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  • AutorenbildHans Genthe

A foiling revolution - and an overview about motoryacht design

Aktualisiert: 9. Jan. 2023

Most of my private free time I spent with sailing and sailboat design, but a month ago I had a new key experience in another world. For half a year I have been living in Dubai now. As fate would have it, the dream job here did not turn out to be what I expected. So I changed to Enata, who among other things are building foiling motor boats. The first test drive turned my ideas upside down...

Professionally I could and had to drive very much on motorboats. Press work for the North Sea Week (NORDSEEWOCHE >>), photo productions, shuttle trips, events. Fast and comfortable in smooth water, but always exhausting in waves and a nuisance for the back. That's why I have always been interested in motor yacht design, looking for a solution to go fast in waves and to be able to take pictures. On the different photo productions I was able to discuss ideas with editors, so it came that I was allowed to write an article about motorboat design in 2009. Briefing: Not highly technical, but simple and clear. So I wrote this story:

"With the lap finish I reduced the chinewalk on my bowrider." My new acquaintance Carlo proudly shows me his new prop - his buddy smiles mockingly: "Knut, just buy yourself a new boat with better balance!" I only understand Spanish. Well, I guess this calls for a little digression into boat design. Two months later, I'm in shape. Now I know that a full displacement hull cannot plan and planning captains need bigger wallets. The boat should fit into the wave it creates. This is an old yacht ydesigner rule. If you want to sail fuel-efficiently, you can use fish as a guide to the shape of the underwater hull. They are optimized in shape due to evolution, depending on the speed the fish has to reach.

But with the boat, a large part of the hull is above water. A full displacement boat displaces the water, it remains trapped in the wave system that it generates itself: at maximum speed this consists of a bow wave and a stern wave. When the back of the bow wave reaches the front of the stern wave, the two waves interact. If the speed now increases, only the generated waves become bigger, the boat does not get much faster. Usually no extra horsepower helps. This maximum attainable speed is called hull speed. Really experienced designers therefore use a trick when dealing with fast full displacement yachts: By using a chine in the boat in combination with concave areas in the cross-sections, or frames, you succeed in binding the bow wave and preventing interaction with the stern wave. This construction is difficult, but leads to fast and fuel-efficient boats.

Fish do not plane

The angle of the hull lines at the stern says a lot about the characteristics of the boat: If the wave is guided steeply upwards through the hull, the boat cannot detach itself from the wave system.

To get a nice shallow underwater hull, the angle at which the water exits the stern should not exceed 15 degrees. Due to the flat lines, the stern wave detaches from the hull at a certain speed and the boat starts to plane. The last part of the hull is then approximately horizontal to the water surface. When a full displacement hull descends a high wave, it can briefly starts planing, because the angle of the water surface at the front of the wave becomes almost parallel to the stern lines of the displacer. Gravity helps the displacer descend from the crest of the wave with a virtual increase in engine power.

From two make one In extremely slender ships, from about a length and width ratio of 1:8, the bow and stern wave interact with more. Since such a slim boat offers extremely little space due to its small width (at 10 meters only 80cm width), the clever designer simply takes 2 or 3 hulls and builds a platform on top of them: ready is the catamaran/trimaran. These boats are fast, but like to go through waves instead over them. But these are only the basics. There is still a long way to go until the Chinewalk.

Surfboards are not good motorboats On the tour I learned about the rocker. Very important. Because until now I had learned that a boat planes well on a surface that is parallel to the water surface. Great, I thought: I'd best take a board or a big surfboard and build a motor on it. Not a good idea: It's fast, but not steerable. A boat needs guidance. And volume, so it can hold people and machines. So you bend the plate and put a fold on the front. The stronger the V-fold (this gives also buoyancy), the more guidance in the water and the softer the boat will go through the wave. "Great!" I think, "so I'm gonna make a boat nice and V-shaped, so it's comfortable and safe." The disadvantage is that it sinks deep into the water and that slows down. You can think of it like this: The hole in the water surface behind the stern, which is created when the boat goes through the water, wants to fill up. And a vacuum is created at the stern that holds the boat back. If the V at the stern is very steep, the hole is deep. That costs fuel. And in order to glide, you also have to accelerate much more than if the boat was flatter at the back. Racing boats can pass tight 180° curves with more than 70 knots thanks to the strongly V-shaped hull. They have plenty of power. With fast water ski boats for inland lakes, this doesn't make sense, a flat one is easy on the wallet.

Conclusion: The V-shape is different depending on the requirements of the boat. Attempts by the American Army to develop fast patrol boats have resulted, for example, in a 12.5-degree V at the stern. Slim front, fat stern

But now for the top of my record. The bow has important functions: It splits the waves. It's to keep the boat from nose diving. And from everyone: minimizes resistance when the boat is moving. A sharp "V" in the bow area is comfortable in a seaway, but the boat will start planing later. Interestingly, it has been found that a concave bow bulkhead does not set in softer than a slightly convex one. At the sharp V the water displacement is too progressive, i.e. the increase of the water to be displaced becomes too large, the submerging movement is stopped later but harder. Since the waterflow breaks off at the stern, the stern is always in the water, and the fulcrum in a seaway is about 80% of the length from bow to stern in a glider, the stern can be wide, much wider than in a displacer. This creates tipping stability and creates space on board.

