©Karver

Upffront.com Structural Furling Forestays

The use of continuous line furlers and torsional cables for main structural forestays

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Contents:

Page No.

1. Introduction 3

2. What is a “Structural Furling ”? 3 a. Description 3 b. Advantages 5 c. Perceived disadvantages 8

3. Wire vs composite furling forestay 10

4. Deck and interfaces a. Fixed forestay length 11 b. Toggles or strops 11 12

5. interfaces 13 a. Luff 13 b. Hoist 14

6. Specifying considerations 15

7. Conclusion 14

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1. Introduction

In this document you will be introduced to the use of continuous line furlers, together with torsional cables, as an alternative furling system for the main structural forestay. This is NOT a “traditional” furling solution i.e. with an aluminium foil over the existing wire forestay, however, it is an increasingly popular, lightweight alternative for both offshore racing and cruising sailors alike.

Traditional Genoa furler / foil system (©Facnor)

We will be describing the key components, advantages and disadvantages of the system, discussing the appropriate use of wire vs composite fibre , setup methods and various sail interfaces and investigating the implications for the boat’s . Finally, we’ll be offering some guidance on correct specification.

2. What is a “Structural Furling Forestay”?

a) Description

The main forestay on a yacht is a crucial part of its “standing ” i.e. primary mast support, without which the mast will fall down! It is a permanent installation, normally with a fixed length and an essential element for maintaining the correct rig tension and tune.

Traditionally, we have accepted the forestay as a “given” and then added appropriate sail handling systems to it, depending on your sailing style. For example, old school headsails are hanked on to the

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wire stay with piston hanks, racers clip on lightweight plastic twin-groove foils for seamless upwind headsail changes and cruisers add aluminium foils and furling drums onto the fixed stay to allow genoa furling and reefing.

Soft Hank (©Equiplite) Tuff Luff Headfoil

A structural furling forestay is different in as much as the forestay itself is an integral part of the furling system. Instead of being attached directly to the mast tang and deck chainplate, the forestay attaches to the furling drum at deck level and a swivel sits between the stay and the mast tang and the sail furls directly around the revolving forestay. Both Drum and swivel are therefore permanent, structural elements in the system and take the full rig load.

Structural furler (©Karver)

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Old style furler / foil systems have evolved to fit on / around the existing wire forestay. However, continuous line furlers were developed for independent code zero and asymmetric furling systems where the drum, swivel, cable and sail all form part of an integrated system. Continuous line furler manufacturers have now evolved their product ranges to take in this new, niche, structural furling forestay system application to offer some significant benefits.

b) Advantages

Weight

The primary advantage over traditional furling systems is weight reduction. The forestay itself is the longest, heaviest individual element of your . Saving 1 kg of weight in the mast and rigging package is equivalent to adding 4 kg to the bottom of the bulb of your and is regarded as one of the most cost-effective methods of improving boat performance. Reduced weight aloft will increase stability, reduce pitching and heel angle, improve acceleration and even reduce rolling at anchor.

The best way to demonstrate the weight benefits of a structural furling forestay system is to run a simple case study example: A 56 ft cruising boat with a 17 m long, 12 mm wire forestay:

Traditional furler / foil setup:

• Wire forestay: 12 mm wire = 0.7 kg/m x 17 m = 11.9 kg • Aluminium foils: at this size, average foils are approx. 1 kg/m x 16 m = 16 kg • Furling drum and swivel: We are not aware of a single traditional furling unit manufacturer which publishes furling unit/swivel weights, but at this size, a conservative estimate would be approx. 6 kg • Total system weight: 33.9 kg

Structural furling forestay setup:

• 17 m Composite K49 forestay (see reference cable here) = 4.2 kg. o This is a robust cruising option…….. a race cable would be approx. 2kg • Furling drum and swivel: a 7-8 t Safe Working Load (SWL) unit would be appropriate and averaging across various suppliers gives an approx. weight of 2.3 kg for drum and swivel • Total system weight: 6.5 kg

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Therefore, the structural furling forestay system provides an overall weight saving of 27.4 kg which is a colossal 81% saving over the traditional furler / foil configuration. This rig weight saving would be equivalent to adding 110 kg to the bottom of the keel and make a significant difference to upwind sailing performance in a seaway.

