A work in progress1
Anchoring
It’s about friction. It’s about drag. It’s about compromise.
It’s about a good nights sleep. Brought to you by the ratio 7:1.
Disclaimer: This material is designed (?) to provide one with the basics of anchoring, primarily in the inland waters of Washington and British Columbia. Anchoring theory and practices are often debated as adamantly as are religion and politics. The difference being that anchoring is an engineering problem with a technical solution. Some take pride in the process of solving the anchoring problem through study and compromise and others, well, don’t. It is a balancing act of selecting suitable locations, using appropriate equipment applied with proper techniques in order to achieve a safe and secure anchoring experience for vessel and crew as well as to avoid putting other vessels at risk.
Anchoring is about the use of mechanical devices, tools and the rules pertaining to the use of those tools. Some rules are quite good, others fall short when logic is applied and others, well, anyway one needs to be able to decide which of these rules are appropriate and how stringently one should adhere to each. I suggest that the further your stray form the rule of scope at 7:1 or any other well founded rule for that mater, the more you place yourself at risk. After all, managing risk is what this is all about.
Anchoring: Securing one’s vessel to the sea floor using line, chain or cable attached to mechanical devices such as an anchor or heavy weights.
The Goals in Anchoring: 1. Restrict the movements of a vessel so as to prevent it from drifting. 2. Reduce the demands placed on the crew. 3. Increase safety.
Anchors come in many forms from stones on ropes to highly complicated mechanical devices. Modern anchors are fabricated from high strength metals, usually steel, but sometimes others including aluminum. They vary from the high weight mushroom to the light weight high strength Aluminum Fortress which is a spin-off of the Danforth. In between are the Admiralty types like the fisherman or yachtsman similar to those seen on very old sailing ships, the Navy hinged fluke type often seen on very large ships, the Danforth lite weight hinged fluke type, the plow type like the CRQ whether hinged of rigged and the Bruce. The most popular are the Danforth, CRQ plow and the Bruce.
General Rules of Anchoring:
Picking a place:
Selecting an anchorage is one of the more difficult parts of anchoring. You need to study the nautical charts carefully, looking for locations that will be appropriate for the type of anchor and ground tackle aboard and be safe and comfortable for the crew and vessel.
Studying the nautical charts, one looks for natural features that provide protection from the prevailing and anticipated wind and sea state for the duration of the anchoring. In Puget Sound, there is usually good anchorage within a couple hours of motoring from just about any point in the sound. As an example, one of the best is Quartermaster Harbor, between Vashon Is. and Maury Is. This Harbor is fairly large being about a mile long and 2/3 of a mile wide. The depth is quite uniform ranging from 15 to 20 feet for most of the area and there are very few obstacles and no anchoring exclusion warning areas like “cable area” or “pipeline area”. The bottom material is mud without excess aquatic plant life that may foul your anchor or prevent it from engaging the bottom material. If I were seeking an anchorage for a storm in Puget Sound, this would be my first choice.
1) The location needs to be protected from the prevailing or expected weather and sea state. This is done by utilizing natural and artificial wind and wave blocking features like islands and breakwaters.
2) There needs to be sufficient room for your vessel to swing on its anchor rode. This is one of the most often violated rule of anchoring as all too often boaters will sacrifice proper scope for a favored location in the anchorage.
3) The sea floor is the source for the holding power of the anchor. The sea floor needs to be of a make-up that will allow the anchor to set properly and not foul on, or become entangled with plant life, submerged objects and obstacles, or submarine cables and pipe. Also, be aware of all hazards like overhead power lines or anything else that may pose a danger. Mud is often desirable, but in some cases like the Chesapeake, the mud is so soft as to have difficulty holding an anchor. Though the anchor barriers completely, every time the boat fetches up on its swing, the anchor gives way a little. So leave lots of room down wind.
For those, not interested in getting too deep in the anchor theory, then you can jump directly too applying rules and having a fairly good success rate.
1. Anchor: Delta, CQR Plow or Bruce. No substitutes. Sized appropriately for the size of the vessel or maybe one size up on the size recomindation charts in the catalogs or at the chandlary.
