In the family of intrepid and original sailors, some are known for their radical options. Among the adepts of short waterlines, I'd like to pay tribute to Yann Quenet, who has just set off around the world on his four-meter sailboat, poetically named Baluchon. Like the Mini class, which is limited to 6.50 meters, many sailors opt for a certain minimalism. However, others, and not the least, have adopted the opposite option. To cut a long story short, I'll cite the famous examples set by Jean-Yves Terlain aboard Vendredi 13, a 39-meter monohull, followed by Alain Colas' delirious 72-meter Clubmed...
The importance of length
The reason for these bold choices lies in a well-known physical principle. This states that the longer a ship is at the waterline, the greater its speed potential. In plain English, this means that for the same weight and propulsive power, the bigger the ship, the faster it will go. With the advent of gliding carènes and hydrofoil carriers, this basic rule is a little outdated, but it remains true, especially for our cruising units.
Dejauger to gain speed
Ãa plane pour certains, mais pas pour tout le monde. In the world of gliding carènes, a boat only a few meters long, but highly motorized and accessible to a wide public, routinely reaches 50 knotsâeuros and moreâeuros! For the user of such a craft, it doesn't matter how long it is, as long as the power-to-weight ratio is maximum. On the other hand, when a vessel is not suitable for displacement, due to certain design parameters, the archimedean mode will always apply.
Displacement hulls are systematically subject to the harsh law of wave resistance. As a result, despite its colossal thrust, a tug is unable to exceed its hull speed, inexorably pushing a monumental mass of water in front of it as soon as it reaches this limit.
This tug, in action, is towing a very heavy barge. With its hundreds of horsepower, it can only reach a modest speed, well below its hull speed. This explains the virtual absence of waves and the flat wake. The beautiful plume of foam across the wheelhouse is due to the encounter with a wave, not to speed
This planing hull evolves at the most unfavorable speed. It has almost reached its critical speed, as shown by its very hollow wave, but on the other hand, it's not going fast enough to lift off. At this speed, most of the fuel consumed is used to create waves and foam, and very little speed.
With a few hundred extra kilowatts, the hull will lift, the ship will accelerate and displace less water.
Explanations of hull speed
So, how does it work? A boat moving forward generates a particular wave system. At slow speeds, a ripple is created on either side of the bow. As engine power and speed increase, this "hump" grows in length and height along the edge. As we continue to accelerate, we'll find that this wave runs the length of the boat, and has deepened in the middle. At this point, we've reached carène speed, and it's going to be very difficult to exceed it.
Inventive hull design
This is where certain types of carènes make all the difference. Naval architecture is a fascinating business! So, a few decades ago, a designer, annoyed by this length-related speed limit, came up with a type of carène shape capable of overcoming (in part) this physical rule. He kept the front end pointed, so as to start off easily, without pushing a lot of liquid (as is the case with a loaded barge). Then he progressively twisted both sides of his bow until it became a flat spatula, adding a panel of edging on each side (to make it float...).
Bingo, the engineer had just invented the carène plananteâeuros! An architectural revolution that turned the boating world upside down. As a result, as the boat accelerates, the bow becomes more and more elevated. This reduces the amount of liquid to be displaced, allowing the stern section to glide easily over the surface of the water, like a ricochet.
A light boat for gliding
What we can learn from it. Many things have become possible with this discovery, but there are limits. While it has proved feasible to reach high speeds using this principle, it is at the cost of a plethora of engines and a great deal of weight control. Clearly, in the world of gliding hulls, lightness is a key performance factor. Fans of maximum speed, who propel their (more or less uncomfortable) machines forward with hundreds of horsepower, often come up against the problem of their propulsion machines being overweight. This is why many fast boats have a derisory range. It's impossible to stay light if you're carrying heavy engines and lots of fuel.
Safety and marine qualities
Let's imagine ourselves in the shoes of the intrepid pilot riding a boat capable of sailing at 60 knots thanks to his collection of outboard motors lined up like a parade: first of all, he'll only be able to sail on almost flat seas. And after a few minutes of exhilaration, he'll have to return to port, if only to refuel. In the rain, as you approach the access channel, a sign often reminds you that it would be reasonable to proceed at a moderate speed...
The hull speed of a 9-meter-long motorboat is around 6.9 knots. At this speed, she's still moving in archimedean mode, which is easy to see when you look back at the monstrous wave she's creating. When decelerating from 50 knots to 10 knots, all of a sudden you really feel like you're moving very slowly. Yet this is often still far too slow. In a speed-limited or heavily-trafficked area, it's simply essential to slow down to 70% of our famous hull speed, or... 3.5 or 4 knots (3 knots for a jet ski). I know, it feels like we've stopped, but it's the only way to go smoothly.
And what about the sailboats?
A number of enlightened architects were quick to exploit the concept of planningâeuros for the benefit of boats powered by motors. In doing so, they had to deploy a great deal of ingenuity. Indeed, when energy can only come from the sails, it is impossible to obtain power levels comparable to those of thermal or electric machines. Add to this the obligation (for monohulls) to carry ballast, and it's a lot less straightforward than advocating additional horsepower. So, architects, builders, equipment manufacturers, everyone has been working on the subject. And they have made such progress that today, most high-speed sailboats are able to get away from the sacrosanct rule of carène speed.
The special case of multihulls
Multihulls, which carry no ballast, are less heavily impacted than other types of boat, but they are still affected to some extent. Much depends on their weight. Roughly speaking, however, the more power a catamaran provides, the faster it goes, without getting stuck on a value linked to waterline length.
And for the ordinary boaterâeuros?
Here, we're talking about those family sailboat enthusiasts who share the joys of wind-powered sailing with companions, friends and children - the yachtsman. When it comes to buying a new yacht, budget and berth are the most important factors in the final decision. So, even if you're convinced that a metre more would be better, it's often your wallet that will decide.
On the other hand, when buying a pre-owned vessel, the customer has to choose between a recent yacht of modest dimensions, or an older, but potentially longer canoe. And it's precisely in these circumstances that it's preferable to opt for a larger-than-expected boat, especially when planning a long-haul voyage. There will undoubtedly be some extra hassle, and even expense, associated with the size, but the level of comfort, safety and trip length will be optimized.
Our valiant "minitransateux" are stuck because a Mini measures 6.50 m and then that's it. Similarly, anyone with a 10-meter berth is likely to be handicapped by a 40' purchase. On the other hand, when it comes to choosing a boat designed for real blue-water cruising, and insofar as the budget allows, we strongly advise you to give preference to the boat with the longest waterline.
