multiple subjects


multiple subjects

Following on from last week’s look at the technical features of lead Barcelona World Race boat, ‘Paprec-Virbac 3,’ Lia Ditton chats to VPLP’s Vincent Lauriot Prévost about everything from the IMOCA’s to the MOD 70’s, AC to Jules Verne. Photographs by Christophe Launay.
Shot at the C Class Worlds in Newport, R.I last year.

On Foncia/Virbac-Paprec
Why do you think Foncia’s mast failed?

VLP: That’s a good question! I didn’t get any feedback from Michel.

LD: ‘Foncia’ went with a wing mast and outriggers, rather than a classic mast with 2 sets of spreaders like ‘Virbac-Paprec 3.’ How did this choice affect the design structure of the boat?

VLP: It does affect the structure of the hull – in the vicinity of the shroud chain-plates. We can say that the Paprec mast is lighter, when outriggers are taken into account for the wing mast, but the height of the center of gravity is a bit lower. We can also say that a pre-constrained structure of a mast such as Paprec’s is more stable than a structure beholden to outriggers.

LD: What are your thoughts on the split coach roof on VP3 versus the more conventional cabin on ‘Foncia?’

VLP: I would say that the slide-able coach roof of Foncia is surely more sheltered than the split coach roofs of Paprec. How much weight do you afford yourself for comfort? That is a question which answer belongs to the skipper!

LD: The IMOCA monohulls are beginning to behave more and more like multihulls in terms of power zones and performance. Do you think this is a result of partnerships such as VPLP/GV?

VLP: I think that IMOCA’s increase their performance by increasing righting moment with canting keels and huge ballasts (tacking and reaching), and having more and more planning hulls and light displacement (downwind). We could say that the more power you get for the minimum weight is a basic principle originally applicable to multihulls that has now shifted to monohulls.

LD: We have seen the focus of foil development go from canting keels and tacking rudders (one kick’s up the other is put down), to angled and curved asymmetrical dagger-boards. Keels, which were previously designed to create lift as well as righting moment, are now only designed to generate righting moment. Dagger boards, which previously prevented leeway now often function as hydrofoils and contribute to righting moment. Hydrofoils can be shuttled from the one side of the boat to the other (like on the IMOCA 60 ‘DSS’). Where do you see foil evolution heading now?

VLP: A lot of new features have appeared these past three years in terms of keel and foil development. We must bare in mind that the boat has to keep some versatility and we don’t want to neglect the side force, to the sole profit of vertical lift. IMOCA’s already have hull shapes, which develop massive vertical force when planning – which is not the case with the concept of fine-hulled multihulls.

LD: What sort of advances do you foresee for IMOCA designs in the future? Where would you like the class to go?

VLP: The IMOCA is a quite restricted class in terms of rules, with some critical tests for stability and safety, maximum righting moment allowed and a limited sail area due to the mast height limit. From that standpoint, it looks like the boats will converge to give a very similar type of performance. From some tests we have made, it is not evident that lift developed by foils compensates the inherent drag. Sure there is still a lot of research to be made on the interaction between the hull, the canted keel and the foils.

MOD 70
LD: The Multi One Design or MOD 70 is the so-called new one design, which will replace the ORMA 60 fleet. You must be very excited to be the designers for this project. What new ideas about multihulls were you able to exercise with this class?

VLP: The new idea is a very pragmatic one: conceive a multihull, which is the synthesis of 20 years of evolution and development. A new generation of hull shapes, more seaworthy platforms, increased safety factors for structure reliability, all that staying in the same type of power as the last ORMA 60 projects.

LD: “A fast, modern design with foils” says your VPLP website. Am I correct in assuming that if these foils are designed to support the same percentage of displacement as an ORMA tri, a single foil could lift up to 70% of the boats weight in "normal" sailing?

VLP: Correct.

LD: At what point would you say a multihull becomes a ‘foiler’ rather than ‘with foils?’

VLP: Good question! No one has ever asked me that! Maybe a foiler would apply to ‘Geant’ whose main rudder was equipped with a T foil with adjustable angle of attack to get rid of hull buoyancy at the stern.

LD: Standardizing and establishing a one design of this size with such a varied design brief (offshore, inshore, full crew, short handed) must have presented a great deal of technical challenges. What sort of design boundaries has the MOD 70 project pushed for you?

VLP: The MOD 70 design brief is clearly for offshore races with 6 people on board. Inshore races will happen but it is not the primary goal of these boats.

LD: Wave piercing bows seem to be the zeitgeist. Can they be seen as one approach to addressing two different problems: reducing pitch for the purpose of stabilizing airflow over the sail plan and moving reserve buoyancy forward in big seas to reduce the chance of pitch poling? If we assumed a fixed LOA then wouldn’t the later of these two benefit by adding reserve volume to the extremes thus allowing a more powerful hull and rig for the length?

VLP: These kinds of bows are possible when, as for the MOD 70, hull length is not limited by any box rule. The volume distribution forward of the front beam to counteract pitch poling is distributed more aft and the extra length allows us to have a finer angle of entry, reducing drag when passing through waves.  

LD: Isn’t the hull length of the MOD 70 limited by the box rule?

VLP: The idea was to design a boat safer than an ORMA, more in terms of structural reliability than in high speed behavior, notably the longitudinal stability, all of that by keeping the load numbers of an ORMA (same range of rig size, same deck gear, same appendages etc…) Increasing the reliability implies adding structure and weight at the same time. To keep the same load values, we have reduced the beam size to end at the same righting moment.

About longitudinal stability, we replaced the bowsprit with a main hull bow extension to reach the same overall length, which corresponds to adding 10 feet. This extra main hull length at the front, combined with the 5 feet increase in the length of the floats adds some important buoyancy forward to delay the time when the boat could capsize. So you are right! I made a shortcut. It is not a box rule, all boats being identical as monotypes.

LD: How did your involvement with BMW/Oracle in the run up to the America’s Cup D.o.G match, further the design office of VPLP? What elements from the project do you think will roll over into other multihull designs or the marine industry in general?

VLP: We have learnt a lot about racing inshore multihull behaviour in general. There will always exist a moderating factor for offshore multihulls to deal with – the seaworthiness and passing through the waves, the range of sea and wind conditions being much wider than for inshore. We have learnt a lot in terms of tradeoffs and from the data recorded on the boats or calculated by the engineers of the design team.

LD: Do you think that having wing sails in the next America’s Cup is a good thing in terms of cost/benefit to the event as a whole?

VLP: Some benefits of the wing will be the accuracy of adapting the power of the sails in gusts and the improvement in performance and maneuverability. If we consider that maneuverability is a key factor for the race format, the wing is a good answer.

LD: What design considerations have led the switch from catamarans designed by the Gilles Ollier/Multiplast team (Gitana 13, Orange II) to VPLP-designed trimarans (such as Groupama & Banque Populaire V), for big boat record/ RTW programs? What are the trade offs of cat versus tri for records/RTW?

VLP: Our studies show that, at equal length and equal righting moment, a trimaran is lighter and faster, more versatile in various wind conditions and boat course. With more longitudinal rigidity due to the main hull longitudinal stiffness and with a wider platform, it is more adapted to foils implantations and the boat is able to perform well either tacking or reaching, from light wind (reduced wetted surface area) to heavy breeze (foils on floats). Lastly, the ergonomics and life on board is more efficient in one single main hull where all the crew is centralized, either in the cockpit, or in the cabin. However, the weakness of the trimaran is the exposure of the windward float in some crossed heavy waves conditions.