In the RC MARINE magazine (n° 117) of December 2000, I had briefly introduced the Hydro Fly, without revealing its secrets. Since then, new tests were made and have consequently improved the amazing perfomance of that regatta boat. I have been practising the radio-controlled multihull Mini 40 for about 10 years. I have always tried to improve this kind of boat in a technological way. As everyone knows, progress is omnipresent these days. In the world of sailing, life-size boats evolve the same way as model boats. However, if it appears that monohull boats tend to make slow progress, things are different for multihull boats.
Texte: Jean Philippe Bellot
photos: F. Langois - A.Keranflec'h et J. P. Bellot
RC  MARINE MAI 2001 N° 122
The famous French navigator Loïc Peyron, in an interview on the LCI channel, recently said that famous monohull navigators intend to join the 60 foot multihull world in the future in order to improve their experience.
At the moment, this tendancy is increasing and a few people, by the research they are doing, are making plans that may look senseless to some.
Alain Thébault, the inventor of the Hydroptère (French word for hydrofoil boat), tried hard for more than 15 years to create and finalize that boat.
Today the Hydroptère turns out to be a very competitive boat due to its technological advances. It will soon try to cross the Atlantic within 6 days and so beat all the previous records achieved so far.
I was captivated by the concept of making a multihull boat sail over the water, and I know you are quite a few to follow the researches made on that matter with much interest.
With this article, I don't pretend to teach you everything about the model multihull trimaran, but I just wanted to explain the main principles applied to that kind of model made for entertainment and competition. 		The technological interest:
It is based on bringing the hulls out of the water and make them stand on supporting planes. The advantage is that there is less friction, which allows the boat to reach a much higher speed.
This is aeronotics at its highest level, from which comes not only the technology but also the means of propulsion of the multihull. It is vertical forces that raise the hulls, pushed up by the submurged supporting planes. These are called foils. Their lifting forces increase according to the squared speed.


The original Hydro Fly concept:
I wanted to put that concept into practice, but using the trimaran I have now as a basis. It is a Mini 40 called "22 v'là..." (the French expression "22 v'là les flics!" means "Here come the cops!"), which has already proved itself: RC MARINE Challenge winner for the 3rd time, French champion for the 2nd time, and medal-holder at the 2000 European Cup. Consequently, the point for me was to take advantage of the performance of my boat and add the necessary parts to it in order to give it the ability to stand up over the surface of the water. Once this was done, I was able to start the whole conception and construction of the Hydro Fly. More than 2 years of research were necessary before I could start to build the model boat, which doesn't really look like Alain Thébault's Hydroptère. The original feature of the Hydro Fly is its diversity.
Actually, one should know that a hydrofoil boat can stand up over the water only when the wind is sufficiently strong. Alain Thébault's Hydroptère raises itself only when the wind makes 20 knots. So with a faint wind, the boat can't use all its potential. The notion of diversity of the Hydro Fly can be defined as follows: the trimaran can be equipped in 2 different ways: one for gentle wind, and another for medium and strong wind.
Vue d"ensemble du système du lets mobile. Implantation des manchons moulés poue les bras de liaison sur les flotteurs La platine carbone de support de foils comprenat l'accastillage spécifique.

In gentle wind:
The boat is equipped the same way as a conventionnal trimaran: a centreboard under the central hull and a rudder at the stern with eventually the use of mobile ballast.
In medium and strong wind:
The centreboard and rudder are replaced by a foil under each side-hull and at the back of the central hull a much bigger rudder with a stabilizer at the bottom of it. The use of mobile ballast is essential as it helps the boat to remain steady and balanced in all the wind directions. With further studies of the project, the Hydro Fly could have been equipped with a system allowing the boat to change from conventionnal to hydrofoil version without changing or adding any part. But that would mean a detrimental gain of weight. Indeed, the Hydro Fly is a multihull of competition weighing only 3 kilos. In addition, it would require a radio-control of a higher level. 		The main interest to compete with the Hydro Fly:
 To conceive a hydrofoil multihull is an exciting idea, but to me, the real purpose of that concept is to be able to compete in regattas. Keeping that aspect in mind, I tried to build it from a Mini 40 hull, with which the conception of a hydrofoil boat is possible. In addition, I never saw any hydrofoil competing with conventional multihulls, that is why I wanted to break new ground in that field, and why not convey that idea to the 60 foot multihulls of navigators like Loîc Peyron.


