11.4 litres at 12 knots.. : Feadship Najiba sets new standard for superyacht efficiency

It always takes several weeks for the final results of a Feadship’s sea trial to be released after a period of analysis. The figures for Najiba were even more keenly anticipated than most as the parties involved – the Feadship Aalsmeer yard responsible for her engineering and construction, the naval architects & engineers at Feadship De Voogt, the designers & naval architects at Vitruvius Yachts, and the inspiration behind the overall concept, Philippe Briand – would discover whether the project’s innovative design had achieved her ambitious targets.

High potential

While every Feadship is an exercise in bespoke teamwork, Najiba was unusual in the sense that her owners had already decided to work with a particular design by Vitruvius Yachts and a specific hull initially conceived by Philippe Briand before they came to Feadship. The slender lines and distinctive vertical bow are based on Briand’s rich experience with designing fast sailing yachts. Najiba is also the also second largest Feadship to be built in aluminium to date, saving weight and adding to the potential savings on fuel consumption.

Many months of meticulously planned naval architecture and several years of construction later, Najiba was launched in January 2019. Her sea trials in February indeed confirmed that the approach chosen by the owners had paid dividends. Najiba’s average fuel consumption is as low as 11.4 litres per nautical mile at her cruising speed of twelve knots, between 20% and 25% less than any equivalent motoryacht of this size. The yacht also has a top speed of 17 knots, outstripping the original goal of 16 knots, while her range came in at over 6000 nm, more than a thousand over the specified figure.

Exceeding expectations

“These results mean that – despite the complexity of her development and build – we have outperformed all expectations with Najiba,” comments Roderick de Vries, technical director of the Feadship yard in Aalsmeer. “It was a real ‘wow’ moment for everyone involved to see this level of efficiency achieved during the sea trials. It is one thing to design such an efficient full-displacement motoryacht but another to translate these lines and shapes into a stunning Feadship that is even faster and more fuel-efficient than we projected.

“Working very closely with Vitruvius, hull efficiency was the leitmotif of the project from an engineering perspective. It impacted every decision made about appendages like the bowthruster, propeller shaft, stabilisers and main exhaust. We had to strike exactly the right balance between Philippe’s desire to minimise all sources of resistance and the Feadship values of ensuring total comfort for clients, ease of maintenance for crew and the smartest construction. It was a fascinating challenge at times but we have managed to combine the best of both worlds and Najiba is a true Feadship.”

Leveraging on experience

As the designers of the original concept, Phillippe Briand and his team were committed to translating the Vitruvius philosophy into a Feadship and create the most efficient possible motoryacht design. “We leveraged on our decades of experience with sailing yacht designs to optimise the hull lines, proportions and weight of the boat,” Briand explains.

“There are various parameters involved in the resistance of the hull. For example, for a given overall length, the optimum is to have a plumb line bow with a maximum wet waterline.

Another parameter is to try and reduce the wetted area – the area of the hull immersed in water – as this has the greatest impact on cruising speed. We also optimised the pressure on the hull and reduced wave drag by ensuring the smoothest possible distribution of the lines.

“At Vitruvius we are used to designing boats in aluminium, which obviously leads to savings in weight and reduces displacement. As the inertia of the boat is less, so too is the wave resistance when underway and the degree of pitching. These, in turn, lead to further reductions in fuel consumption. Aluminium also brings extra benefits in terms of motion comfort, as was instantly perceptible during Najiba’s sea trials.

“Together with Feadship we believe that this project has been a crucial step on the road to greater superyacht sustainability, a path that we expect more owners to follow in the years ahead. Najiba is a genuinely innovative motoryacht and she illustrates what can be achieved when two expert parties share their knowledge and experience to attain a greater goal.”

Matching programs

Annemarie Steenbergen, who led the team of naval architects at Feadship De Voogt,

thoroughly enjoyed the close working relationship with Briand and Vitruvius. “Najiba has a different hull shape from any previous Feadships, showing curvature in all degrees. The hull shape isn’t straight from the waterline up to the main deck. A cross-section halfway through Najiba’s hull shows she is like a half moon, with a smaller beam on the waterline than at deck level. The bow area is also different. Feadships usually have a horizontal keel with the radius in front, especially with straight bows, before rising vertically. Najiba has a raised keel from approx. one third to the bow, resulting in a reduction in resistance without any compromise on maneuverability.”

Feadship already has its own efficient engineering and naval architecture procedures, and the Najiba project proved that these can also be adapted to different designs from different sources. “We received the lines plan in the Vitruvius program and transformed it into our own,” adds Steenbergen. “The hull design was very much Philippe’s but we ensured it could be built according to Feadship procedures and standards. Height stacking was another challenge as the sleek design sought to push the vertical centre of gravity down and save weight. We successfully accommodated the smaller distances between each ceiling and the deck above.”

Looking ahead

As a technical director at Feadship, Roderick de Vries is impressed with the potential that the Najiba project opens up. “This was the first yacht we had built with Vitruvius so it involved both lots of discussions and building significant margins and tolerance into the system. Any future builds would inevitably see a reduction in these margins.

“Looking beyond the individual parties involved, this project also illustrates how we might move forward with other efficiencies with respect to electric propulsion. The lighter and more efficient a design is, the easier it will become to fit electric engines. One of the key obstacles to using batteries is their power density, which is lower than diesel fuel. If we can achieve a very efficient hull and power consumption, less battery capacity will be required to achieve a given range or speed. Who knows what the future may hold!”