Inside Gurit’s Structural Innovation for Crédit Mutuel 2

The colours were the same, as was designer David Raison, but in almost every other respect Globe40 winner Ian Lipinski’s second Crédit Mutuel Class40 was a very different beast from its highly successful predecessor

JEAN-MARIE LIOT
Happy chappy… Ian Lipinski dominated the Class40 for two years with his first scow, Crédit Mutuel 1, then once the performance of his ‘novel’ design became obvious the rest of the fleet rushed to follow designer David Raison’s lead and eventually improve on it; remarkably, following the launch of No 158, every single Class40 launched has been a scow. No157 now - and probably for ever - holding the distinction of being the last ever ‘pointy-nose’ Class40.

Winner of the Globe40 that concluded in April, the latest David Raison-designed Crédit Mutuel 2 Class40 proved as reliable as it was obviously fast in the expert hands of Ian Lipinski, Amélie Grassi and Antoine Carpentier.

A clear evolution of Lipinski’s previous boat, No 158, which had first introduced a then rather daring scow bow to the Class40, the current racer (No 202) has been conceived with a focus on medium conditions, while her predecessor excelled in stronger winds. Talking to Gurit engineers Paolo Manganelli and Nicolas Poidevin reveals just how big a step has also been made in terms of structural engineering for this latest Raison machine.

As skipper Ian Lipinski puts it, ‘The hull shape has changed and offers more power, with notably more beam towards the bow. What is also obvious are the tall side panels protecting the cockpit. But it is inside that we have developed the most important feature: an all new longitudinal structural layout to enhance global rigidity.’

Collaboration between scow pioneer David Raison and Gurit had already started, albeit with a lighter touch, for Crédit Mutuel 1; now there was a deliberate intention to deepen the relationship for the second boat. ‘We were already super happy with the first collaboration, working for Ian Lipinski,’ says Manganelli.

‘When he decided to build CM2, Ian had a bit more budget and asked us if we would consider being more involved in the design, working more closely with David.’

The brief was ‘push hard in all areas’. Much new work has been done on the rig and CM2 also has more advanced lifting rudders. In development terms the team were going to leave no stone unturned.

Gurit’s biggest contribution to this was to table the idea of a novel longitudinal structure: ‘It may not sound like much to write home about,’ says Manganelli, ‘but examining the solution in detail reveals critical nuances and significant gains.’

The team started from the observation that Class40s are getting wider towards the front as well as flatter. Added to this, one of David Raison’s signature features is coachroofs that tend to extend further and further towards the bow. ‘And this set us thinking…’ continues Manganelli.

‘As Class40 rules forbid carbon we’re always fighting to gain longitudinal stiffness, key to keeping sufficient forestay tension. Our idea was to make greater use of that distinctive coachroof by turning it into an integral part of the structure, to end up with a “thicker” boat essentially.’

To really grasp the idea, one has to do two things: first, picture the boat’s profile, looking at it side on. Second, consider the boat as a beam whose stiffness is directly dependent on its thickness (ie its depth from coachroof top to hull bottom).

Now allow me to add another image: the hull is essentially like a crossbow, flexing at its extremities under the tension of forestay and backstay, the top of the mast figuratively being the finger of the bowman. Now, on a boat whose coachroof is not integral to the structure but rather added to it, the effective thickness is measured (still looking at the boat’s profile) from the hull bottom to the side deck. But in the case of Crédit Mutuel 2, it goes from the hull bottom to the top of the coachroof - this is why Manganelli talks about a “thicker” boat.

‘This is where we then needed to look at the adoption of longitudinal bulkheads,’ continues Manganelli. ‘Theoretically the wider you go at deck level the stiffer the boat… yet, due to shear lag (the non-uniform distribution of stress in the flange of a beam), the further you move from the topsides the less efficient the added material becomes. One solution would then be to go heavier on the exterior contours of the “box” (hull shell) and maybe also add material to the side deck…’

‘At the same time,’ chimes in Nicolas Poidevin, ‘the coachroof is usually considered non-effective and so is ignored when calculating a yacht’s overall stiffness.

‘That’s where the new Crédit Mutuel is quite different, since the coachroof’s longitudinal elements extend under the deck to connect to full height longitudinal bulkheads. So now we’re moving the side panels (the outer portions of the box, the “shear webs” in structural jargon) in towards the centre of the boat (Figure 2).

‘This brings another advantage, which is to provide a more efficient support for the hull bottom structure - all the more important because these boats are now so wide and so more prone to slamming.’

‘In a traditional structure,’ says Manganelli, ‘ringframes and bulkheads will span laterally from topside to topside.

‘But by introducing deep longitudinal bulkheads you drastically reduce the unsupported span of the ringframes which can therefore be lighter. The gain is most notable around the keel box area, as well as where you are resolving mast compression: instead of unsupported transverse elements that span the width of the boat, we now have a strong x-shaped structure.’

In terms of weight, the added mass of the longitudinals is offset by gains in the ringframes, which allows CM2 to still hit Class40 minimum weight (4,500kg). ‘And today this is less and less easy to achieve,’ adds Manganelli, ‘because as the boats become wider the surface area increases, which means more weight of material.