The construction:
Once I had my information listed and plans made myself, I started to build the Hydro Fly.
The first step was to have the 2 symmetrical floats and central hull cast in fibreglass and epoxy resin, with the help of the Thiais (77) Club. So far, I hadn't realized yet the originality of the prototype.
But when we moved on to the second step of the project, that is the building of the 2 asymmetrical carbon foils, the stabilizer at the stern, the carbon plates holding the foils adjustment shanks, the mobile ballast system and the specific rigging, then I fully realized that I had started a very exciting project. 		The basis: simplicity and efficency!
 simplicity and efficency! The technology used in a hydrofoil boat may look complex and too technical. As a result, before I started the Hydro Fly concept, I decided that my motto would be to combine simplicity with efficiency.
I wanted to use not more than 3 controls on my radio-control, which would mean a reasonable weight and a moderate cost. Here are the 3 controls :
- Control 1: a servo-winch fot the sails
- Control 2: a servo-steering for the rudder
- Control 3: a servo-winch for the mobile ballast

   

The equipment of the Hydro Fly:
The rigging: of swing-ring type, quick and simple to use. A few seconds are enough to change rigging according to the strength of the wind. The supporting elements (foils, rudder and stabilizer) can retract or be dimantled in about 30 seconds. The multihull: it can be fully dismantled and easily carried in a car (I've got an Opel Corsa!).

The building in detail:
The first step consisted in building the following pieces: - the 2 floats and central hull - the 2 big foils - a bigger rudder with its stabilizer - the 2 carbon plates holding the adjustment shanks (necessary for the foils adjustment) - the mobile ballast system - the specific rigging The hulls of the Hydro Fly were cast from the same mould as my previous boat and are rather conventional in their shapes. They were made in fibreglass and epoxy resin. They all are about the same length. Yet, Alain Thébault's hydrofoil is different: it has 2 small floats situated much ahead of the central hull, probably to reduce again the contact with the water and the weight. Once the boat is up, the floats just help the foils to support the boat. I had to take a different point of view, otherwise it would have meant for me to build my central hull in a different way and, as I told you earlier, I wanted to keep my former boat as a basis for the Hydro Fly. The second step consisted in casting fibreglass shafts accross the floats, so that they can be inserted into the 2 carbon connection tubes (diameter: 16 mm) of the central hull.

The supporting elements:
they were made in balsa and carbon layers gone to press. These were: - the pair of foils - the 2 anti-drift planes (playing the role of centreboards when the boat is up) - the 2 carbon plates on which the foils rigging is set - the rudder - the stabilizer

The constructin of the 2 foils:
once I had defined the best profile for my boat thanks to the various tests made in the lake where the Voile Val d'Europe Club is always training in Magny le Hongre (77), I decided to have a flat surface on the supporting side (the side out) and a Naca profile on the other side (the side in). The foils length was defined according to the height I wanted to reach for the Hydro Fly when sailng, so that the hulls never get into contact with the waves in strong wind. The next step consisted in cutting the foils out of a 5mm thick balsa sheet and giving them the necessary shape mentioned before. Once it was done, each foil was wrapped in 3 layers of carbon fabric with epoxy resin, the whole going to press. But when I made the first few tests, I realized that the flow of water on the foils was running in an anarchichal way, creating eddies detrimental fot the boat as it was slowed down a little bit. To solve the problem, I decided to put at regular intervals a few kind of teeth starting from the front of the foils back to half the width of the foils. These elements are used on Alain Thébault's Hydroptère as well. I noticed that they not only helped to lift the boat but also reduced the cavities caused by the lift. In addition, it was obvious that the Hydro Fly was better balanced

