When sizing an anchor or anchor rode, it’s useful to have starting point and to think of the whole system. This is a table developed by American Boat and Yacht Council (ABYC) to approximate the expected load on the anchor and the anchor rode for given wind speed and boat length. The complete table appears in Section H-40 of ABYC’s Standards and Technical Reports for Small Craft available at abycinc.org. Many experts in the filed question that these values are over concervative.
The ABYC Horizontal Working Load (lbs) Table
ABYC data was developed in 1950s to inidicate the strength required of an anchor bitt or a cleat. Tom Hale a past ABYC technical director said it is not unreasonable to apply the figures in the chart above to the entire anchor system. Hale said the ABYC tests ( done with a 40 foot boat by Bob Ogg, who developed the Danforth anchor assumed the worst case situation including sea state and surge.
Boat Length – Feet
There are many that questioned these estimates as being too conservative:
Robert Smith’s analysis of the force applied to sailboats of different sizes in wind and waves based on observations in the Columbia River. See “Anchors Selection and Use” 2d ed. (1983) differ markedly from ABYC data, see below. When interviewed by Practical Sailor in 1996 and asked about the discrepancy, Bob said ” I do not know how they got their figures: mine are real developed from actual testing”
Loads Expected on Anchor Load (ABYC vs Robert Smith)
Wind Condtions (knots)
ABYC standard document H-40 last updated 2014. Defines Design Loads for Deck Harware, and recommends for strong points to use these numbers with the safety factor of 2.
Design Loads for sizing deck hardware
Design Loads (lbs)
How do you intend to use your boat? Do you sail only on calm days with fair weather (Lunch Hook) or are you outfitting for an adventure (Storm) where you might be exposed to hazardous weather at anchor? For example, if you are outfitting a 35 foot boat for you should size the working rode and anchor to safely handle loads up to 2,700 lbs if planning a cruising adventure, but the same margin of safety is not necessary if operating day trips from the safety of a marina.
RODE CHOICE AND SIZING:
The ratio of the length of rode to the water depth is known as the scope. Anchoring with sufficient scope brings the direction of strain close to parallel with the seabed. In general the longer your rode the better. 10:1 Scope is preferable in windy conditions. It’s recommended that an average cruising boat carries at least 400 feet of Rode, which will allow for 10:1 scope in 25 feet of water (5 feet allotted for freeboard height). This will ensure that most of the time you can anchor with 10:1 Scope.
|Scope||% Anchors Maximum Holding Power||Angle between Rode and bottom|
Sizing for Strength
When sizing your gear you should make sure that the Working Limit Load (WLL) of your rode is higher than the loads expected in a storm. Sizing the working rode for a hurricane might not be practical but sustained winds greater than 40 kts are commonly encountered by cruising vessels on anchor. Sizing for such conditions is a reasonable way to approach the problem and is the basis for our recommendations.
WLL: Working Limit Load /Safe Working Load (SWL)/Normal Working Load (NWL) is the load that the rode can safely support without fear of breaking. Usually marked on the equipment by the manufacturer and it is often 1/3 to 1/4 of the Minimum Breaking Strength (MBS) for chain and 1/10 to 1/12 the Minimum Breaking Strength (MBS) for rope.
Chain Grade #:
The chain Grade is based on the nominal stress in the link at the design breaking force strength. (It is calculated by taking the minimum breaking force load and dividing by two times the nominal cross sectional area of the link.) Another words the higher the grade the stronger the chain. Chains Grade 80 and above are considered to be ok for overhead lifting. These higher grades are usually not easily accessible. Most common grades for anchor chain are Grade 30 (BBB & Proof Coil) & Grade 40 (HT).
Chain Link Geometry:
BBB: Short, compact links. The short link makes the chain more flexible and ideally suited for a windlass.
Proof Coil: Proof coil chain is a general purpose chain, frequently used for light duty tow chains, tie down chains and logging chains. Links are longer, thus chain is lighter than BBB and Grade 40 HT for equivalent length.
Grade 40 HT (High Test Chain): made for applications requiring higher strength to weight ratios than Grade 30 chain, the short links make it more flexible and ideally suited as a windlass chain.
Stainless Steel Chain:
Imported and US Made stainless steel chain is available made to various NACM specifications
Chain Type Choice:
Main focus should be placed on sizing the chain for the expected loads. Using higher grades of chain can offer some weight saving however. For example, 300 feet of 3/8 BBB weighs 468 lbs vs. 300 feet of 5/16 G4 HT which weighs 310 lbs and has higher strength. Thus if taking 160 lbs off the bow is considered important, using higher grade chain definitely has its advantages. Another issue is matching the gypsy on the windlass to the chain size and type. A windlass gypsy designed for 5/16″ High Test chain will not work on 5/16″ BBB, which has shorter, more compact links. Many windlasses have a selection of gypsies, which can be Special Ordered to fit the rode on your boat. Often availability and the cost of the right gypsy enter the calculus of which chain size and type is right for you. It is important to remember not to compromise on the strength of the system when making such decisions.
