Stability and foils on IMOCA sailboats

In my last post on this blog I wrote about stability on sailboats, how does it changes with different hull shapes and different center of gravity shifitng, we pointed our attention on stability curves, their meaning and limits

Now let’s make several steps in a completely different world , making an analysis on dynamic stability concept on new generation IMOCA ocean racers provided with foiling technology ; we’ll discuss later why I wrote here dynamic stability and not static stability or simply stability; these sailboats are going to start within one week for the “Vendee Globe”, the superfast and superfamous round the globe singlehanded nonstop regatta. The new foiling generation of IMOCAs makes about one third of the entire fleet, pointing out that most of the new generation boats have chosen to follow this kind of technological leap

let’s rewind the tape to understand what IMOCA racing sailboat are and how their stability works:

to put it simple IMOCAs are 60 footer super beamy (16-70 feet beam) skiffs, flat bottomed, over-canvassed, specifically built and tuned to scream at huge speed mostly reaching or downwind (top distance on 24 hours is around 530 nautical miles, that’s to say 22 knots of AVERAGE speed, wow…) ; IMOCA class is a so called “open” class, so basically you can do whatever you want as a boat designer within a given box rule defining several aspects , mostly about dimensions of the boat and materials that can be used on them; in the last version of the rule few components have been standardized to try to keep down skyrocketing costs;



a sailing view and a sideview of new generation IMOCA provided with foils

let’s come now to examine the submerged control appendages of these boat up to the last vendee globe generation boats; during the last ten years IMOCAs assessed themselves on a configuration made by: 2 rudders, one canting keel with lead bulb, 2 asymmetrical daggerboards fitted on the half of boat length , let’s start to explain what lies behind this setup:

  • twin rudders: easy to understand, these boat s are super beamy and flat bottomed, upwind and reaching a good part of the centerline is out of the water, a single central rudder would be highly ineffective, ventilated and partially out of the water ; twin rudders very far from centerline work perfectly, with the one leeward totally submerged; this is the standard configuration for all the classes of offshore racing sailboats, ranging form minitransat 6,50 , class 9,50, class 40, VOR65 and IMOCA; these solution is becoming increasingly widespread also in sport-cruisers sailboats (whether it’s fashion or real technical need, that’s a whole different story)

  • canting keel : that’s to say the keel is fitted on a pin (a HUGE one…) and can be canted upwind, with its 3 tons lead bulb at 4,5 meters under the waterline, thus shifting significantly the centre of gravity (see sketch) and increasing the Righting moment, apart form a little bit of lift “on the wrong side” given by the keel wing ; the undesired fact is that canting the keel in this way makes it loose the capability of generating the amount of sideforce required to keep the boat on track, so this force must be supplied by a different appendage, and here they comes…..

  • twin retractable daggerboards : two retractable daggerboards, in a slot, like the one on the Laser you sail, just 7 meters long , 3 meters area under waterline, asymmetrical both in position and in profile; their duty is to generate enough sideforce to keep the boat on track upwind and reaching

in the sketch an analysis of the forces created by submerged appendages in this setup; you may notice that, apart form the small amount of lift given by the canted keel , all other forces contributes to create sideforce and righting moment



small digression n.1 : the “mantra “ of all the modifications on last offshore racers generations, chined hulls, canting keel , water ballasts, foils, is simply : INCREASE THE RIGHTING MOMENT , up ot the class limit (32000 kgm for IMOCAs) without making the boats heavier, that’s to say increase the ability to sail with a lot of sail area hoisted without heeling too much, and do this in stronger winds, to squeeze all the thrust available form super-stiff high modulus carbon rigs and hi-tech laminated sails , and rectangular high aspect mainsails more similar to an airplane wings than to old-school triangular shaped or slightly roached mainsails.