N.B. Lighter not only means faster, but also safer. Reduced rig weight means lower loads, less wear / fatigue and more comfortable sailing……….. Lighter, faster, safer and more fun, equals a real win-win scenario!

Offshore sail plan

A structural forestay furler lends itself to an offshore sail plan. Continuous line furlers are not designed to sail with the headsail reefed, so the genoa is either all-out or all-in (see Perceived disadvantages section below) which means it works well for a smaller high clew / yankee used in combination with an inner .

This style rig configuration has been a popular choice with offshore cruisers for centuries and increasingly for offshore race boats as well.

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Take a look at the latest IMOCA Open 60 and Volvo Ocean Race fleets and they will often be seen with three reaching headsails, all on continuous line furlers: the inner staysail, genoa (a structural furling forestay) and Code Zero.

Three-sail reaching (©Volvo Ocean Race)

Windage

Not a major factor for most cruising sailors, but like weight, anything that can be done to reduce overall windage will improve your sailing performance and comfort. A continuous line furler is a much lower profile unit and generally closer to the deck than traditional furling drums set on, or above, forestay turnbuckles. However, the main windage gain is the removal of the aluminium foil which sits over the forestay.

Using our 56 ft cruising boat example: average aluminium foil section diameters for this size boat would be 50 mm. Multiplied by the length (16 m) gives us a cross sectional area of 0.8 m2 or 800,000mm2. Compare this to the cross-sectional area of 246,400 mm2 (0.25m2) for the recommended 15.4 mm diameter torsional cable in a structural furling forestay setup, and you have 70% lower windage.

Maintenance

The main maintenance advantage with a continuous line furler on a torsional cable is the removal of the aluminium foil. The traditional furler / foil system sits on, and rotates around, the fixed forestay beneath, which creates plenty of opportunity for interface issues, abrasion and damaged foil sections.

Add to that the possibility for external impact damage or getting the sail luff tape caught on the 6-8 joins along the luff length groove……..

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Just the fact that there are so many more individual parts in a traditional system makes them more prone to damage.

This parts list diagram for Harken’s MKIV Furling system is representative of the numerous elements that make up a traditional furling system.

Conversely with a structural furling forestay – there is one drum, one cable and one swivel – with simple interfaces, which provides a basic, effective and low maintenance system. The cable also has a soft, abrasion resistant outer cover which is kinder to your sail.

c) Perceived disadvantages

All-in OR All-out

This is the main question to be answered when considering a switch from a traditional furler / foil to a structural forestay furling system. Many production cruising boats have a single 140-150% furling genoa which has been designed to be reefed in order to cope with the full range of sailing conditions.

A continuous line furler and torsional cable however, are not designed to be used with the sail reefed and therefore the compromise is that the sail is either used at full size or furled completely away. As discussed above, this is not necessarily a problem but a decision to switch to a structural furling forestay cannot be taken in isolation and needs to be seen in context with your wardrobe. Your old 150% furling genoa would need to be cut down or replaced, you would need to be considering the use of a staysail (for heavy airs) and, if you haven’t already got one, a Code zero to give you more power in lighter airs, both upwind and reaching.

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Bearing the load

The traditional furler / foil is set up on a fixed forestay. The forestay takes all the tensile rig load and the furler and foil, effectively “rest” on the deck chainplate / turnbuckle. The bearings in the drum are just taking the weight of the foil and sail above it, while the upper swivel takes the load and both are allowed to rotate freely with little resistance.

On the other hand, with the structural forestay furler setup, the furling drum and upper swivel take the full tensile load of the forestay which is holding the mast up. Whilst the bearings are designed to take these loads, require very little maintenance and can give years of trouble free use, there is the risk that if the bearings seize it will mean that the sail cannot be unfurled, or furled. However, this is the same situation if the bearings of a traditional furler seize and fixing a problem with the drum bearings will usually require the removal of the forestay regardless of which system you have, so in reality there is little difference.

