Sailboat Composite Standing Rigging – Material Options

Having established the significant advantages of reducing rigging weight aloft (view previous blog here), next we would like to look at the composite rigging options available on the market. However, before we get to that point, we need to take a little diversion into basic material properties to provide some background context.
Physical material properties are a technical subject and numbers vary considerably depending on manufacturer, test procedures and samples used. The table below represents a correlation of data across multiple sources and reflects the relative characteristics of a range of different materials. N.B. These are theoretical numbers for the material itself and finished products (cables, rods, wires) will have their own unique properties which will generally be lower than theoretical.

Sources: ^òToyobo Zylon Datasheet, ^tToray T800, ^YDSM Dyneema Datasheet, *Matweb

Terminology

    - Youngs Modulus – this is a measure of the stiffness of the material. The bigger the GPa number - the stiffer the material.
    - Density – provides an indication of relative weight for the given strength/stiffness
    - Ultimate Strength – is the amount of load to failure
    - Elongation at Break – this is the % increase in length to failure point (of a lab sample)

Discussion

Nylon/Cotton/Polyester/Hemp are shown for interest. Hemp and Cotton were traditional sailing ship materials for standing rigging and sailcloth and show just how far things have moved forward.

Steel wire replaced Hemp standing rigging from the mid 1800’s and is still the default material for boats sailing around today. Wire is 6x stiffer than Hemp and double the strength, but this does come at a significant weight penalty. Durability, reliability, ease of use and mass production techniques led to wire's domination. However, in pure strength-weight terms, Hemp was not a bad material!

The performance metallic standing rigging option is Nitronic 50 rod. Nitronic is a nitrogen-strengthened steel alloy and was developed in the 1960’s. The basic material properties are stronger and lighter than stainless wire plus the fact that it is a solid rod, rather than a twisted wire, leads to a significant diameter reduction for equivalent stiffness, which reduces windage.

The synthetic rigging revolution started quite slowly with the introduction of Aramid fibres in the early 80’s. A Kevlar 49 cable, built to the same stiffness, would be approx. 70% lighter and 7x stronger than Nitronic 50. The other major development was the extremely low elongation at break (2,4%) which means that the fibre will sit under high load and not deform / lengthen until it breaks. This is very important for standing rigging in terms of maintaining rig tune. The one downside with Kevlar is that it takes a lot of fibre to achieve the same stiffness, which adds significant diameter compared to rod.

PBO (Zylon) arrived on the sailing scene in the early 90’s with carbon rigging products coming in the early 2000’s. These two materials have very similar properties and are approx. 45% stiffer, 80% lighter and 8 times stronger than Nitronic rod. There are challenges in manufacturing these materials into standing rigging cables but, for a stiffness equivalent cable, they are comparable in diameter to wire/rod, whilst providing major weight and strength gains.

For the last 20 years therefore, Carbon, PBO and Kevlar have established themselves as options for lightweight, high strength standing rigging cables. Whilst it is possible to build lateral rigging from Kevlar, it is relatively uncommon due to its diameter. For example, a Nitronic Dash 17 rod has a diameter of 8,4mm – in PBO the stretch equivalent cable would be approx. 9,5mm but Kevlar would be more like 14mm. Kevlar is mainly limited to backstays on larger yachts where it can still deliver a 70% weight saving over wire/rod.

Dyneema®, which is a household brand name for an Ultra-high-molecular-weight polyethylene (UHMWPE), was developed around the same time as Aramid fibre. It is a bit stiffer than Kevlar (depending on the type), similar in strength but extremely light. However, UHMWPE has not been widely used in standing rigging, to date, due to one weakness - it creeps. At 30 degrees under constant load (10% of break load) it physically grows (SK75 - 0,02%/day, SK78 - 0,006%). On a 15m cable that equates to approx. 8% / 1.1m in 365 days for SK75, and 2.2% / 330mm for SK78.

Having said that – Dyneema is a cost effective and incredibly reliable / durable fibre and, on smaller boats (under 45ft), there is a growing community of sailors who are adapting and finding ways to manage these challenges. This can be done by using long lashings for terminations, so that creep can be taken up, and also by over-building the cable, which reduces the rate of creep. On larger boats it becomes physically impossible / impractical to develop the required rig tension using a lashing.

Summary

For performance sailors and larger yachts – PBO and Carbon are the primary material choices available for standing rigging. For sportsboats and smaller yachts these materials/products can become cost prohibitive and there is an increasing trend towards DIY Dyneema® rigging solutions.

If you would like to explore the options for a retrofit to composite rigging on your boat please use the link below, or contact us at support@upffront.com