Anchoring theory

[asteven221]

Been boating for quiet a long time and like many people hop around marinas or use moorings. Don't anchor much as I don't feel happy the boats going to stay in one place. Why? Cause I don't really understand the theory, despite reading the books. I am sure someone will be able to deal with my query. Right here goes! All the books and magazine articles explain about chain, lenghts, depths, bottom and setting the anchor. Fine so far! Where I have a problem is that all the theory is based on a nicely set anchor with chain going along the botton and up to the bow of the boat shown in a nice drawing. That's fine if the boat is going to keep pulling on the anchor as per the drawing. Trouble is the boat can for various reasons decide to swing 180 degrees, which will have the theoretical result of tripping the anchor. Then it's the lap of the Gods if it decides to set itself again. In other words the anchor is only any use when the boat is pulling in the direction of when you set the anchor in the first place. This is difficult to put into words, but hopefully someone will get my drift - oops my point. Obviously I am missing something in the theory but it's been bugging me for ages.

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[Deleted User YDKXO]

Well I guess you're right in that if the anchor drags it will have to set itself again but last season I wondered myself what happens to the anchor when the boat sheers about on the surface and I used to swim out to check it (OK it was the Med not the Channel). What I saw was that the vertical part of the chain moved with the boat but the further the chain was from the boat particularly the part that lies on the seabed, the less movement there was and at the anchor there was little or no movement of the chain and hence no movement of the anchor
So, providing you have at least the reccomended length of chain paid out (3 x depth but I use 4-5 x depth depending on wind and holding), the flexibility and weight of the chain (catenary effect) will take out most of the boat's movement. Obviously, though, if the wind is strong enough or the chain too short, the chain will tend to lift off the seabed altogether and put a load on the anchor which could disturb it

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[powerskipper]

Its normaly the weight of the chain that holds you, the anchor is a well anchor [sorry for pun] for the chain, and does not hold you as such if you have put out the required length of chain. or so I understand it.

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Julie

 

[flaming]

You are quite correct, it can trip the anchor. This is especially true of the lightweight flat rubbish that most manufacturers supply as standard. That type of anchor is designed to be great in a straight pull, and the fortress in particular does seem to have a fantastic holding power, but this type of anchor will simply pop out and skate along the surface when the pull direction is changed. Most decent anchors will bury completely in a sand or mud bottom so when the pull direction changes the anchor is already half burried and should simply turn and dig deeper in the new direction. If you want to see how your anchor will behave when the pull changes take it to the beach and try it in the softer sand.

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[Medskipper]

Without a doubt you will always sleep with one eye open when at anchor!

My brother has been in the Med for the last four years now. He is a cruising yachtsman and loves the life, but during the summer months his nights are always spent at anchor because the marinas are too expensive. He has a huge amount of ground tackle and chain but has had to sit up for three nights in a row before now with his engine gently motoring into the wind because of the strength of the winds in the Med! while many other yachts have been dragged large distances and even dumped onto beeches and rocks!

i.e. its not a exact science!

Barry


<hr width=100% size=1>I just want to retire with my boat to the Med!

 

[Sunnyseeker]

If your in a situation where the wind or tide will change, swinging you around on your anchor....put out a kedge, we've hedl our boat at 90degrees to 25 knot wind so that she sat into the swell, and didn't roll, kept spilling the drinks. we staid on that anchorage for 5 nights didn't move an inch, and felt very secure, it was good sand though.
To feel completely happy and get a good nights sleep, you'll need two anchors from time to time, two out the front at 30-40 degrees apart is good for most swinging around...or in the med one anchor and a long line to rocks ashore..
Anchors will usually re set but you wont sleep if you think youve swung round suring the night...Sweet dreams

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[Dave1258]

Anchoring
With so much investment literally riding on your anchor, your boat's ground tackle system is no place to cut corners. Your choice of anchor depends on the size and type of your boat, and the weather and anchoring conditions you generally encounter. Boats with heavy displacements or superstructures that present a lot of wind resistance need heavier gear. The same is true of cruising yachts that brave a wide variety of conditions and may sometimes have to anchor in open waterways. Even if you're a confirmed fair weather boater, remember that the true test of your ground tackle will come in adverse conditions when you need it most. So, when talking anchors, bigger is better, and there's safety in numbers. No anchor can be all things to all bottoms, so have aboard at least two anchors of different designs to handle varying conditions. Finally, keep an anchor close at hand. There are stowage systems available for every type of anchor-brackets, bow rollers, chocks-don't let convenience outweigh common sense if it means your primary anchor has to be assembled before it can be deployed.

Which Anchor?