Corrugated iron bottom and Chinese crepe

For the performance and comfort of a fast planing boat, the longitudinal stringers, the triangular folds that run all over the underwater hull, are very important. They break the wave and reduce the splashing water that would come on deck. The water is deflected to the water surface by the downward facing flanks of the stringers. This increases buoyancy and reduces hull friction. In addition, the stringers stiffen the hull surface like a corrugated sheet. When cornering, the stringers/deflectors increase the guidance. In the stern area, in the surface on which the boat glides, the stringers must run parallel to the midship line, but the Petestep® spray deflectors are a new concept (2017) that surrounds the running waterline above the water in a triangular shape: "Crepe de Chine", Chinese crepe I knew from school and handicraft time. I would never have associated it with the sharp bend in the hull, which runs from bow to stern. It causes the wave to break off, just like at the stern. Without this chine, the own bow and stern wave and the swell would stick to the hull. Because if you make the transition from the bottom of the hull to the side wall round, the wave is guided upwards, as with the displacement stern. This increases the resistance and the amount of spray that comes on deck. Now we are finally at the "Chinewalk". These are now the high consecrations of fast motor boating: At high speed the boat comes out of the water further and further and plans on the rear bottom. If it is not properly balanced, it tips over and plans only on one side of the V-bottom, until one wave, steering movement or weight shift to the other. In the process, it can build up and start to tip back and forth all the time. This is not only unpleasant, but dangerous. What can help here is an improvement of the longitudinal trim, the weight distribution or an optimization of the propeller, called "lap-finish".

To increase speed with fast motorboats a stepped hull can help by reducing wet surfaces and lowering drag. A step is an abrupt across change in hull-height running from chine to chine, and ending with a notch of the hullsides, which are drawing in air when the boat begins to plane. The air reduces the wetted surface. The limitations of motorboat driving A motorboat quickly reaches its limits when going against the wave. The higher the wave and the faster the boat, the harder the impact on the next wave. This leads to the fact that at 1m wave height life itself becomes hell even on an offshore rip optimized for these conditions. Instead of 40 knots, there are then only 20 or less, and even that is more than exhausting - if you don't have a sprung seat, very painful for your back. During a transfer to Helgoland at 5 Beaufort with a rib that was actually 45 knots fast, only 12 knots were left in it.

Many thanks to Toni Gerken and his great offshore rib for the support during the NORDSEEEWOCHE!! Now the new experience: On my second working day I was allowed to test drive the foil motorboat for the first time.

Driving a supercar The experience was amazing. Unexpected. Impressive. This thing combines the racing abilities of a supercar with the cruising comfort of a luxury limousine in a motoryacht. The foils and steering lead to an unknown performance - driving a 12m circle with more than 30 knots is pushing the limits to a new level without drifting out of the turn. This 32-foot foiling yacht flys five feet above the waves offering passengers a smooth and quiet experience - no chop-chopping through the waves. The hydrofoils enables a highly comfortable sailing even in rough conditions, without slamming, spray and no seasickness. The joystick control in the bow, which can be activated from the traditional wheel-and-throttle helm console, is a another impressive experience. Like other joysticks, pushing forward or pulling back powers up or dials back the speed of the Foiler from a standstill up to 40 knots. Tilting to either side mandates direction... At least this is the best boat for photo- and video productions I can imagine.

Why is this so revolutionary? The patented foiling system has a long history that goes back to the world record breaking famous foiler „Hydroptere“. The Enata Foiler is developed by the Enata Design Team - now based in Dubai - which took part in the development of Hydroptere and the foiling America´s Cup Catamarans. The foils work silent and efficient. As the water is 840 times denser than the air, the foils can be much smaller than the wings of an aeroplane and still lift the boat. As the boat speed increases, the foils start to lift the FOILER, like an aeroplane on a runway. At 18 knots, the lift created by the foils entirely lifts the boat, and the hull flies at 1.5m above the water. So the yacht can handle waves up to 1.5m in foiling mode, while still providing unexpected comfort. In more extreme conditions, you can retract the foils and use it as slow as a classic boat. The FOILER is completely made of carbon and extremely strong. The new 740 hp hydrostatic propulsion system and ENATA's inhouse developed torpedoes deliver more than sufficient power. With a simple push of a button all foils and the hydrostatic pods with the propellers can be driven out of the water staying in the harbour. The whole boat is steered and controlled by software developd by enata. All parameters are available on an app, on clients wish the system can be accessed by the enata yard for maintenance und control.

With the options of a cabin, toilet, bar and more, this boat is the perfect shuttle between a super yacht and the nightlife on the shore - independent from bad weather and rough seas: Check this video >>

U-Shape full displacement yacht Very seaworthy and fuel-efficient hull shape for displacers. Offers plenty of space below deck. Seaworthy motor yachts for long-distance cruises and expeditions are preferably built with the U-shaped frame.

Classic semi displacement yacht, German "Wellenbinder" (wave binder) The bow wave is "bound" by the concave areas, the hull speed can be far exceeded without the boat going into planing. This means that heavy, comfortable yachts are still fast.

Classic V-Frame concave Popular hull shape for classic motor boats for many years. Behaviour at sea is not as good as expected. Nevertheless, due to its high aesthetic appeal, this shape has not only been retained in boat building until today, but has also had a renaissance in recent years.

V-frame flat planning hull

This form is more suitable for areas without waves. The boats are fast, start planing early, but set hard into the waves. The very flat stern area gives little course stability.

deep convex V planning hull The further development of the concave frame is a frequently encountered boat form for fast boats today. The wave behaviour is very good, the boat starts softer than the concave form.

modern V-frame deep Fast planing hull for high motorization. During slow speed strong turbulences at the stern brake. This shape has proven to be very seaworthy and is used today on very fast offshore boats.

Foiling hull Fuel efficient and comfortable. Due to ventilation at the foils is the speed limit at appr. 40 knots. A 30" boat is very seagoing and comfortable in waves up to 1,5m.

More Information: AEOLOS YACHTS

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