Forestay fatigue

A concern for some sailors might be that the main forestay is being used for both holding up the rig and for furling purposes, and that this dual function could result in increased fatigue on the primary headstay.

For the majority of boats, we are talking about replacing a traditional wire / rod forestay with a composite torsional cable which derives its overall strength and stiffness primarily from the core unidirectional fibre (commonly PBO for race boats and K49 for cruisers) whilst all the torsional performance is delivered by the multiple braided covers that are applied around the core, load- bearing, fibres.

Any composite standing rigging element will be specified based on its stiffness rather than strength. The rationale for this is that the mast designer and sailmaker need the mast to behave / bend in the same way regardless of whether wire, rod or composite rigging is used. One of the significant advantages of using composite rigging, other than its 70-80% weight advantage, is that for the same stiffness, composite cables are generally 3-4 times stronger than their wire / rod equivalent. This has x2 important benefits:

• Increased Factor of safety o The difference between working load and break load is significantly increased • Lower fatigue rates o The composite cable is working at a significantly lower percentage of its break load which reduces fatigue rates. o Obviously, different materials have different fatigue rates i.e. steel wire can operate at 50% of its break load for much longer than most fibres. However, the specification

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of the composite cable can be fine-tuned to reduce fatigue relative to its wire / rod counterpart, without sacrificing much of its 70-80% weight advantage.

Therefore, given that the two functions of the structural furling forestay (tensile load and torsion) are handled independently by the core and covers of a torsional cable, their safety factors are much higher and fatigue rates can be managed, we would contend that the weight benefits and simplicity of this system exceed any “perceived” risks.

3. Wire vs composite furling forestay

Structural forestay furling systems using a normal wire stay are available on the market for boat lengths of up to approx. 10-11 m and a maximum of 7 mm wire diameter. If you look closely at the way a wire cable is made, it is wound in a spiral which lends itself quite nicely to handle torsional loads. At the upper 7 mm limit, a Dyform wire construction is preferable as this has a much more robust construction with fewer individual wires which are prone to break resulting in potential tear hazards on standard 1x19 wire.

Structural wire furling forestay with halyard swivel (see section 5)

Above 11 m boat length and 7 mm wire diameter a composite torsional cable should be used as the structural furling forestay.

It should be noted that wires must be furled in one direction only. Although this is no different to composite cables, which rely on off-axis braid fibre alignment for their torsional strength and for optimum performance should also only be used to furl in a single direction.

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4. Deck and mast interfaces

Fixed forestay length

One of the first mental adjustments required for cruising sailors when considering a structural furling forestay system is that there will NOT be a turnbuckle on the forestay. Most traditional furling / foil systems allow for a forestay turnbuckle below the drum or, on more advanced systems, integrated within the drum body. Whilst it is theoretically possible to put a forestay turnbuckle between the deck chainplate and a continuous line furling drum, there really is no point and it also defeats the objectives of weight reduction and a low profile / windage system.

Raised drum of a traditional system Low profile structural furling system

Turnbuckles on forestays are there to allow flexibility in the system, in terms of rig setup and tune. However, once you have established the correct forestay length for the boat (during initial sea-trials on #1 of a new production boat!) – this will rarely change, and the forestay turnbuckle becomes superfluous, actually creating an unnecessary complication. Race boats all have fixed length forestays. They decide what mast rake they want (sometimes using a number of strops to vary rake for different wind conditions) and this dictates the fixed forestay length. The forestay is attached first during mast stepping and then the rig tension and mast tune is achieved using the lateral rigging (vertical / diagonal) turnbuckles.

Once you have determined your optimum forestay length, the need for a forestay turnbuckle becomes obsolete.

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Toggles or Strops

In the same way that we are conditioned to believe we need a forestay turnbuckle, perceived wisdom would suggest that we need metal toggles to connect a structural forestay to the deck chainplate and mast tang. The toggles are there to allow articulation of the stay, to minimise metal fatigue on these critical rig interfaces. Installing extended toggles with multiple hole positions can also allow some length adjustment and “de-risk” the length calculation of the forestay.