‘The temptation is to reduce grammes per square metre but then you pay a price in stiffness - a problem we avoided by rethinking our structure. Of course it wasn’t that straightforward, because now David had to adapt his roof design which brought additional complexity!

‘We also needed to make sure Ian was onside too, since freedom of movement below is clearly reduced.’

Both parties were enthusiastic about the idea, but first a full-scale mock-up was created as a precautionary step. This soon validated the idea as it immediately felt ‘liveable’.

The first sea trials also revealed another unexpected advantage to the longitudinal structure, for life onboard, Ian finding it rather reassuring to have something to lean against or limit a fall, bearing in mind these boats operate at greater heel angles than, say, the Imocas with their canting keels (forbidden in the Class40); heel angles of around 30° being common for a modern Class40.

Next the openings in the shell were factored in and dimensioned according to the size of the sails, to ensure stacking remains as trouble-free as possible. ‘We’re talking today about all these innovations, but they are all driven by Ian’s enormous experience of winning big races,’ explains Manganelli, ‘which is different from presenting untested concepts - now we can honestly say these are not theoretical gains.

‘During the Globe40 Ian felt he could really push the boat hard and remain confident, and he partially attributes his victory to this peace of mind.’ While other contestants suffered structural damage no major issues were reported on Crédit Mutuel 2. David Raison, for his part, has already said that, should there be another boat at some point, he’d embrace this new structural approach from the outset.

However, both Manganelli and Poidevin are very open about the fact that the principle behind this approach is really nothing new, and already widely understood. The reason its application remains limited is primarily because it is only fully relevant for these very wide boat types.

But the design evolution, towards flatter and beamier forward sections, makes it so logical that we will certainly see more longitudinal bulkheads in the future. ‘We have considered applying the idea to Imocas, but the benefits are not as obvious; the Imoca coachroof is set back aft and proportionally smaller compared with a Class40, plus the skippers we approached did not seem very keen…’ says Poidevin.

‘We could gain a bit in terms of shear lag, by adding structure in the middle, but not in terms of overall “thickness”, since the Imoca coachroofs are not only small but they also do not extend forward.

‘When it comes to ergonomics, our solution also constrains movements in the cockpit somewhat, since the sides of the coachroof are quite high where they meet the deck and go through to connect with the “inside walls”. The sides also had to be extended aft for the runner attachments, which changes the circulation for the skipper in and out of the cockpit.’

But on CM2 that aft extension contributes to giving the boat, viewed sideways, a nice convex shape on the top, precisely what is needed for optimal flex resistance - where the maximum ‘beam’ thickness needs to be close to amidships.

To return to the crude construction beam, if one was to design it optimally instead of sticking to the off-the-shelf rectangular section, a typical beam would end up tapered towards the ends. (This doubletaper shape is often seen on bridges.) ‘And the double taper is a staple of structural engineering vernacular,’ says Manganelli.

For the boat’s builders, compared to a traditional structure, the team at Gepeto (who built the hull) had to deal with quite complex intersections between transverse and longitudinal elements - an unusual set-up. Poidevin notes that the longitudinal bulkheads were placed first, and the crossbeams were then built in three parts to fit between and on either side of these.

The deck and coachroof combo was crafted by MerConcept, who did a textbook job in terms of resin infusion, much to the Gurit team’s pleasure. ‘The overall build quality of the whole boat is superb,’ says Manganelli. ‘All involved delivered a level of quality that is superior to anything I’ve seen before on a Class40.’

An interesting lesson, again perhaps unexpected, is that the extra global stiffness is enough to require more delicate tuning of the standing rigging; it initially proved difficult to generate forestay sag in light conditions because the hull cannot be counted on to provide flexibility. ‘But perhaps this is actually a good outcome in so far as it encourages us to put less tension in the rig to begin with?’ muses Poidevin.

These new questions are interesting for designers and engineers alike, as new possibilities emerge to make gains where it previously wasn’t an option. ‘Due to this unusual arrangement,’ interjects Manganelli, ‘we also found ourselves able to suggest subtracting material from other zones including the keel and mast areas…

‘However, that led to raised eyebrows from both David and Ian - these are sacrosanct zones considered paramount for reliability and safety. So in the end, crossing that psychological red line was never an option. Still…’

But the point remains that having the possibility to go lighter forces us to think creatively (but still inside the box - not metaphorically): what can be saved somewhere can either be done away with completely, or reinvested as reinforcement somewhere else.

‘Certainly, as part of the initial brief, we also set ourselves the task of addressing slamming, which was becoming a real concern with the previous generation of boats, hitting the limits of their robustness due to higher speeds, fiercer impacts and the flatter bow sections. So some of the weight gains were deliberately reinvested to increase volume and density of core material,’ Manganelli explains.

‘We already touched upon the trust that Ian had in the boat, but for us it’s a great source of pride… and also of peace of mind, as we can’t help but worry when our boats are at sea. Ian sent us a nice message just after finishing the Globe40, saying “Not the tiniest of worries after all these miles in some difficult sea states, the boat really inspires confidence”.’

Arguably the best compliment the engineering team could get, together with a level of high performance that speaks for itself.
Jocelyn Blériot