Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

Example: For Hold Fast,we would get

0. 2 +(36 lb. ÷185) = 0. 394 in. diameter

or

5. 08 +(16.4 kg ÷ 3 .3) =10 mm diameter

This is close to^3 / 8 inch (0. 375 in.), which is

what we could use, or we would use 10 mm

nylon three-strand or brait. The one excep-

tion is when you’re installing a combination

rope-chain windlass. To ensure adequate

strength in a rope-to-chain splice, you should

increase the rope diameter by one standard

size. In this case, increase to^1 / 2 - inch (12 mm)

three-strand nylon.

Anchor Rode Length

The total length of the anchor rode is con-

trolled by the scope required. Scope is the

length of rode paid out relative to the depth

of water. Generally, a 3:1 scope is the bare

minimum for any kind of holding, and this

is in light weather. Most anchors do best

with a 5:1 scope in most weather, going to

7 :1 for serious storms or bad holding

ground. Even a 10:1 scope is not unheard of

in very severe conditions. One way to get

this length is by attaching two rodes to-

gether. Chain can be connected using

shackles or midlinks, while rope can be

tied together with 90 percent strength using

the Carrick bend (Figure24-1).

The standard minimum rode length

should be suited to anchoring at 5: 1 in 30 feet

(9 m) of water for small inshore craft and at

5 : 1 in 60 feet (20 m) of water for most cruising

boats. This is a minimum of 150 feet (45 m)

of rode for small craft and 300 feet (100 m) for

cruising boats. Larger cruising vessels would

do better carrying 400 feet (120 m) of rode.

You often see a distinction made in

scope for an all-chain rode versus all-rope or

a rope-chain hybrid. My recommendations,

however, apply to all rode types.

Keep in mind that when it comes to

scope, what we’re after is to keep the angle of

pull on the anchor as shallow (as close to

horizontal) as possible. The real scope is

relative to the depth of water plus the free-

board at the bow.

Example:A boat with 5-foot freeboard

at the bow in 22 feet of water would need to

pay out 135 feet of rode for a true 5:1 scope

22 ft. depth +5 ft. freeboard =27 ft.

27 ft.× 5 =135 ft.

Or

A boat with 1.5 meter freeboard at the

bow, in 7 meters of water would need to pay

out 42.5 meters of rode for a true 5:1 scope

7 m depth + 1 .5 m freeboard = 8 .5 m

8 .5 m× 5 = 42 .5 m

Wire Anchor Rode Diameter

Assuming we had special deep-water anchor-

ing requirements, we might want to go with

the wire-rope anchor rode on the hydraulic

reel-drum windlass. The WLL for the required

BBB chain was 1,900 pounds (862 kg). Refer-

ring to the wire-rope table (Table 23-14), we

see that this would mean^7 / 16 - inch (11 mm)

7 × 19 316 stainless wire rope.

Because the wire has no give, it is usual

to install a leader of chain twostandard

chain sizes larger than standard BBB or

proof coil for a boat of this size with

all-chain rode. We found^5 / 16 - inch (8.73 mm

wire dia.) BBB, so this would be^1 / 2 - inch

(13.48 mm wire dia.) BBB. Length of the

chain leader should be between 1 and 1. 5

times boat LOA. Hold Fastis 32 feet (9.7 m),

so you would use between 32 and 48 feet

PART SEVEN: ANCHORING SYSTEMS

Figure 24-1. Tying
the Carrick bend
(From Field
Manual No. 5–125
“Rigging
Techniques,
Procedures, and
Applications”
Department of the
U.S. Army)