Yet again however, modern materials are challenging these old preconceptions and offering exciting new solutions with significant benefits. Strops / loops and even lashings provide a viable, and some would say much better, interface for this primary structural stay.

Structural furling forestay with lash termination

By way of example, furling unit manufacturers didn’t enjoy the 2008/2009 edition of the Vendee Globe. There were a number of forestay failures / dis-mastings which were initially attributed to continuous line furling units. At the time, the standard setup on IMOCA 60’s was to use metal

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toggles to connect the drum to the chainplate. However, following extensive post-race investigations it was found that the rapidly increasing speeds of these boats, and the subsequent slamming through waves when power reaching, was significantly increasing the articulation at the bottom of the forestay, to a point where the toggles would bind up. This resulted in rapid metal fatigue in the base/stem of the drum, ultimately leading to premature and catastrophic failure.

Since that edition of the Vendee, all boats now use strops to connect the drums to the chainplate and failures of this type have all but disappeared.

Soft strops / lashings naturally self-align to variable load angles and modern Dyneema® has incredible fatigue resistance. Its most recent reincarnation, DSK99, almost eliminates the historical issue of creep and provides a viable, cost effective, simple solution to standing rigging interfaces. It reduces fatigue, extends the working life of the mast and fittings…… oh, and not to sound obsessive at all…… coming in at a fraction of the weight!

The only downside with using soft strops for drum / swivel connection to deck / mast is the potential for twist, thereby reducing furling performance. This can be minimised by making strops / lashings as short as possible, but this does increase the importance of forestay length calculation and construction accuracy.

Most continuous line furlers, from 5 t SWL and above, will be supplied with a lashing eye as standard for use with soft strop connections. For the bulk of the market, using 1.5 t – 2.5 t SWL units, the default connection method would be metal toggles. However, if you are feeling brave and want to make the switch to soft terminations then the 3:1 friction sheave accessory, provided by most manufacturers, provides an excellent substitute for a lashing eye to enable the use of a soft termination.

5. Sail interfaces

Luff

Traditional furling genoas are finished with a luff tape which runs up the single or twin groove tracks in the aluminium foil. Obviously on a furling forestay, there is no track and so the sail can be finished with either soft hanks or a tube up the sail luff with a zip closure. From a purely practical perspective, soft hanks are the simplest, cheapest and most reliable system and are favoured by professional cruising yachts and race teams.

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There are many varieties of soft hanks available on the market and it is a relatively quick/simple alteration to make to a luff tape sail.

Hoist

As the forestay itself will now be rotating, there has to be a modification to the halyard system to manage the transition between the furling sail and the fixed halyard point on the mast. Depending on whether you have a wire or composite furling forestay, different options are available to deal with this problem.

However, we would suggest that the simplest, cheapest and most reliable system for attaching the head of the genoa is to hoist it on the dock and lash it permanently to the top of the furling cable, at the correct luff tension. Again, this is counter-intuitive for many cruising sailors but, for a sail that is normally permanently mounted on the headstay, it is the cleanest and lightest solution. It does require a trip up in the Bosuns chair for installation and removal but once it is up, you can forget about it.

However, if you would like to retain the ability to hoist / lower the genoa without a trip up the mast, then there are a couple of different solutions.

For wire furling forestays the halyard swivel works very well using the existing halyard / hoist system with no alterations. The forestay runs through the centre of the halyard swivel, so it slides freely up and down the wire.

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The lower half of the swivel attaches to the head of the sail and the upper half is attached to the halyard. The sail is hoisted as usual and then the sail / headstay can be furled without affecting the fixed halyard exit point.

With the introduction of composite torsional furling forestays, many manufacturers proposed the same solution. Whilst the theory is sound, and the system can work well, practical issues with cable diameters vs halyard swivel internal diameters and the need to put the halyard swivel on the cable during manufacture, severely limited its use.

In response to this, some alternative solutions are starting to appear on the market. One example is Karver’s forestay Link which is designed to be used with a small halyard lock. The link and lock are mounted just below the upper swivel and the hoist line is run up, and then back down the luff of the sail. The halyard tail is kept in a pocket at the of the sail.

