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.
Last edited by NCC320; 08-21-2012 at 10:14 AM.