Matrix evolution !
|21 January 2013||Posted by Alex from Lagoon under Technical world|
Following the launch of the Lagoon 52 and 39, many questions came about our technical choices for the evolution of the Lagoon range: “why is the mast so much aft?”,”why is the boom smaller?” “what is the point of having a genoa track?”… To answer these questions I suggest to drop in the aero and hydro world.
First, the new position of the mast is located quite aft compared to other boats. This is the most visible difference, which also has a significant impact on the general layout of the boat. Below is a comparison between the Lagoon 380 (red line) and the Lagoon 39 (black line).
So the next question is why have we moved the mast back? The fact is that our boats are more and more comfortable with an equipment “like at home”. This leads to a substantial extra weight which can become negative for the sailing performance. We made this comment to VPLP (our naval architecst) as we wanted to improve performance without neglecting comfort. VPLP has an unequalled offshore racing experience and they came with architectural proposals that you only see in the racing world as the mast aft centered. In fact there is no difficulty in realizing the difference of mast position between a cruising boat and a racing boat : here a Lagoon 620 compared to a 60 feet IMOCA 2008.
But what does bring this new rig:
A well balanced boat!
Quite logically, the more the mast amidship is, the more the overall weight of the boat is centered. This improves the balance of the boat thus avoiding “nose diving”. This also allowed the architects to refine the bows and the first part of the hulls so as to reduce the wetted surface of the boat and obtain a better wave penetration.
A better repartition of the sail area centre of effort !
This mast position gives a better distribution of sail plan. This allows to better adjust the sail repartition according to the conditions of navigation. Indeed, the more the centre of effort of the sail plan backward is, the more the boat tends to have weather helm (trend of the boat to come upwind) and vice versa when the sailing centre of effort is forward. Note that during the first test of the Lagoon 39, the boat had a slight weather helm which made her more “alive” and therefore more enjoyable to steer. Indeed racers often prefer a boat with a slight weather helm to “feel it better” when steering.
More foresail possibilities!
The new Lagoon generation boats enjoy many different solutions of foresails thanks to the fact that the mast is located more aft. Foresails can be bigger and various. Why this fact is really important? Foresails are really powerful because unlike with the mainsail, the wind flow is not disturbed by a mast or another profile, hence the generation of large jibs in the 70s. This is why the Lagoon 39 and 52 have the option of a code 0 and the Lagoon 52 can also carry a 105% genoa.
Let’s stop for a moment and take a look at a few figures to better understand the evolution of the management of the sail centre of effort by comparing the data of a Lagoon 380 and Lagoon of 39:
|Sail||Lagoon 380||Lagoon 39|
|Square top mainsail + jib (wind >10 kn, close hauled course)||52 m2 + 30 m2||50 m2 + 32 m2|
|Here, we note that both boats have quite the same sail area but the distribution is slightly different with 2m2 more area for the Lagoon 39 jib,, which is nearly 10% of the foresail area…|
|Square top mainsail + foresail (wind < 10 kn between 55° and 90° of apparent wind)||52 m2 + 30 m2||50 m2 + 60 m2 (code 0)|
|Here, we note a complete new repartition of the sail area for the Lagoon 39 with a bigger foresail of 60m2 ,her code 0. It gives a better capacity to sail in light conditions.|
In addition to having different foresails solutions to be more efficient whatever the wind, these new catamarans have the distinction of having a foresail fitted with a self-tacking jib. The aft position of the mast allows to install a rail on which the car of the jib sheet is placed. As you can see on this video, the functioning of the self-tacking jib is very simple and allows easy cruising with a reduced crew.
Greater aspect ratio!
The shorter the boom, the higher the “aspect ratio” of the mainsail. The aspect ratio is the relation between the sail span to the sail surface area.
|Aspect ratio||Lagoon 380||Lagoon 39|
|span 2 / sail area = aspect ratio||15,012 / 52 = 4,33||162 / 50 = 5,12|
But why is this aspect ratio so important?
o The first reason is that the wind blowing close to sea level gets friction due to the surface of the water which interferes, creating a speed reduction and a change of direction. This is called “atmospheric boundary layer”. This disturbance diminishes with altitude so the wind is more stable and efficient higher up on the sail. With a high aspect ratio, the upper part of the sail is much more efficient. And as mentioned earlier in this article, the square top mainsail uses this phenomenon at its best.
o Second reason: when the wind flows around the mainsail, several whirlpools or “vortices” in technical terms are created: one at the top mainsail and one at the boom. Each vortex is created because the air particles on the inside of the sail, the “intrados”, are “sucked” by the depression on the “extrados”. This combined with the movement of the boat creates vortices that increase the drag and thus hinder the progress of the boat. With a high aspect ratio vortices are reduced, indeed the area where the vortices are created is smaller and therefore slows the boat less.
A lighter rig!
The rigs of the Lagoons 39 and 52 are light, and even more, I would say they have suffered a diet. Not that we wanted to save money on this important element at the detriment of safety, but this is the result once again of the position of the mast. With this aft centered position, the angle between the forestay and the mast is more open. So far nothing very innovative. But this wider angle induces a lower compressive effort on the mast than on more conventional rigs. This decreased effort has allowed us to reduce the mast, shrouds and forestay sections. Moreover, the mainsail is narrow so the battens are short. So we save a substantial number of kilogrammes in the upper part of the boat, which is excellent to reduce pitching.
“Voilà”! You now know the secrets the new lagoon cats in detail! I hope I have been as clear and simple as possible to explain why the Lagoon Lagoons 39 and 52 are technically quite different of the rest of the range. The final aim is to have a more versatile boat which combines the best of both worlds: comfort and speed.
For those who are interested in the topic, I suggest this article of the thesis Damien Lafforgue the University of Southampton on the experimental conditions sails.