boat owner

How we tested them

Spreaders

Backstay

Cap shroud

Lower shroud

Forestay

Andrew Simpson

Rig tension gauges

➜

Adjusting cap shrouds by

the ‘folding rule method’

Seldén, in their excellent publication Hints and advice on rigging

and tuning of your Seldén mast (also to be found on their website),

suggest this method as an alternative to a rig tension gauge. It

works on the principle of measuring the extension of the shroud

under load. You will need:

■ 2m-long measuring rod (a 2m folding rule is ideal)

■ Vernier callipers

■ Adhesive tape

METHOD: Start with both cap shrouds hand-tight. The rig is stayed

with the lower shrouds and the forestay and backstay. Attach the 2m

rule with the adhesive tape at the top to the starboard shroud about

5mm from the upper end of the wire terminal. Measure the distance

accurately using the Vernier callipers. Call this measurement x.

Tension the cap shroud until the distance is x +1.5mm, noting

the number of turns required to do this. Move across to the port

shroud and tension the rigging screw the same number of turns.

Return to the starboard side and adjust the tension until the gap is

x + 3mm. The shrouds are now tensioned to 15% of their breaking

load. If the mast is not straight, adjust the lower shrouds. The folding

rule method can be used on other stays, such as the backstay and

forestay (without jib furling system). It can also be used for Dyform

or rod rigging, but you will need to take the difference in stretch into

account compared to 1x19 wire (see the Seldén website). This

method works for different diameter wires because the extension

is proportional to the cross-sectional area and the break load.

In our test, we wanted to see how the rig tension gauges performed

over a range of loads. Yacht Production & Surveying student Matt

Turner (below) and I took a length of 5mm-diameter 316 stainless

steel rigging wire that had both ends fi tted with a swaged eye. We

set this up in an Olsen tensile testing machine in Southampton

Solent University’s material test laboratory. We

programmed the machine to apply fi ve different

loads progressively on the wire, ranging from

100kg to 500kg. This represents around 4% to

22% of the wire’s break load. At each load we

measured the reading on the different gauges

to see how they performed.

Shroud construction

method and material

Most yachts have shrouds made from 1x19 wire:

this comprises 19 wires of the same diameter. One

wire runs up the middle, six wires are wound round it

and then 12 wires are wound in the opposite direction

around them.

A variation is Dyform wire, which uses shaped strands

which fi t together in a more compact way. Another is to

use wire that is itself wound: this can give, for example, a

7x19 wire. Racing yachts also use solid rod to form their

stays and shrouds, a more expensive option.

Most stays and shrouds these days are made from 316 stainless steel,
which is specially produced for marine applications because of its ability
to resist pitting corrosion. While not completely rustproof, the alloy is
more corrosion-resistant than other common stainless steels.

A larger mast will require more
spreaders spaced up the mast
to support it. Spreaders on
masthead-rig yachts tend to be
angled transversely across the
boat, while fractional-rig boats
usually have the spreaders
swept back at an angle to
help support the mast in a
longitudinal direction.

Mast theory
In purely engineering terms,
yacht masts are structural
columns designed to resist the
compressive forces and bending
moments applied by the loads
they are subjected to by the

rigging and the wind. Leonhard

Paul Euler (1707-1783), a Swiss

mathematician and physicist,

provided the basic theory that

structural engineers and mast

designers use.

Euler established that a column

will buckle long before the applied

load would cause it to fail in pure

compression, and went on to

prove that the load at which

buckling failure will occur

depends on the ‘slenderness

ratio’. This is expressed as

the effective length of the

column divided by the radius

of gyration of the column

section – the l/r ratio.

The main

components of

the standing

rigging