How many sails do the Imoca have? Why are trees a weak point? Word to Charal’s “sailmaker”

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Charal

The Vendée Globe gets into full swing, and it’s time for the fleet to push south, contour the St. Helena Anticyclone and begin the great ride into the Southern Ocean. In the lead was Thomas Ruyant, who passed Alex Thomson, but we will discuss this in our next chronicles. Today it’s time to talk about sails, and masts, with one of the “gurus” of the class Imoca, the North Sails designer Gautier Sergent who oversaw the plans for many of the Imoca 60s leaving for the Vendée Globe, Charal in particular, and is now part of Ineos Team UK, the British challenge to the America’s Cup.

Gautier Sergent

That of sails, and reflexively also of masts, is a hot topic at this Vendée Globe, interesting to understand what the potential of the new Imoca foilers is. The inventory of sails aboard an Imoca 60 includes 8:

  • Mainsail
  • A storm jib of the minimum size of 20 sq. m.
  • J3: a windward jib for upwind, which can also be used as a staysail in front of asymmetrical offshore
  • J2: Upwind from 12 knots and up, and very tight slack
  • FR0: it is a fractional Tails, it can be used upwind with little wind or downwind with strong wind
  • J0: is the masthead gennaker, for light air, under 12 knots, on the slack
  • A7: asymmetrical from winds over 22-25 knots
  • A3: asymmetrical for winds up to 25 knots

That said, it should be noted that the Imoca class, now since Vendée Globe 2016, has been using standard masts throughout the fleet. These are designed for loads, generated by the combined righting moment between keel and foil, within which the older generation Imoca or the first generation of foilers lie. The discussion changes if we talk instead about the latest generation of foilers.

In fact, the regulations left room for designers to “indulge” in foil sizes. This has resulted in XXL appendages, never seen before, but under certain conditions such as tight slack in high winds they come in some cases to generate a righting moment greater than the loads allowed by the mast. A risk, however calculated over nearly two years of testing, that designers took in light of the enormous benefits of large foils. And this, for those who are in the business of designing sails instead, is a problem to be taken very seriously.

How have the sail plans of the latest generation of Imoca foilers changed?
There is one less sail allowed in the regulations than last time. So up to 8 sails in total, including the storm jib. This in itself forces a review of the sail inventory. If you add the new foils design that drastically changes the boat’s polars, the new sail inventories are drastically different from last time. Then each skipper makes the specific choices based on his boat, but if we try to summarize the differences for new foilers:
– Generally there are no more symmetrical spinnakers
the so-called J1 (masthead jib walled at the extreme bow) has disappeared. That sail generated high loads by being masthead. The rig, which is of the One Design type and limited to 30t.m of righting moment, has become the fuse or weak link in the chain now that foils can generate a huge dynamic righting moment (close to 40t.m launched at tight slack). So the trend is to favor fractional sails. This also helps the stability of the boat when foiling(lowering its center of gravity and center of effort). As you can imagine the boats are quite unstable in pitching by not having elevators on the rudders. Just like a rocking chair! And because boats start foiling very early, they will have to reduce sail area much earlier, so as to shift the various “crossovers” between sails to lower limits of the true wind.

What is the difference, and why, with the sail shape of an older generation Imoca?
In general, the sails have become flatter especially at the top. Boats need the square footage to take off but then you try to “unload” aerodynamic power as the boat rises on the foils and accelerates (with the apparent soaring). Because you are racing solo, you will not always have perfect sail trim, so making the topsails flatter means they “automatically” depower when boats accelerate. The shape curves are also more advanced to maintain the balance of the boat and realize fast airfoils.
All sails that are not jibs use “load sharing technology” (Helix) (there is no anti-twist cable, but the sail has “structural” reinforcements that replace it and absorb the loads by lightening the rig ed.). This helps to reduce crushing of structures and to project forces forward rather than sideways. It also reduces loads on the rig, which is a key element, as explained above, as the rig is the limiting factor in terms of power.

What are the maximum speeds that a new Imoca can reach?
Maximum speeds are limited by sea state. What is impressive is what we call sustained speed. The previous generation could reach high speeds on a surf along a wave. These new foilers are most impressive in the medium breeze, where they can really use the full power of the foils and sails and be stable. For example, in TWS15 (TWS is the real wind ed.) and in reasonable sea conditions we will have a boat at 25kts speed 120 real angle to the wind. We have had cases where guys have been able to sustain a boat speed of 25kts for a very long time. Hours, not seconds.

On the topic of masts we will come back to it again, with an in-depth study in the coming days devoted to all the load “alarms” there are aboard an Imoca, on foils in particular, to try to understand how the skipper moves to stay within the “limits” of what his boat allows him.

Mauro Giuffrè

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