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.
An average cruising boat is recommended to carry 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.
||% Max Hold Power
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
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.
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.
BBB: Short, compact links. The short link makes the chain more flexible and ideally suited for a windlass, usually made in Grade 30 only.
ISO G43 Hi Test Chain: Most common specification for Hi Test Chain sold in the USA.
DIN 766 (Common Standard for Metric Chain)
ISO4565 G70 Chain
Grade L Chain (AS2321-2006) Grade 30 Chain specified for the Australian Market
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 usually made to DIN776 dimensional standard.
National Association of Chain Manufacturers
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.)
So, the higher the grade, the stronger the chain. Chains Grade 80 and above are considered to be ok for overhead lifting. These higher grade chains are usually not easily accessible for most sailors. The most common grades for anchor chain are Grade 30 (BBB & Proof Coil) & Grade 40 (HT).
Note: the WLL (Working Load) for chain is usually defined 1/4 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.
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 especially 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 your decisions.
When sizing your chain remember that it’s recommended for an average cruising boat carry at least 300 feet of chain. This 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 will reduce the likelihood of rode failure.