Chain Strength Table
Note: the WLL is ¼ of the MBS expressed in pounds for all chain types except G4,
G4 HTChain has the WLL defined as 1/3 of the MBS
|1/4 (7mm)||5/16 (8.7mm)||3/8 (10mm)||1/2 (13mm)||5/8 (16mm)|
|Proof Coil |
Chain Sizing Recommendations:
How Much Chain:
It’s recommended that an average cruising boat carries at least 300 feet of chain, which will allow for 10:1 scope in 25 feet of water (5 feet allotted for freeboard height). This set up will ensure that most of the time you are anchored on chain and reduce the likelyhood of rode failure.
Line is subject to degradation with time from chafe and UV exposure, further this damage is not easily identified nor can be easily quantified. Trying to envelop such uncertainty would require you to use very large line (For example: 7/8 line to substitute 3/8 BBB chain) and even then frequent inspection would be necessary to insure line integrity. Thus is it is our recommendation to use chain as the primary rode when possible as it is more forgiving and durable. Line could be used as shock absorber in the form of a bridle or a snubber system and it is reasonable to extend the length of chain rode with line to accommodate deeper anchorages and/or increase in scope. A typical cruising boat carries 300 feet of chain, which will give one 10:1 scope in 25 feet of water (5 feet allotted for freeboard height). This set up will ensure that most of the time you are anchored on chain and reduce the likely hood of rode failure. However anchorages deeper than 30 feet are plenty and it is often impractical to carry more than 300 feet of chain, in such situations it is important to use line that is appropriately sized and well inspected.
The SWL is a guideline for the use of a rope in good condition for non-critical applications and should be reduced where life, limb, or valuable property is involved, or in cases of exceptional service such as shock loading, sustained loading, severe vibration, etc. The Cordage Institute specifies that the SWL of a rope shall be determined by dividing the Ultimate Tensile Strength of the rope by a safety factor. The safety factor ranges from 5 to 12 for non-critical uses and is typically set at 15 for life lines.
Assuming 1:8 safety factor we generated this table:
Understanding Line Material Choice:
Anchor Line is commonly available in Nylon and Polyester
Nylon and Polyester are similar in strength; however Nylon has more stretch for an equivalent size and plait. For this reason nylon is recommended material for a snubber/bridle line, and both are reasonable choices for anchor rode.
Understanding Line Weave Choice:
When talking about rope construction, there are two general categories, twisted and braided. Twisted rope is formed by coiling three strands together in the same direction. The fibers within each of the three strands must twist in the opposite direction as the corresponding strands. Twisted rope hangs straight and resists kinking.
The other type of rope construction is braided. There are three different kinds of braided construction though, diamond braid with a core, diamond braid without a core (also known as hollow braid) and solid braid.
Diamond braid is produced by weaving ends of yarn over and under. If there is a core around which the rope is braided, it cannot be spliced. If a core does not exist, the rope is called “hollow braid”. The leading feature of hollow braided rope is its ability to splice easily.
Solid braid is tightly woven which produces a very firm and rounded construction. The unique lock-stitch construction prevents unraveling when cut or torn. When the rope, as well as the core, are braided, it is known as “braid-on-braid” or “double braid” construction
Anchor line most commonly is offered in Twisted Three Strand, Double Braid, 8 Strand, 12 Strand Solid Braid. Braided ropes have many advantages over twisted three strand: they do not kink and are easy to coil/flake and they have less stretch. Thus when the entire length of the rode is line – sufficient stretch can be achieved with braided line.
Three strand offers the best resistance to abrasion, and is more affordable. Three strand offers more stretch and is ideal for use in (bridle/snubber) application.
Sizing Your Line:
The most important detail is to size the line appropriately for the expected loads on the rode.
**Different Braids of line have similar strength for the same line diameter**
Sizing a Shackle: Click here to learn more about shackles
Generally Shackles Come two grades:
- Industrial Grade (Type IV A)
- Specialty Alloy Shackles (Type IV B)
If you are trying to match the strength of your chain you can use these two rules:
- If sizing for BBB or Proof coil chain use Industrial Grade (Type IV A) shackle, one size bigger than your chain
- If sizing for HT (G4) chain use Specialty Alloy Shackles (Type IV B), one size bigger than your chain
- If sizing for stainless steel chain use a forged SS shackle that is one size bigger than your chain.
***Shackles are sized by their body dimension, the pin is usually one size bigger than the size of the shackle***
***Chain and shackle sizes are presented as a fraction of an inch***