2. Rode: 110 ft of chain followed up by 300 ft of 3 strand nylon, of adequate size and strength.
3. Select your location, lower the anchor to the sea bed and back away while laying out the rode. When the scope is 7:1 as measured from the bow roller to the sea bed, tie off the rode and continue to back away till the vessels stops backing. If you expect conditions to worsen, advance the throttle to about cruise RPM or higher, in reverse and check for anchor drag.
4. Set Anchor conditions, like anchor light, and/or anchor ball.
5. Periodically, recheck your gear and position in the anchorage and proximity to other vessels.
6. In bad weather, set an anchor watch and monitor the situation, react if necessary.
How anchors work:
In the early days, rocks on ropes worked (sometimes) because of weight induced friction with the sea floor. Today’s equivalent to this is large concrete blocks used for moorages like the State Park mooring buoys and as anchors for US Coast Guard buoys. Instead of mechanical advantage, the rely on the friction brought about by there weight alone. Works well if there submerged weight is a significant percentage of the boats weight, but then are not very portable as they require heavy equipment to set of move them. I have watched a 32 foot boat travel downwind in 40kn wind with an 800lb engine block as anchor.
The modern anchor are mechanical devices designed to hook, snag or dig into the sea bed. It the anchor can grab a hold of something very heavy like the sea bottom, it anchor itself need not be very heavy. In other words, we are trading mechanical advantage for pure weight. But to make this work, the anchor must ENGAGE the bottom somehow and that is where the skill comes into play. As in all trade-off’s there is a give and take, so boat anchors require more attention then just dropping a massive weight on the bottom. Most of us are anchoring in sand and/or mud and so that is where I will focus. The better know anchors of this are Plow, Bruce, and Danfort. Though there are always new things on the market, some work and some do not, we will stick with the standard. In order for them to work, they need to engage the bottom material when they are drug across the bottom. This requires some rather particular angles and some sharp edges. Danforth is a type of anchor common to boating and it is easy to see the angular relationship in the parts to that anchor which make it work. Where the angles come into play is the angle at which the flukes ( those things that did into the bottom) come into contact with the sea bottom and that one is 11 degrees. The other angle that is important is the angle that the shank ( tha part of the anchor where the rode and thus, the boat are attached. That angle is also very consistently close to 22.5 degrees.
Snagging is the mode in rocks and coral, though we should avoid doing so in coral, but re us not very reliable and is most ovteThis is achieved through snagging, being buried in, digging into or plowing up the bottom material. Most often, modern anchors dig into the sea bed, increasing there friction with the bottom by so doing. The key to this is to deploy the anchor properly so that it can engage the bottom material effectively.
It’s about angles:
The first angle is the angle at which the anchor rode pulls upon the shank of the anchor. This is the angle over which the mariner has control as it is determined by the scope, the catenary of the anchor rode and the force being exerted on the rode by the boat. Each of these factors are complex.
Scope is the ratio of the length of the rode to the height. Rode is measured from the attachment on the anchor to the point on the vessel where the rode make first contact. Usually first contact is the bow roller, or sometimes a chock or fairlead. With a bow roller that is 5 feet above the water and a water depth of 15 feet and 140 feet of rode measured from the roller to the anchor, we have a scope or ratio of 7:1 which is generally thought to be the optimum for good holding in medium to strong weather conditions. This calculates out to an angle of pull on the anchor shank of 8.21 degrees with an all rope rode. The problem is that the environment in which we anchor is not static. The scope changes with tide and the dynamics of the vessel as it rises and falls as waves pass beneath. The tide, as predicted in as much as the relation of the Earth to the Sun and Moon are predictable and this relationship is the primary determinant of the tide. The other elements affecting the tide are barometric pressure, wind locally and on the nearby high seas, and rain with its associated river runoff, all of which can account for up to a 10% change to the predicted tide. In areas with hurricane, storm surge can add ten feet or more to the depth of water as well.
