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 Power||Deg|
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 Name||Boat Length – Feet|
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.
|Proof Coil Chain||Chain Size (in)||Wire Diameter (in)||Inside Length (in)||Inside Link Width (in)||Inside Link Length (mm)||Inside Width (mm)|
|(Usually Grade 30)|
- 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 30||Chain Size (in)||Wire Diameter (in)||Inside Length (in)||Inside Link Width (in)||Inside Length (mm)||Inside Width (mm)|
- ISO G43 Hi Test Chain: Most common specification for Hi Test Chain sold in the USA.
|Iso G43 Hi Test Chain||Chain Size (in)||Wire Diameter (in)||Inside Length (in)||Inside Link Width (in)||Inside Length (mm)||Inside Width (mm)|
- 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)|
- 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)|
- Grade L Chain (AS2321-2006) Grade 30 Chain specified for the Australian Market
|Grade L Chain||Chain Size (mm)||Wire Diameter (mm)||Inside Length (in)||Inside Link Width (in)||Inside Length (mm)||Inside Width (mm)|
- 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)
|Proof Coil Grade 30||5200||7600||10600||14800||18000||UBS|
|BBB Grade 30||5200||7600||10600||14800||18000||UBS|
|Stainless Steel 316||6280||9600||14200||21600||26000||UBS|
|G43 High Test||7800||11700||16200||21600||27600||UBS|
|Grade 60 Dupplex||8400||14160||22480||37090||UBS|
|Boat Size||0- 14 feet 1,000 lbs||14-20 feet 5000 lbs||20-30 feet 11,000 lbs||30-35 feet 15,000 lbs||35-40 feet 20,000 lbs||40-45 feet 40,000 lbs||45-50 feet 50,000 lbs||50-60 feet 60,000 lbs||60-70 feet 70,000 lbs||70-90 feet 110,000 lbs|
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.