These boats feature a huge amount of sails, and a huge percentage of them up high on the mast (29 meters high), where winds are stronger and more regular, so all this righting moment is the only condition to fight the heeling moment from the sailplan

small digression n.2 :

talking with several persons involved in offshore racing scene, the sensation is that races like Vendee globe will be heavily determined by performances in “weather system transitions areas” , that is to say areas with medium to light conditions ; those parts of the races are becoming more important than going full throttle in southern ocean with the constant risk of shattering the boat in terrifying sea conditions. So these new modifications are tuned to push boats at 104% of their potential in light-medium conditions, without the risk of cracking the boat jumping on high waves

now let’s take a look ot the new IMOCA foiler setup, the new kid in town for this edition of vendde globe:

  • twin rudders: same old story, no significant news compared to previous generation boats

  • canting keel: here too, no revolutions, apart a general optimization of hydraulic machinery and mechanical parts of the system, that suffered several failures in the previous editions of the regatta;

  • twin retractable daggerboards: PUFFF, VANISHED !!! foil generation boats simply doesn’t need them, let’s try to understand why

  • foils : after a first period of trials , now it seems that all the foiling boats have adopted a L shaped set of foils (see pictures); given small differences among boats, seems that this configuration gives the best all-round performances; foils are L shaped monolithic carbon appendages that can be retracted in slots within the hull, so when you don’t need them you’ll end up two small carbon moustaches out of the hull (not a big problem for added drag in the water in pot-au-noir super-light conditions, since this boats have yet a wetted area similar to a tennis playground)

what kind of forces do these L shaped foils generate ?

They ‘ll have to provide the same amount of sideforce generated by daggerboard in the previous configuration; this task is mostly provided by the vertical part of the foil , resembling the old generation boats; the horizontal part of ther foil generate a force pointing approximately upward , increasing again the righting moment (see picture) ; the important fact is that we’re generating righting moment through speed and not increasing the weight of the boat, as happended with water ballast; this the ideal solution for light conditions, light, boat, maximum righting moment and a lot of speed;  just one note: this is an article for sailboat amateurs and weekend sailors, so many details have been simplified;



Foil and speed: I won’t crunch equations here, but there is one important concept that must be pointed out: LIFT generated by a wing (foils are wings, as the ones of an airplane ) is proportional to squared speed, so when an IMOCA pass form 12 to 24 knots (typical speeds for these boats) , lift generated by foils became 4 times the starting one; this why we’re talking about DYNAMIC STABILITY , because it’s speed that generates righting moment , a little bit like in F1 cars, where speed generates the downforce required to steer the cars at 250 km/h without flying away from the track;

foils “side effect” : the videos that emerged until now , even if they’re provided much more to generate media buzzs than to show real boat’s potential, show one thing for sure: these generation of boat is really much more stable than previoius ones, especially in pitching movements, they tend to slam in waves less than old boats, this something fo r sure related to foils and to bow sections featuring  more volumes

everything fine: no concerns ?  well…not exactly …. foils have of corurse por and cons, one of the answers that will come form this Vendee Globe is if pro will overcome cons, or if “nofoils” boats can still compete in this kind of races at the top level ; for sure new foiling boats will pay the price of thenew projects in terms of reliability (there’s alwalys been a high attrition rate in past editions  when new technology has been used) ; one great concern is UFO collisions (no, not the green samll men form Mars, not those UFOs, but the Unidentified Floating Objects like semi-submerged containers or floating timbers) , since foils are very much stronger and stiffer than hull sides , and the fear is that in case of high speed collision foil may open the hull side like a hot knife in the butter; The IMOCA boats are superpowerful and superlight boats, and Vendee Globe is a so much important race that almost no one will trade 200 kg of additional structural reinforcements for a fraction of boat speed.



IMOCAs design is driven by super detailed Vendee Globe simulations: the “virtual boats” run hundreds of “virtual Vendee GLobe” with several combination of appendages, hull shapes, sailplans, displacements ;  design teams struggle to understand which configuration has the best chances to win an “average Vendee Globe” from the weather conditions point of view ; several Vendeeleaks emerged, rumoring that foil provided boats may gain a one knot speed advantage in light-medium winds comparing to previous boats ; one knot is a HUGE advantage, we’ll soon understand which perfomances enhancements feature the “no foil” boats fleet;

Definitely this year the Vendee Globe is gonna be a even more fascinating race,  just a few days more and all the rumors and chatter will be forgotten when the race will start.

official Vendee Globe site



a big THANKS to Sergio Frattaruolo for the suggestions !!!   ( by the way, if want to have a great experience sailing on a modern class 40 offshore racer “Calaluna” contact him  here )