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PRACTICAL SEAMANSHIP

WIND

30% chain

70% rope

30 % chain

70 %

rop

e

LIGHT WIND

Chum

Chum

With the chain already virtually

straight due to wind speed, a

violent upward motion as the boat

rides a wave applies a powerful

force directly to the anchor,

pulling it upwards and possibly

to one side too, as the boat yaws.

These are both very diffi cult for

any anchor to deal with. Anchor

tests (see YM December 2006)

suggest most new generation

anchors will hold more than

two tonnes without dragging,

a fi gure that exceeds the safe

working load of all chain up to and

including 12mm. I suspect that it

also exceeds the strength of many

deck fi ttings, which could pull out

in the worst conditions.

The usual solution is to rig a

snubber that restores elasticity

to the chain (see YM September

2014). With suitable rope

diameter and length matched to

the boat size, these shock loadings

can be reduced considerably.

Mixed rode?

An alternative, if slightly

controversial, solution is to

substitute rope for some or almost

all of the chain. There can be no

catenary in this arrangement

but elasticity is provided by

the warp. Renowned yachting

mathematician Alain Fraysse

Sailing mathematician Alain Fraysse

pioneered the mixed rode. A short

length of chain dampens the nylon

rode’s elasticity and improves its

angle of pull on the anchor shank

showed that an all-rope rode

suffers from excessive elasticity

and suggests that a short,

heavy length of chain, weighing

perhaps 18kg, placed adjacent to

the anchor would be the most

effective method of damping its

elasticity. As wind increases, up to

the point where the rode is taut,

the chain’s weight helps to reduce

the angle of pull on the anchor.

Perhaps the best-known user

of mixed rodes was the late Alain

Poiraud, inventor of the Spade

anchor, who suggested that the

chain’s length should equal the

average depth of water in which

the boat anchors. In later years he

spent time in the Magellan Strait

and Beagle Channel where his

ground tackle coped well in often

violent conditions. Mixed rodes

need a high scope, rarely less

than 10:1 in windy conditions.

The disadvantage of mixed

rodes comes in lighter winds

when boats skitter about with

every slight wind shift. However,

owners of small, light boats may

fi nd the lower weight of a mixed

rode provides better sailing

performance, and for them, a

chum weight lowered down the

rode to the seabed can help to

reduce a boat’s swinging circle in

crowded anchorages.

In light winds, running a

chum down the mixed

rode helps to prevent

your boat skittering

around the anchorage

Simulating

Force 6 winds

The photo opposite shows my

15kg Rocna anchor on a

sandy seabed with a

scope of between

3-4:1 and the boat

motoring astern at

2,500 rpm to dig

in the anchor. The

8mm chain is entirely

off the bottom, as it

would be in a Force 6,

applying an upward force to

the anchor shank. I have seen the

same effect on a 27kg Manson

Supreme with 12mm chain on a

4:1 scope holding a 55ft Oyster in

a Force 6-7. This shows a heavier

chain won’t preserve catenary.

Losing catenary

In the picture top right, we see

my 15kg Rocna with 8mm chain

at a scope of about 4:1. The

indentations in the sand are clear

and un-smudged, which shows

the chain has been lifting clear of

the sand as the boat yawed.

These are relatively short

scopes, but I have anchored in

2-3m with a scope of 10:1 to ride

out a blow south of Bonifacio

Strait. Even there, catenary was

absent. Over four days I swam

over the anchor many times,

watching the whole chain lifting

while the shank of the anchor

moved as the force on it varied.

Snub and stay safe

Modern understanding is that

chain catenary has limited

effectiveness in fresh and strong

winds. It is essential to provide

the rode with some elasticity to

help prevent anchor break-out

and possible damage to deck

fi ttings. The most appropriate

method for most yachts is to fi t a

snubber tailored to the boat’s size

and weight to give considerable

stretch in gusts and waves. W

In strong winds, when catenary

is likely to disappear, snubbing

uses the elasticity of nylon rope

to dissipate the snatch loads

PHOTO: COLIN WORK/YM

PHOTO: ALAMY

My 15kg Rocna is well dug in with

8mm chain on a 4:1 scope. The

clear imprints of the chain on the

sand prove that the chain has

been lifting clear of the sand,

which means no catenary

PHOTO: VYV COX

Failure to snub your chain in

strong winds could lead to

your boat careering

helplessly across

an anchorage

MARCH 2016 http://www.yachtingmonthly.com 35

Anchoring myth busted