A lower tech alternative would be to replace the lock at the head with a and then mount a small purchase system at the tack of the sail and maintain halyard tension that way. can be constructed in two parts to minimise the amount of line to be stored at the tack.

Without “active” halyard control from the cockpit / mast, if luff tension control is required a simple system can be set up on a ring / loop within the tack patch on the genoa. However, you must remember to detach the cunningham prior to furling!

6. Specifying considerations

Specifying a traditional furler / foil system is relatively easy as the manufacturers have done all the background calculations and standardised their ranges. Sizes are driven by existing forestay diameter and length. Continuous line furlers, on the other hand, have come from code zero and asymmetric furling where there is no existing forestay and specification is driven by Safe Working Loads (SWL).

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But what is the SWL of your main forestay and does that give you the correct spec for a structural furling forestay system!? Interestingly, when you start digging into the technical data, your chainplate pin sizes can often be the primary driver for the correct furling unit size.

See the table below referencing Industry standard data for 1x19 wire / Rod dash sizes and using some relatively standard continuous line furler sizes as an example:

* Assumed working load using a Factor of Safety of 2,5

Let’s just run through an actual example to demonstrate some of the complexities involved in arriving at a reasonable specification. A customer had a 37 ft cruiser/racer with a 15 m long, Dash 10 forestay. The Dash 10 forestay would suggest that a 2500 kg SWL unit would be the correct choice…. and from a pure working load perspective it would be.

However, when we got into the detail, it was established that the chainplate / tang pin sizes were (as per standard industry specs) 12,57 mm, whereas the toggles supplied by the furler manufacturer were only suitable for 8 mm pins. When asked if the furler toggles could be adapted to fit a 12,57 mm pin, the manufacturer replied that this was not possible as they could not maintain a reasonable factor of safety for their toggle. The maximum they could increase the hole size in the toggles was 11 mm.

Why are the working loads bigger on the furler, but the pin / hole sizes so much smaller than on the boat? ……. it doesn’t seem to make sense. The answer lies in the origins of the different products. Wire and Rod rigging manufacturers have created standards which the rest of the industry (boat / mast builders) have had to conform to. If you are approaching the safe limit of one rod size then you have to jump to the next standard size up, which may be oversized but it’s your only choice.

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Continuous line furlers on the other hand have only developed since the emergence of composite rigging which is NOT constrained by industry standards. A composite cable can be designed and built to have a SWL of 1800 kg, 1850 kg, 1900 kg or even 1937 kg, if you want. This technology was created for race boats and the manufacturers focus was, and still is, to make their equipment the smallest and lightest possible, using the latest materials.

In this instance, the client was forced to upsize to a 5 t SWL unit to be able to interface with the deck chainplate and the mast tang. BUT this was the correct unit for his boat for x2 reasons:

• The 2.5 t unit would have looked out of place on his – way too small for the surrounding hardware / fittings. • The additional drum diameter is a major benefit for delivering the power to start the genoa furl.

Specification using strops

If you are planning to use strops for mast and deck attachment, then the pin / toggle issue disappears and you can be guided a little more by SWL, but appropriate drum diameters, to deliver power for the start of the furl, will generally be the guiding factor.

In, conclusion, structural forestay furling units can rarely be specified on SWL alone. Decision criteria are more based on mast/deck interfaces and an appropriate drum size for delivering the required furling power.

As a rule of thumb, a structural furler should be one size up from the manufacturers recommended code zero furler.

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7. Conclusion

Various structural forestay furling systems have been around for many years for wire forestays on dinghies and small keel boats. These days, with the advent of torsional composite cables for code zeros and asymmetrics, structural furling forestays are now a viable choice for all boat sizes, from beach cat to mega-yacht. And for offshore cruising boats the performance benefits over traditional furler / foil systems can be significant.

If you are interested in looking at whether a structural furling forestay could be the right choice for your boat and sailing style, then why not contact us. We will just need some basic information about your boat; type, existing forestay length and diameter etc. and we can provide you with some options and discuss all the potential implications of transferring from your current system.

We look forward to talking to you.

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