It’s vitally important to ensure that the tension of your boat’s rigging is within the correct limits: Alex Bell tests four rig tension gauges to assess their accuracy
Getting the correct tension on the shrouds on my various boats over the years has always worried me, writes Alex Bell.
Too slack, and I know that the sails won’t be working as efficiently as they can: and, worse, the rigging and mast fittings can be subjected to shock loads and fatigue.
Too tight results in extra strain on the various attachment points of the rigging and the shrouds themselves, and can distort the hull as the loads are transmitted through the chainplates.
Most mast breakages occur when the mast is overloaded, either by the wind or, more frequently, a collision.
Modern stainless steel shrouds themselves rarely break, but the fittings to the mast or the boat do fail, with consequent failure of the mast.
Fortunately, there are devices available on the market that enable one to measure the tension and make sure it is set within the right limits.
There are currently three makes of rig tension gauge on the UK market designed for yacht rigging:
- The Loos, which comes in Standard and Professional models, each in three different shroud diameter ranges, from 2.5 to 10mm.
- The SureCheck, which comes in four sizes, from 2 to 10mm.
- At the time of going to press, the newly-launched Spinlock Rig-Sense was only available in one size: 2 to 5mm diameter. A larger version of the Spinlock Rig-Sense for 5 to 8mm diameters has since become available.
Whether a yacht has a masthead or fractional rig, they all have common rigging features: a forestay and a backstay to support the mast longitudinally (i.e. forward and aft), and the lateral rigging supporting the mast across the boat.
Many yachts will have an adjustable backstay in which the tension can be varied: such adjustments will always affect the tension in the forestay, particularly with a masthead rig. The lateral rigging, known as the shrouds, requires a fixed tension to keep the mast upright in the transverse plane.
Most sloop-rigged yachts will carry two sets of shrouds. One set will terminate just below the spreaders – the lowers. The other will pass through the ends of the spreaders and terminate at the masthead (masthead rig) or lower down at forestay level (fractional rig) – the cap shrouds.
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.
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.
Shroud construction method and material
Most yachts have shrouds made from 1×19 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 fit together in a more compact way. Another is to use wire that is itself wound: this can give, for example, a 7×19 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.
How we tested them
In our test, we wanted to see how the rig tension gauges performed over a range of loads. Yacht Production & Surveying student Matt Turner and I took a length of 5mm-diameter 316 stainless steel rigging wire that had both ends fitted 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 five 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.
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 1×19 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.
Readings are based on 1×19 stainless steel cable: two models are available, each covering three wire sizes. Manufactured from anodised aluminium, the gauge cannot be left on the shroud when tensioning as it has to be
held in position. The gauge comprises two aluminium arms which are riveted together at the bottom end. The wire is placed in a groove at the riveted end, then the gauge is placed up against the shroud and a piece of string is
pulled so that one of the arms extends to a maximum value. The reading is then taken from the other arm on a scale: this reading is then transferred to a scale on the arm which enables the tension, in kg, to be read on the appropriate diameter scale.
This gauge is straightforward to use, and an instruction sheet is provided.
The Loos Professional gauges are designed to have a higher accuracy than the Standard gauges. Readings are based on 1×19 stainless steel cable, and six models are available – three sizes in either metric or imperial. The gauges are manufactured from anodised aluminium, and supplied with instructions and a conversion chart.
The gauge comprises a flat aluminium plate with two fixed spools at the bottom and a sliding nylon hook at the top which engages the stay. The hook is attached by pulling on a string attached to the slider, which works against a coil spring. The slider has a pointer which runs along a scale. A reading is then taken from a table on the plate graduated to wire diameter.
It is simple to use, but care is needed to eliminate friction in the slider. The shroud tension can be adjusted with the gauge attached to the wire.
The SureCheck is calibrated for 1×19 wire.
Manufactured from stainless steel, the gauge comprises two arms pivoted at one end, the other being held together by a coil spring.
A grooved nylon spool is rotated until the correct wire size is selected; the gauge is then attached to the stay by pulling on a ring so that the grooved spool is hooked over the wire. The two arms should be squeezed together and released. The reading is taken against the last red dot exposed by the left-hand arm. The load reading is obtained from a graph provided as a percentage of the break load, based on a figure of 2,000kg for 5mm diameter wire.
The gauge can be left on the shroud while adjustments to the tension are made. At the top end, the graph provided was difficult to get percentage break load readings from because the curve flattens out. This is a robust and good-value gauge, but it is important to ensure the correct wire diameter is selected on the spool.
The Spinlock Rig-Sense is a new rig-tuning tool developed to give consistent and repeatable measurement of loads on different wire composition as well as fibre.
It is manufactured from anodised aluminium with glass-filled copolymer components and a composite leaf spring. The wire grips are stainless steel, which should reduce wear and give constant readings over time. The leaf spring is calibrated to output load readings to a direct linear scale in kg. The Rig-Sense allows one-handed operation, and comes with a carry bag: an app can be downloaded to smartphones or tablets to enable readings to be recorded and shared for future reference.
This is easy to use and gives a direct load reading: it can be left in position while adjusting the shroud tension. The design eliminates friction, which should lead to consistent readings. It is currently only available for 2 to 5mm wire.
PBO results and verdict
As we expected, the readings taken from the gauges varied slightly from the actual loads. There will be a number of reasons for this. Firstly, some readings require some interpolation between two marks on the scale. There is some skill in operating the gauge: the effect of friction, which can lead to a false reading, has to be reduced.
From the graph, the SureCheck final reading looks suspect, but this was probably because the conversion graph tended to a horizontal line at the upper range which made for larger reading error.
The Loos Standard gauge was simple to use, but could not be left on the shroud when it was being tightened. The Loos Professional gauge gave a more accurate reading than the Standard gauge and could remain on the shroud during tension adjustment. The SureCheck was straightforward to operate, but it was more difficult to obtain results at the top of its range.
The Spinlock Rig-Sense was the only gauge that allowed the load to be read directly and whose design eliminated the effect of friction in the mechanism. For ease of use it gets the best buy, but until the model range is extended to accommodate larger-diameter wire, it has limited applications.
As published in the April 2016 issue of Practical Boat Owner magazine. Prices correct at the time of going to press.
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