So what angle does it take to make this all work? Some say 5:1, some say 7:1 and then there are others, well we won't even consider them. What it really boils down to is that the pull on the anchor has to be low enough so that the flukes can dig in. At a rode ration of 3:1, the flukes will be parallel to the bottom at which they will not dig in. Just like a kniff lying flat on butter will not dthe flukes will slide along hte bottom like a k niff would slide along a peace of cheeze if it were laying
Next there is catenary which is the sag in a line between two points, a hyperbolic cosine function. Rode, being line, or chain or, more often a combination thereof, exhibits the sag defined by this function. We are interested in the sag that rode exhibits due to its weight in water because it reduced the angle of the rods pull on the anchor. For nylon line which weights little more then sea water, the sag is minimal. As for anchor rode, most include chain which is of sufficient weight to reduce the rode angle at the anchor.
Then there are the forces of nature pushing on the boat: Wind; Current and Wave Action.
Wind pushes against the entire vessel and rig that is exposed to it. The power of the wind is ‘square law’ meaning that, as the wind speed doubles, the force against the vessel is quadrupled. Engineers have calculated that a 40 foot modern cruising sailboat, facing head to wind, with the wind at 20 mph, the wind will push against the boat at 170 lbs. When the wind speed is increased to 40 mph, the force of the wind, exerted on the vessel by the wind would be 680 lbs. Please note that these are engineering calculations, for a boat facing head-to-wind. Problem is that sailboats do not behave that way. They hunt back and forth because of inherent in-stability. This is due to the center of lateral resistance of the boat in the wind being forward of the center of lateral resistance in the water. If one anchors stern to the wind, the vessel will lie strait away and not swing. Back to anchored head to wind, as the wind increases in strength, this hunting will become more pronounced. The course that the boat takes is the form of a figure 8 that is lying in the inside of a ¼ circle with the anchor at the center. As the boat inscribes the lopsided figure 8, the loads are maximizes as the boat swings outward and is headed up then reduces as it nears head to wind, at which it heads back toward the center of its swing. It is kind of like a water skier as they cross back and forth behind the tow boat.
I cannot think of an area in the Greater Puget Sound, where one would wish to anchor in an area with much more then minimal current. If you do have a need to anchor in a high current area, one need be very careful about your anchoring as the current will typically change direction 4 times in a day and your anchor may pullout and need to reset up to 3 times. Also, in current, the flow of the water may wash under the anchor making it difficult to keep it barred in the sea bad. It’s not infrequent to come across a tide shift line in an anchorage with the boats in one area of an anchorage lying to the current in one direction and those in another, facing in another direction. If the vessels are anchored inside each others swing circles, they can and sometimes do cone in contact with each other. Off of the waterfront of Port Townsend is one place where the back eddy will slowly progress through the anchorage causing boats to swing in opposite directions. I experienced one early morning un-anticipated rendezvous in this very spot.
Now we are up to the sea and swell and its effect on scope. Waves are energy traveling through water in the shape of a sine wave. This is true in theory, but in practices there are many forces in nature that modify the wave shape, a few being wind, current, participation, and most impotently for anchoring is the influence of the sea floor. In general, the sine wave prevails up to the point where the sea bed is less then 10 times the wave height. As the wave progresses into water shallower than this, the wave’s energy is reflected off of the sea floor and the wave starts building vertically until it becomes unstable and breaks.
Anchoring in a sea is to be avoided, but when so doing, be very careful to use (at the very least) the recommended 7:1 scope ratio, measured at the peak of waves at high tide. More scope is better, and much more is much better. As a wave passes beneath the boat, the motion on the bow is not a smooth sine wave, but is exaggerated as if the wave were shaped more like a sharp peak. The boat is also being pushed aft, in the direction of the wave’s travel. This combination of actions occur at the moment of least depth, so they combine to applying a shock load on the anchor rode and anchor when the scope ration is least. Under these conditions, nylon rode acts as a very good shock absorber, but you need to have enough nylon deployed so that the shock loads may be absorbed. Nylon can safely stretch about 10% as a working load. If you stretch it more then that, it heats up and if too much heat is generated, it melts and fails. If the loads are high, chafe protection needs to be employed where nylon line can come in contact with objects.
