Rupert Holmes talks to the build and design teams behind the 81m Royal Huisman flagship Sea Eagle II, which recently completed her sea trials off the coast of the Netherlands

Royal Huisman has an enviable track record of producing superlative sailing superyachts, with hundreds of projects completed to date. Yet the latest vessel to leave the shipyard is extraordinary even by these standards. She is the world’s largest aluminium yacht and is one of the top ten biggest sailing yachts ever built. 

Sea Eagle II is a magnificent 81m/266ft three-masted Panamax schooner, created by the same Dykstra and Mark Whiteley Design collaboration that produced the stunning 56m/186ft Royal Huisman ketch Aquarius just over a year ago. 

Her very experienced owner has sailed all his life and spent a lot of time at sea. “Sailing around the world in the weekends is a phrase often used to outline his days on board,” says Royal Huisman project manager Arjo Spans. He is also a repeat client – the original Sea Eagle is a 43m/143ft Frers/Rhoades Young design launched by the Dutch yard in 2015. 

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Her 3,500 square metre sail plan is evenly distributed over her three 62m carbon masts. Photo: Tom Van Oossanen

Initially the brief for the new boat was for a more classical yacht, similar to the 79m/259ft three-masted gaff schooner Athena, which Royal Huisman built for Silicon Valley mogul Jim Clark in 2004. However, the owner quickly realised he would prefer a more modern yacht: a fast-looking hull shape with straight lines, long waterline and plumb bow. The concept of Sea Eagle II was born. 

“Having worked with him to build the original Sea Eagle, we understood the owner’s priorities from the start,” says Spans. “These are: low maintenance, functionality, and safety, including ease of moving around the yacht, even when heeled at sea.

“For each of the key aspects of the boat he asked for three proposals, he would then choose one of them and leave us to implement it,” Spans adds. “He put a huge amount of trust in the shipyard and in our craftsmanship.”

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The scale of this yacht is nothing short of astounding. The sleek hull lines belie the 4m/13ft freeboard and the two-tier deckhouse, including a half-raised bridge, looks entirely in proportion. The main deck-level accommodation has a huge expanse of glass, with the 360º view interrupted only by minimal mullions. There are also acres of cockpit space, with room left over for a long sweep of uncluttered aft deck.

Mark Whiteley was responsible for developing the interior and refining the exterior styling. He created a simple and modern style for the interior, with straight lines. Lacquered Alpi walnut wall panels, brushed natural oak floors and dark stained wenge trims are complemented by light colour leather handrails and upholstered wall panels.

Part of the brief was for the yacht to be able to host business meetings – the forward part of the superstructure therefore includes a large saloon with expansive seating, plus a 16-seat table for formal dining.

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The largest winches are capable of a tremendous 18-tonne pulling load. Photo: Rondal

A technical challenge 

Dykstra Naval Architects drew a narrow and efficient hull shape with a maximum beam of only 12.4m/40ft, while displacement has been kept down to a relatively light 1,050 tonnes at full load. The final shape was achieved following extensive computational flow dynamics (CFD) calculations, velocity prediction programming (VPP), and testing in both tow tanks and wind tunnels. These all helped to determine sail balance, keel position and predicted rudder angles.

The plumb bow is balanced by a modest, but graceful, stern overhang, with a contemporary, relatively wide transom at deck level. Twin knuckle lines extend aft from near the bow, merging into one amidships and then forming a chine aft. Right aft the hull has negative curvature on the flare below the chine, which gives interesting reflections in the gleaming paintwork.

The lead naval architect, Dykstra’s Erik Wassen, told me he introduced the knuckle lines to give the hull “a bolder and more interesting shape.” The owner liked the concept and Mark Whiteley helped to refine the detail. Wassen describes the underwater shape as being: “a very modern round bilge shape, with a nice slender hull that gives good sailing capabilities.” Appendages are a conventional fin keel and balanced rudder.

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Photo: Priska van der Meulen

The latter presented an interesting technical challenge, according to Wassen. Initially they looked at fabricating the rudder stock in steel, but that alone would weigh three tonnes. Screening a number of alternatives led to a decision to use carbon for the stock, with a foam core section clad in a relatively thin composite skin for the blade. 

