what kind of friction that might generate. Any suggestions on how to best do this are welcome.

Last edited by CapnRon47; 08-21-2012 at 01:42 PM.
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