How they are fixed on the floats:
the foils are fixed by means of an aluminium rail fastened along each float. A tenon equipped with a screw at each end comes sliding in the rail. The upper part of the tenon is indented in order to receive the upper part of the foil. A small metal shank comes accross the whole system. This mechanism is a way to adjust the foils along the floats allowing to determine the best position. The rail is fixed along an aluminium piece which is fastened on the float by means of a screw at the back, whereas at the front the piece comes against a small aluminium square with 2 holes in it. This system permits to adjust the lift angle of the foils. This lift angle, which had been determined in the primary tests we made, is about 5 degrees inward. Below 5 degrees, the foils act as centreboards and cannot bring the boat out of the water. Over 5 degrees, the lift is too important and the boat gets slowed down by bigger eddies along the longitudinal axis of the foils. The adjustment shanks are fixed as follows: the top is held by a small tenon sliding in an aluminium rail under the carbon plate whether the bottom is held by another small tenon fixed on top of the foil, on the side in. Consequently, the 2 foils are adjustable in 3 ways: - longitudinally - to adjust the lift angle - to adjust the foils angle (more or less 45 degrees)

The construction of the winglets (anti-drift planes):
during the first tests, I noticed that without winglets, the boat tended to skid on its foils. This problem prevented the boat from sailing correctly when facing the wind. I rapidly put an end to that drawback by inserting a winglet (or anti-drift plane) at the bottom of each foil. Both have a symmetrical Naca profile and are made with carbon material, the same way as I mentioned earlier.

The construction of the 2 carbon plates:
still made with carbon and balsa layers, they have a significant function. They are placed on the deck of each float, in line wih the front linking shafts. They play the role of support for other pieces: - a rail to set the adjustment shanks - a pulley mounted on ball bearings and receiving the mobile ballast sheet - it also has a reinforcement hole in which the front linking shaft is inserted

The construction of the rudder:
its length has been calculated in accordance with the foils length. Once again, the rudder was made in carbon and balsa layers. Then, an aluminium piece has been stuck at the bottom of it so that the sabilizer can be fixed on it. .

The construction of the stabilizer:
during the tests, I tried stabilizers of various dimensions and profiles. I finally opted for a 24cm long wing with a symmetrical Naca profile. I decided to fix the stabilizer with a 5 degree angle downward on each side, notably to compensate for the heeling movements. A kind of aluminium socket was made on the stabilizer in which comes the rudder. Thus, the rudder and the stabilizer are distinct elements that can be dismantled. What I wanted first was to make a system that permits adjustments of high precision so that I could set the exact lift angle during the tests. By the way, I noticed that the lift angle was around 1 or 2 degrees upward. I never had to change it during all the tests, as a result I didn't feel the need to radio-control that function


Les deux foils asymétriques et la gouverne
avec son plan porteur pour la version hydroptère.
La gouverne et l'empennage sont des éléments séparés et réglables.

   


The mobile ballast system:
It has an essential function on the Hydro Fly. To compare with, Alain Thébault's Hydroptère uses a ballast system that puts weight on the float exposed the wind. This device helps the boat in keeping a horizontal level, so that the immerged supporting planes can fully play their parts. Therefore I decided to use a similar system for the Hydro Fly. On a technical point of view, it seemed difficult for me to reproduce such a system using water ballast on a model scale. However, I chose to use lead mobile ballast sliding along a carbon rod stretching accross all the width of the boat.

The conception:
The ballast:
it is made in lead and has a cylindrical shape. It is hollowed out in its centre. Ballast of different weight can be used but I use a 300gr. ballast.
The carbon rod:
it is made of 2 elements: a 6mm tube stuck against another 4mm tube. The egg-shaped hole in the ballast prevents it from turning round on itself, otherwise the ballast would often hit the 2 metal props on the central hull, which use is to hold the carbon rod.
How the rod ends are fixed on the floats:
it is fixed thanks to 2 small plates stuck on top of the floats. Each plate is equipped with a pulley mounted on ball bearings, and receiving a sheet linked to the servo-winch on one side, and the mobile ballast on the other. Another pulley, loose and fitted with an elastic, maintains a constant tension on the sheet.
Moving the radio-controlled mobile ballast:
I use the 3rd control of my radio-control. I have a MC-15 Graupner transmitter on which I added a kind of sliding switch. With it, I can move the ballast and stop it at any accurate point as soon as the boat has found its balance. .
The main characteristic of this system:
in the end, it remains simple and efficient. Furthermore, the whole device is fully adjustable along the floats, which helps to find the best balance for the Hydro Fly. All the elements of that system can be quickly adjusted or dismantled


The tests:
The first tests were made in March 2000. They have been video-taped and some photos were taken. The Voile Val d'Europe Club rubber dinghy (equipped with a 9.9cc engine) played a major role. It helped in towing the Hydro Fly when necessary, and in observing the boat and listing all necessary measurements. Thus, step by step, the Hydro Fly was progressively modified and finalized.


