Sizing your

CHAIN

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 is recommended that an average cruising boat carries at least 300 feet of rode, which will allow for 10:1 scope in 25 feet of water (5 feet allotted for free-board height). This will ensure that most of the time you can anchor with 10:1 Scope.

It is generally accepted that in a storm situation we strive to put out 10:1 scope, but why? See how anchors maximum holding power varies with changing scope.

Many cruisers find that 10:1 scope is a nice compromise between rode length and maximizing anchors holding power.



Scope% Max Hold PowerDeg
2/110 %30
3/140%19.5
4/155%14
6/170%9.5
8/180%7
10/185%5
20/189%2.8

ABYC data was developed in 1950s to indicate the strength required of an anchor, capstan, rode 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.

After comparing this data in this table to work that has been done since we come to the conclusion that the values presented here represent loads expected on the anchor rode if anchored on chain only without the use of the snubbing system. Further using a line rode or and adequately designed snubber you can reduce the loads by a factor of 3, by eliminating the shock loads. This is an argument for why its so important to use a snubber but when sizing anchor rode or your anchor, it is important to consider loads as stated in the table. The reason is that we can not rely on the integrity of a snubber in a storm.  Should the snubber fail, the integrity of the rode should be sufficient to handle the loads without the snubber.



The ABYC Horizontal Working Load (lbs) Table

 

WindSpeed(Knots)Anchor NameBoat Length – Feet      
  20253035405060
15Lunch Hook90125175225300400500
30Working360490700900120016002000
42Storm72098014001800240032004000
60Violent Storm1440196028003600480064008000

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 link

  • Proof Coil Grade 30: 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.

 Proof Coil ChainChain Size (in)Wire Diameter (in)Inside Length (in)Inside Link Width (in)Inside Link  Length (mm)Inside Width (mm)
(Usually Grade 30)      
   1/40.27910.50725.412.9
   5/160.3421.10.50727.912.9
   3/80.4041.230.6231.215.7
   1/20.5121.50.82238.120.9
   5/80.6281.91.0248.325.9
   3/40.7792.21.155.927.9
  • BBB: Short, compact links. The short link makes the chain more flexible and ideally suited for a windlass, usually made in Grade 30 only.

BBB Grade 30Chain Size (in)Wire Diameter (in)Inside Length (in)Inside Link Width (in)Inside Length (mm)Inside Width (mm)
    1/40.2740.870.4322.110.9
    5/160.32810.525.412.7
    3/80.3921.090.6227.715.7
    1/20.5091.340.7534.019.1
  • ISO G43 Hi Test Chain: Most common specification for Hi Test Chain sold in the USA.

Iso G43 Hi Test  ChainChain Size (in)Wire Diameter (in)Inside Length (in)Inside Link Width (in)Inside Length (mm)Inside Width (mm)
    1/40.2740.8450.421.510.2
    5/160.331.030.4926.212.4
    3/80.3991.220.5831.014.7
    7/160.4671.40.6635.616.8
    1/20.511.590.7740.419.6
  • DIN 766 (Common Standard for Metric Chain)

DIN 766 Dimensions (Grade 70 and Grade 80 chain are usually DIN 766)Chain Size (mm)Wire Diameter (mm)Inside Length (in)Inside Width (in)Inside Length (mm)Inside Width (mm)
 660.730.3118.58
 770.870.35229
 880.940.392410
 10101.100.552814
 11111.220.553114
 13131.420.713618
  • ISO4565 G70 Chain

ISO – G70  ISO4565 – (ACCO)Chain Size (in)Wire Diameter (in)Inside Length (in)Inside Link Width (in)Inside Length (mm)Inside Width (mm)
    1/40.2790.870.4417.122.1
    5/160.3321.110.5058.428.2
    3/80.4041.2260.57610.331.1
  • Grade L Chain (AS2321-2006) Grade 30 Chain specified for the Australian Market

Grade L ChainChain Size (mm)Wire Diameter (mm)Inside Length (in)Inside Link Width (in)Inside Length (mm)Inside Width (mm)
 660.710.30187.5
 880.940.392410
 10101.180.493012.5
 13131.540.643916.3
  • Stainless Steel Chain: Imported and US Made stainless steel chain is available made to various NACM, ISO, DIN specifications . Stainless Chain is available in several grades:  304, 316, Duplex. Duplex stainless steel chain is the strongest option available in Grade 60 and is more corrosion resistant than 304, 316 stainless steel and is ussually made to DIN776 dimensional standard.

National Association of Chain Manufacturers
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)

Note: the WLL (Wroking load) for chain is usually defined ¼ of the UBS (Ultimate Breaking Strength) expressed in pounds for all chain types except G4, Grade 40 HTChain has the WLL defined as 1/3 of the UBS. It is unclear why the WLL definition is different for Grade 40 chain. To make the comparison easier we defined Working Load below for all chain as 1/4 of the UBS.
 
Chain Size
  1/4  5/16  3/8  7/16  1/2
Proof Coil Grade 3052007600106001480018000UBS
13001900265037004500WLL
BBB Grade 3052007600106001480018000UBS
13001900265037004500WLL
Stainless Steel 31662809600142002160026000UBS
15702400355054006500WLL
G43 High Test780011700162002160027600UBS
19502925405054006900WLL
Grade 60 Dupplex8400141602248037090UBS
2100354056209272.5WLL
Grade 701260018800264003500045200UBS
315047006600875011300WLL
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.
Boat Size0- 14 feet 1,000  lbs14-20 feet 5000   lbs20-30 feet 11,000 lbs30-35 feet 15,000 lbs35-40 feet 20,000 lbs40-45 feet 40,000 lbs45-50 feet 50,000 lbs50-60 feet 60,000 lbs60-70 feet 70,000 lbs70-90 feet 110,000 lbs
ChainBBB, Proofcoil1/41/45/165/163/83/87/167/161/25/8
ChainHT1/41/41/45/165/163/83/87/167/161/2

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.