Back to angles:
The Second angle to be aware of is the angle at which the flukes (those large flat, shovel like devices) engage the bottom. For most of the general purpose anchors I have studied it is 11 degrees + and – one degree. It is most easily seen on Danforth type anchors, but studying other types of anchors, often this same angle appears. In other words, engineers have found that this angle gives the best results for engage and digging into the bottom instead of sliding over the surface from the angle being too shallow or if this angle is too grate, it will causing the anchor to become unstable and flip up on it’s beams end. There are exceptions to this 11 degree angle, one being for very soft mud bottom and the other being very heavy anchors like the navy type as seen on larger ships.
The make-up of the anchor rode.
Most often, rode is mad up of chain, shackles and sometimes swivels, connected to three strand nylon line. Some use all chain, or cable and there is always the possibility of combinations.
Nylon line/chain combination, it is by far the best rode for the following reasons. Chain adds weight to the rode and will tend to hold the rode on the bottom at the anchor shank. Nylon 3 strand is very elastic and will absorb shock loads by stretching. A friend of mine Dr. Jay McGrew, a sailor and scientist, published an article in Waterline June of 1988 titled ‘Is Your Life A Drag & Boat Adrift?’ which delves into the mathematical relationships in the anchoring problem. The conclusion of this study and further studies, he has determined that the optimal chain length is 110 feet followed by adequate nylon line to achieve at least 7:1 scope in all expected anchorages, plus extra line for the unexpected. His studies indicate that this combination is the best compromise as the catenary of this chain will leave the anchor shank on the sea bed as the winds approach 40 mph with 7:1 scope. And yet, in shallow anchorages, there will be enough nylon line deployed in the rode, to absorb the shock loads as the boat hunts in the wind and heaves to the sea.
All chain rode transmits very high shock loads to the anchor and deck hardware. Nylon, when sized correctly, will stretch too absorb shock lodes. I have read articles suggesting that 10% stretch under working load is reasonable for nylon and others suggesting 20% as safe. As nylon stretches and retracts, it generates heat from friction between the fibers in the line. Too much heat and the nylon melts. So here we are, back to compromising between sizes and length of nylon in order to come up with the best balance of stretch and ware. Often you see a short length of nylon line attached to chain as a snubber in order to absorb shock loads. Looking at the photos and illustrations, you can see that the nylon is in the order of 10 to 15 feet and thus will only add a foot or so the rode length for the shock load. So, when a short piece of nylon is added as a snubber to an all chain rode, it is usually too short and will be absorbing too much energy over too short a distance and will heat up and fail, or will not absorb enough energy and will still transmit shock loads to the anchor and deck gear. Nylon is susceptible to weakening or failure due to abrasion as the line works, and by heating from friction as the line stretches and retracts under load. Resent investigations of failures seem to point to heat build-up as the failure mode. Also, rock and especially coral are very abrasive and will ware through nylon quite quickly.
Now, finally, we get to the anchor and how it works.
As the anchor is lowered to the bottom, it may or may not lie flat against the sea bed. But, as tension is applied to the rode, the anchor should roll over and pierce the surface of the sea bed and start digging into the bottom material. As the vessel continues to pull on the rode, the anchor will (should) continue to dig into the sea bed to the point where it is able to stop the vessel. If conditions are optimal, the anchor should berry itself up to where the shank of the anchor is held tightly against the sea bed or even baried so that the rode disappears into the mud or sand. As long as the scope is adequate, this should lead to a successful anchorage.
Less successful would be the anchor not being able to dig into the sea bed fully, but the flukes are still offering quite a bit of resistance to the pull on the rode. The bad part of this is that the anchor may be working is way along the bottom and in the process picking up seaweed, eelgrass and other objects in its path. If the anchor picks up too much material, it may not then be able to berry in the bottom or reposition its flukes if the anchor flips over. This is the reason it is not a good idea to keep dragging the anchor around hoping it will eventually engage the sea bed.
