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PRACTICAL SEAMANSHIP

Vyv Cox is a retired metallurgist who spends six months a year in the Med living onboard his Sadler 34, mostly at anchor

Anchoring

myth busted!

WIND

In conditions of light to

moderate wind and limited

sea state, a length of chain

between the bow roller and

the anchor will form a curve

that starts by being almost

vertical at the boat end and

horizontal at the anchor,

where a signifi cant proportion

of it will be lying on the

seabed. The shape of the

curve is dictated by a number

of factors, including wind

strength, scope, weight per

What is catenary in an anchor chain?

Catenary is the curve formed by the anchor chain between bow

roller and anchor. It was thought to be ever present but recent

thinking indicates catenary disappears in winds of Force 6 or more

metre of chain, shape of the

boat and others. This curve

is known as the catenary. In

most cases the only short-

term variable is wind strength,

until the sea state begins to

exert an effect when the wind

becomes stronger.

I

n the typical gusting

conditions of most

anchorages, the wind

comes off the land from a

variety of directions due to

frictional effects. The weight of

a boat’s anchor chain provides

a certain amount of elasticity as

the boat moves forward in the

lulls, increasing the ‘catenary’ of

the chain, and aft in the gusts,

reducing it. Yawing, or moving

side-to-side, also affects catenary

as the wind acts upon the greater

area of the boat’s topsides.

Traditional anchors are very

sensitive to angular loads, relying

heavily on a large scope of chain

to keep the pull horizontal and

dragging quite readily when

pulled from any other direction.

New generation anchors cope

far better with angular pulls and

some remarkable claims are

made for the latest types, scopes

of less than 3:1 (chain:depth) being

quoted. A more common scope of

4:1 for modern anchors produces

an angle of 14° between chain and

seabed, if the chain pulls straight.

When catenary stops

Until recently it was thought that

catenary was always present,

but during the 1990s some

mathematicians began studying

it in earnest. Others began

watching what actually happened

in stronger winds. Both found that

catenary effectively disappears

when the wind reaches about

Force 6. There will always be a

slight curve in the chain due to

gravity, but essentially it draws

a straight line between the bow

roller and the anchor shackle.

Things get far worse when wave

motion is added to the equation.

With my Sadler 34

Straitshooter motoring

astern at 2,500rpm,

simulating the effect of

Force 6 winds in an

anchorage, it is clear

that the chain’s catenary

has disappeared

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

In winds above force 5, catenary disappears.

If you don’t snub the chain, says Vyv Cox, you risk

dragging your anchor or tearing out deck fittings

ALL GRAPHICS: MAXINE HEATH. PHOTO ABOVE: NOELEX