Sailing the Hydro Fly:
In its conventional version, the Hydro Fly sails like a traditional multihull and shows no originality when sailing. As it was built on the same basis as my previous trimaran, the Hydro Fly is still quick and efficient (in its first regatta, the Hydro Fly ended 1st in the last regatta of the year 2000 - see the RC MARINE review, n°117, December 2000). In its hydrofoil version, the multihull lifts itself into the air onl when the wind raches 18 or 20km/h, which corresponds to number 2 on Beaufort's scale. Its favorite directions have to do with all the winds in the back up to the winds on the beam. Sailing close to the wind remains a handicap for the Hydro Fly which behaves almost like a conventional trimaran in that situation, thanks to its deep anti-drift winglets. The ballast can't fully play its role of counterbalance on the float exposed the wind. The solution would be to be able to change the weight of the ballast when sailing, which looks hardly conceivable on a model scale. There you can see the main interest of the water ballast system on Alain Thébault's Hydroptère, as the ballast weight can easily be controlled. The boat takes off instantly as soon as the wind is strong enough. Once up, the Hydro Fly shows a perfect stability and remains perfectly parallel to the surface, at about 10cm over the water. Even when there are small waves, the supporting planes cut them like blades, preventing the boat from touching any of them. The speeds we observed are amazing and are almost twice those of conventional trimarans sailing in the same wind. Once the Hydro Fly is into the air, it remains in symbiosis with the wind, speeding up or slowing down according to the gusts. I would even say that the Hydro Fly is as quick as the wind but I won't assert it, as it has never been measured. As for the speed, it is difficult to evaluate as it varies with the wind strength. And it can be quite high as the boat never plunges. If we consider the scale of the boat, we can say that its speed is about 18 knots (about 30km/h). If you bring it back to a 60 foot multihull, the equivalent speed would probably be higher than 100 km/h. In regattas, the Hydro Fly tacks like a conventional multihull and it rarely misses a tack. It is not always necessary to gibe each time it has to turn round a buoy, which means a considerable gain of time in regattas.

Driving a hydrofoil version:
As you may have guessed, the Hydro Fly is highly sensitive and remains very easy to drive, even accepting some driving mistakes thanks to its great stability. A regular driver of conventional multihulls can, after a few minute training, control and drive the Hydro Fly. The main difficulty is to use the 3 controls at the right time and simultaneously.


Conclusion:
The Hydro Fly is a radio-controlled model opening up the way towards new practice of model boats. You are sure to find pleasure in driving it, and with little training many could manage to drive it. However driving the Hydro Fly is much similar to driving a plane, and it is essential to have some notion of either boats or planes. It is ecological and has little maintenance cost. It is quite a novelty that is easily transportable. This model boat concept remains a well-made and reliable boat. However it could be improved, for instance to have it stand up on its foils when sailing close to the wind. We have already planned to do research on that matter and a less sophisticated type of multihull may come into being in a version for leisure and accessible to everyone. The name "Hydro Fly" and the model plans have been registered at the Institution Nationale de la Protection Industrielle in Paris.



If you want more information on the Hydro Fly, you can e-mail me at: jpbellot22vla@aol.com or write to me at:
Jean-Philippe Bellot
11 rue de l'alouette
77700 Bailly-Romainvilliers
France

Translated from the French version by Mario Fernandez

Fiche technique
HYDRO FLY
Type: trimaran hydroptère radiocommandé
Classe: Mini 40
Longueur: 1220mm
Largeur: 1220mm
Gréement: balestron
(quatre jeux: A,B,C et C1)
Surface de voile maximum: 90 dm2
Nombre de voies : 3 voies
Début de construction: janvier 2000
Fin de construction: septembre 2000
Temps de travail: 150 heures environs
Particularité: bateau comprenant deux versions
Première version: conventionnelle par vent faible à modéré
Tirant d'eau: 32 cm
poids total: 2700g
Deuxième version: Hydroptère par vent moyen à fort
Tirant d'eau: environ 40 cm
poids total: environ 3000g
Architecte et constructeur: Jean Philippe Bellot, membre du club Voile Val d'Europe (77)
Palmarès: première participation en régate le 22/10/2000 (endurance de 4h).
Première place

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