So, it is probably best to lay the anchor on the sea bed and then back away slowly down wind while continue to let out rode until the correct scope is paid out. Once this is done, secure the anchor line and see if the anchor has set. In a known anchorage with a high degree of familiarity and good weather predicted, this may be sufficient. However, in a strange location, or with the possibility of weather, backing away more vigorously is recommended. If the anchor is skipping, you may be able to feel it by pressing your foot on the rode.
A little about Anchors.
Modern anchors attempt to replace the friction generated by weight resting on the bottom, with catching or snagging rocks and coral or digging into the bottom material. Let’s look at the Danforth since it is a very commonly used light anchor. The Danforth has a shank, to which one end is connected to the anchor rode (and vessel) and the other is connected to the stock, fluke and crown assembly through a hinge. This hinge allows the flukes to drop down to an angle where, when the anchor is pulled from the rode end of the shank, they (the flukes) will dig into sandy or muddy bottom material. The plates on the crown, which lift the crown end of the anchor off of the bottom, determine the angle at which the flukes touch the bottom. This angle is everything in a Danforth anchor, where too little angle and the flukes will skip across the bottom and too high an angle and the anchor will roll over and not dig in. The Danforth (style) anchor is more critical of scope ratios then other anchors as it does not have a lot of weight to help it dig in. It is susceptible to skipping, if traveling too fast along the bottom, sliding on eel grass and other bottom foliage and will not work very reliably in gravel, coral and rock. It is light for its holding power, which when the anchor is set, is very high.
The Danforth works best with mud bottom without much bottom growth. It is best with some chain, usually at least the length of the vessel being anchored and 3 strand nylon of adequate size of course. Be very cautious and leery of anchoring with scope shorter than 7:1 with this anchor. I have drifted anchor three times, twice times with Danforth anchors and both times, because of short scope. They were both my fault. The Fortress anchor is very similar in style to the Danforth, but made from aluminum. The Fortress anchor can be disassembled and stowed in a very small space and also the fluke angle can be changed for different bottom materials.
Next are the Plow and its variant, the Delta. Both of these anchors act like a double-sided farm implement plow, thus digging a ditch along the bottom. Farm tractors expend a lot of energy dragging a plow through the soil in order to turn it over. So do the plow type of anchor. It’s just they are designed more to generate drag than to turn soil over.
The Bruce anchor was developed to anchor North Sea oil rigs so the story goes. It is a one piece, galvanized cast steel device with a good reputation as a general use anchor. It tends to engage the bottom material quickly and seems to cut through some bottom vegetation. Because of the unique design, it also resists pulling out on shifts.
Next is the Kedge or fisherman. This is the anchor most commonly found on old larger boats. Its likeness is often used for nautical symbols and flags. It is a very good anchor, but is not as popular due to its squared shape requiring special handling for storage and recovery. It is best in rock and coral and in heavy bottom foliage. It also requires some understanding in its deployment since there is a possibility that the shank has snagged instead of the anchor rolling over so that the fluke has dug in.
Now for the “others” category of anchors. As there are a multitude of anchors out there, some quite bazaar and others that are sort of reasonable look and a few that actually work very well. Anchors are not certified to a standard of performance, reliability or quality by any agency, government or not. So be ware.
The anchor and rode should cost about 1% of the value of the vessel. This is not the place to skimp. Use name brand hardware of the correct size and configuration. Anything else is to place all at risk. In this age of faster, better cheaper being perceived as desirable, do not loose track of the track the fact that if it dose not work you have lost your boat. It should look like something that cannot be broken without extreme stress, and look like something one would get muddy. If it is a spin-off, make sure it copies the name brand very closely in shape and form and is very robust. In this study, I broke an anchor by walking around on the beach towing the anchor by hand in the sand. Upon closer inspection, it was clear that the quality and workmanship was totally inadequate. Anchors should look good and strong and have no evidence of inferior workmanship.
Ref:
Rode Angle: ignoring dip =180*((asin/’anchor rode ratio’))/3.1415926
Developed for SWSA education
By Brian Guptil, sailordude (at this domain name)