Rode Sizing

Rode Length/ Scope

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 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
2/1 10 % 30
3/1 40% 19.5
4/1 55% 14
6/1 70% 9.5
8/1 80% 7
10/1 85% 5
20/1 89% 2.8

Rode Strength:

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
20 25 30 35 40 50 60
15 Lunch Hook 90 125 175 225 300 400 500
30 Working 360 490 700 900 1200 1600 2000
42 Storm 720 980 1400 1800 2400 3200 4000
60 Violent Storm 1440 1960 2800 3600 4800 6400 8000

 

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 Rode

 

Chain Specification:

ISO

 

 

Chain Grade:

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).

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. 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.

1/4 (7mm) 5/16 (8.7mm) 3/8 (10mm) 1/2(13mm) 5/8(16mm)
Proof Coil Grade 30 1,300 WLL5,200  UTS 1,900 WLL7,600 UTS 2,650 WLL10,600 UTS 4,500 WLL18,000 UTS 6,900 WLL27,000 UTS
BBBGrade 30 1,300 WLL5,200 UTS 1,900 WLL7,600 UTS 2,650 WLL10,600 UTS 4,500 WLL18,000 UTS 6,900 WLL27000 UTS
Grade 40 2,600 WLL7,800 UTS 3,900 WLL5,850 UTS 5,400 WLL8,100 UTS 9,200 WLL27,600 UTS 13,000 WLL39,000 UTS
Stainless Steel 316 2,000 WLL8,000 UTS 2,400 WLL9,600 UTS 3,750 WLL14,200 UTS 6,500 WLL26,000 UTS 11,000 WLL33,000 UTS

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 Grade 30 1,300 WLL

5,200  UTS

1,900 WLL

7,600 UTS

2,650 WLL

10,600 UTS

4,500 WLL

18,000 UTS

6,900 WLL

27,000 UTS

BBB

Grade 30

1,300 WLL

5,200 UTS

1,900 WLL

7,600 UTS

2,650 WLL

10,600 UTS

4,500 WLL

18,000 UTS

6,900 WLL

27000 UTS

G4 HT

 Grade 40

1,950 WLL

2,600 WLL

7800 UTS

2925 WLL

3,900 WLL

11,700UTS

4050 WLL

5,400 WLL

16,200UTS

6900 WLL

9,200 WLL

27,600 UTS

9,750 WLL

13,000 WLL

39,000 UTS

Adjusted for 15%

G 7 Chain

2890WLL

4550 WLL

13650  UTS

4350 WLL

6825 WLL

20475 UTS

6024 WLL

9450 WLL

28350 UTS

10260 WLL

16100 WLL

48300 UTS

14501 WLL

22750 WLL

68250 UTS

Stainless Steel 316 2,000 WLL

8,000 UTS

2,400 WLL

9,600 UTS

3,750 WLL

14,200 UTS

6,500 WLL

26,000 UTS

11,000 WLL

33,000 UTS

Duplex  Grade 6 8650 UTS 14000 UTS 22,000 UTS 37,000 UTS

 

Chain Sizing Recommendations:

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

Chain

BBB, Proofcoil

1/4

1/4

5/16

5/16

3/8

3/8

7/16

7/16

1/2

5/8

Chain

HT

1/4

1/4

1/4

5/16

5/16

3/8

3/8

7/16

7/16

1/2

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.