Fluke (Danforth-type) or Light-weight
This popular burying anchor has wide, sharp flukes. Its excellent holding power-to-weight ratio means it can be lighter than other types of anchors used for the same conditions. High-tensile strength aluminum models are lighter still. Fluke-type anchors hold very well in soft bottoms like mud and sand, but tend to slide on grass and skip on rocks. The projecting flukes can be fouled by seaweed, shells, etc., preventing anchor set and are also rather unwieldy, although some models like the Fortress can be disassembled for easy stowage. The Fortress also offers variable fluke angles that can be set for either mud or sand.

Plow
An efficient anchor made of three steel-drop forgings, the plow features a single swivel at the shank base which prevents it from breaking out when the direction of pull changes. The plow is very effective in grasses, weeds, sand, and mud, but opinion varies as to its effectiveness in heavy grasses. Although its shape is somewhat awkward and hard to stow, it can be handled easily with bow rollers. And because it has no projecting fluke, it won't foul the anchor lines. Delta anchors have a similar design, with the added benefit of single-piece construction. The Delta's more streamlined shape also affords deeper penetration and prevents rolling.

Claw
A relative newcomer featuring a proven three-claw scoop design, effective in mud and sand. It sets quickly and reliably in most conditions, including gravel. The stabilized, single-piece design rolls over rather than breaking out when the angle of pull shifts. Its one-piece construction is easy to handle, although it must usually be stowed below, or on rollers.

Ground Tackle Holding Requirements

Now that you have an idea about the type of anchors you need, check the chart to determine the holding power you require. Since modern anchors are so efficient, it's their holding power, not their weight, that proves their adequacy. In the chart below, find your boat's length or beam, whichever produces the greater load. For example, a houseboat would use the load for the next larger powerboat size. Next, match it to your intended use for the anchor (lunch hook, working, or storm anchor) to determine the minimum holding power you need. Cruising boats should consider carrying all three types of anchors.




Boat Dimensions Horizontal Load (lbs.)
Length Beam (Power) Beam (Sail ) Lunch Hook Working Anchor Storm Anchor
10’ 5’ 4’ 40 160 320
15’ 6’ 5’ 60 250 500
20’ 8’ 7’ 90 360 720
25’ 9’ 8’ 125 490 980
30' 11' 9' 175 700 1,400
35' 13' 10' 225 900 1,800
40' 14' 11' 300 1,200 2,400
50' 16' 13' 400 1,600 3,200
60' 18' 15' 500 2,000 4,000





Anchor Rode
Depending on the size and type of your boat, your choices are either an all-rope anchor rode, combination rope/chain, or all chain. Rope should be nylon, either three-strand or braided. Nylon rope is elastic, making it a great shock absorber for sudden loads caused by wind and waves. Three-strand line should be medium lay, which has more twists per foot than soft lay. This is particularly important for use with a windlass, as the soft lay strands can untwist and separate, fouling the equipment. Three-strand rope can become awkward to handle, especially when hardened by salt saturation. An occasional bath in fabric softener will help to keep it supple.

Chain, used alone or in combination with rope, offers great benefits: it decreases the angle of pull on the anchor allowing it to set and hold more effectively, it's unaffected by chafe from rocks or sharp surfaces on the bottom, its weight forms a curve that absorbs shock loads in heavy weather, and, in the case of all-chain rode, requires much less scope-roughly half-for the same holding power as rope.

All chain is not created equal, however. Due to the inconsistent quality and often substandard galvanizing of many imported chains, we recommend that you avoid chain from foreign manufacturers. Proof coil is the most commonly used chain for anchor rodes. Made from low-carbon steel, it is identified by "G-3" imprinted on each link, and is adequate for most marine applications. BBB chain is slightly stronger than proof-coil, and has short links allowing a snug fit into windlass gypsies. BBB is also preferred by many cruisers using all-chain rode. Hi-test chain is made from higher carbon steel that has been surface-hardened. It has a higher strength-to-weight ratio than proof coil and is favored by the weight-conscious, as it can reduce the weight in the bow by up to 30% without sacrificing strength.



Suggested Minimum Working Rode Sizes

(For winds up to 30 knots.)

L.O.A. Beam (Sail ) Beam (Power) Nylon Rode Chain Rode
10' 5' 5' 125’-3/16” 3/16”
15' 7' 7' 150’-3/8” 1/4”
20' 8' 9' 150’-3/8” 1/4”
25' 9' 10' 150’-3/8” 1/4”
30' 10' 11' 200’-7/16” 5/16”
35' 12' 13' 300’-1/2” 3/8”
40' 13' 14' 400’-5/8” 7/16”
50' 14' 16' 500’-5/8” 7/16”
60' 16' 19' 500’-3/4” 1/2”


Suggested sizes assume fair holding ground, scope of at least 5:1 to 10:1, and moderate shelter from high seas. Boats that operate generally in shallow waters, may get by with shorter rode lengths.