The result is what it’s producer, Royal Huisman’s sister company Rondal, believes to be the biggest ever carbon rudder, weighing only 1,250kg, which helps to keep weight out of the ends of the boat. The weight saving here also enabled the intended fore and aft trim to be achieved with the ballast in its optimal location.

The rudder includes load sensors to record and verify torque, side forces and bending moment while sailing. “There is not much load data available for rudders of this size of sailing yacht, so it’s very conservatively engineered,” says Spans. Fibre-optic sensors are incorporated to enable a database to be created that will help with designing future composite rudders for very large yachts.

Due to the distance of the helm stations from the rudder, the steering is via an electric-hydraulic system, which means there’s no feedback from the helm. The plan is to investigate whether data from the sensors can be used in a feedback system to give a more natural feel to steering the boat under sail. 

Structural engineering

This is one of the most interesting elements of yacht design, yet is all too often under appreciated. Extensive modelling showed a couple of unexpected problems that needed clever solutions during Sea Eagle II’s design and engineering. 

Instead of the hull flexing smoothly under load, like an I-beam, the presence of a forward intermediate deck introduced hard spots in this area, resulting in stress concentrations at the forward end of the superstructure. This required a lot of design work to create a smooth transition of stress from the lower deck to the intermediate one and then on to the upper deck.

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Structural engineering was extensively modelled for Sea Eagle II

The roof of the superstructure also posed a challenge, in that the inside surface of a beam needs to become shorter as it flexes. But the original plan for the aluminium roof wasn’t able to accommodate this, with the result the modelling showed stresses concentrating in the corners of the mullions and in the glass itself. 

The solution was to avoid welding the top of the mullions to the roof, opting instead for a flexible joint that allows for movement when necessary. Wassen likens it to the joints in large buildings that need to be included to allow for thermal expansion and contraction.

Whiteley also helped to refine the exterior styling. In addition to the detail of the knuckles, his input helped to refine the shapes – particularly curves and softening corners – in the superstructure.

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COVID-19 restrictions meant her delivery date had to be pushed back by 3 months. Photo: Crea Fragma / Rondal

Panamax rig

The requirement to be able to clear the Bridge of the Americas at the Pacific end of the Panama canal called for the schooner rig with three equal masts. All are of carbon and are made by Rondal, with integrated sail handling systems and Carbo-Link carbon standing rigging.

Each mast has in-boom furling, while the aft two have staysails for use when reaching. The tank and wind tunnel testing was used to verify different headsail options, particularly to determine the differences between a large blade jib versus a large staysail or yankee.

“Obviously you have better windward performance with a large blade,” says Wassen, “but as soon as you bear away the yankee is much more forgiving in trimming, so we decided that made more sense.” A further advantage is that when the yankee is part furled the sheeting point remains the same. 

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Rondal’s Integrated Sailing System consists of the three carbon Panamax masts with furling booms, hydraulic boomvangs, headsail furling systems, deck winches and equipment, captive winches and continuous solid carbon standing rigging by Carbo-Link, all equipped with load sensing capabilities. Photo: Rondal

In addition, a blade jib requires much higher sheet loads to maintain leech tension. Even though Sea Eagle II is a three-masted yacht, the yankee sheet loads are still predicted to reach 18 tonnes – but this still allows standard-size captive winches to be used, rather than larger custom-made units.

The Panamax limitation obviously poses the question as to whether sail area needed to be compromised to fulfil this criteria. However, this is clearly not the case for Sea Eagle II. Hull speed is close to 20 knots and the yacht is clearly capable of surfing at much higher speeds, despite being designed as a pure cruiser. This role, of course, suggests keeping heel angles to a maximum of 10-15°. 

Nevertheless, the VPP figures suggest potential boat speeds of 17-18 knots in 16 knots of true breeze with a true wind angle of 70°. “As soon as you can ease the sheets a little bit, we have a lot of sail area,” says Wassen, “so on a beam reach, I wouldn’t be surprised if a steady 20 knots is possible.” 

Specification

LOA: 81m (266ft)
Air draught: 62m (205ft)
Hull: Aluminium
Guest accommodation: 11
Crew accommodation: 14
Exterior styling: Dykstra Naval Architects and Mark Whiteley Design
Naval Architecture: Dykstra Naval Architects
Interior design: Mark Whiteley Design
Classification: Lloyd’s MCA (LY-3)

First published in the April 2020 edition of Supersail World.



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