Chain Length

There are two schools of thought on figuring the proper chain length. Chapman’s suggests using one half foot of chain for each foot of boat length. But Earl Hinz, a former aeronautical engineer and inveterate Pacific Ocean sailor, in his work The Complete Book of Anchoring and Mooring, says “the chain lead should weigh at least as much as the anchor whose weight it is supplementing”. From his experience, he has determined that the length of the chain lead has nothing to do with the depth of the water or the length of the boat. He suggests using the following formula to help you determine the minimum
length of chain lead you will need:


Minimum Anchor weight in lbs. Anchor materials factor =
length of = x Anchor materials factor 1 for steel and
chain lead Unit weight of chain, 1.6 for aluminum
lbs./ft.


For example: If you have a 7-lb. Fortress aluminum anchor with a 1/2” twisted three-strand nylon rode, you would need to use at least 11.2 lbs. of 5/16” proof coil chain, which weighs 1.15 lbs. per foot. Therefore, you will need at least 9’8” of chain.


Setting the Anchor

For a firm set and a good night's sleep, follow these simple guidelines:

Choose your spot carefully, considering the shelter it offers, whether it's a well-holding bottom, and the proximity of other boats. Remember that boats of different sizes may swing to a different scope than yours, so give other boats as wide a berth as possible to avoid any wee-hours fending off.

Approach your spot slowly, and put the boat in reverse when you're over the desired location. When the boat begins to gather sternway, lower the anchor to the bottom, and gradually pay out the rode. Take a turn around the bitt, snugging up the line and causing the anchor to "bite." Then pay out the rest of rode to the appropriate scope. Finally, check the set of the anchor by one of these methods. Using a buoyed trip line, reverse slowly and note if the buoy bobs up and down in one spot (you're OK), or is being pulled backwards with you (you're not OK). Another method is to choose two objects abeam that form a range, and check your position periodically in relation to them. Any change in their bearings means you'd better try again.

Determining Scope

The amount of anchor rode paid out depends on the type of rode you're using and the weather and bottom conditions. Scope is the ratio of rode paid out to the depth of the water. The proper scope for all-rope rode in average conditions is between 7:1 or 8:1; as noted before, you can be comfortable with a mere 3:1 or 4:1 if you're using all-chain rode. The key is to keep the angle of pull as close to horizontal as possible. Heavy weather or adverse anchoring conditions demand longer scope; however, keep in mind how you'll swing in relation to your neighbors.

Retrieving the Anchor

Break out your anchor by powering up to it slowly (with your engine, not your windlass), taking in the rode as you go. Once over the anchor, move all the crew to the bow, snub the anchor, and move aft. You should be able to lift it vertically. If it resists, snub the rode around the bitt and power forward slowly, taking care not to carve up your topsides. A handy device is an anchor retrieval system, which uses a buoy attached to the rode to help break the anchor free.

Whatever system you choose, remember that your boat's ground tackle system is also its greatest safety feature. Buy the best you can, and check the gear often for signs of chafe and wear. With so much at stake, don't let the anchor and chain be your system's weak link.


Hope that helps!




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[rich]

The Fortress. Take a look at my night of "HELL" in herm. ;click on hermees

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[duncan]

great post however when there is so much inevitably there will be things to pick up on!

Anchor types - I think it well worth considering the newer designs. These are of course developments on older designs but that is exactly why they should be considered. SARCA, Spade and the long german name I can't remember (but it's basically the same as the SARCA.

Chain length - I am certainly not going to argue with the eminent opinions you quote but I think there may be mixed issues (both of which you refer to elsewhere) in there. 1.5 times the boat length takes in chafe and holding whilst the weight calculations concern holding alone - a couple of feet of 30mm chain won't stop chafing! Therefore a combination along the lines of a given length of chain of such a weight to equal or exceed the anchor weight would seem a reasonable guide.

Scope - once you get to 7:1 you gain nothing by going longer from the perspective of angle of pull.

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[asteven221]

Just back from a long weekend trip from the Clyde down to Ireland. Marinas and moorings as usual. Thanks everyone for the input, especially dave1258 for the amount of information. Very helpfull. It seems to me that I am not the only one with the same thoughts about anchoring! Although everyones input has been very helpfull, it still leaves the issue that if the boat shears around and pulls on the anchor enough, it will trip it and then it's the lap of the Gods to what happens next. I guess the only answer is two (big) anchors for a good nights sleep!

Thanks again for the help.

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