Tech Talk: Frank Mulder on Spectre’s hull design

Januari 10, 2019

Tech Talk: Frank Mulder on Spectre’s hull design

With exterior styling by Giorgio M. Cassetta, 69-metreSpectre is the third Benetti yacht owner John Staluppi acquired after Quantumof Solace (now Elysium) and Diamonds Are Forever. Her naval architecture is by the team at Mulder Design in the Netherlands, who have worked on multiple projects for Staluppi, starting with 38-metre Octopussy, the world’s fastest yacht at the time of her launch in 1988.

Times have changed and instead of focusing on outright speed, Spectre is the first monohull superyacht to be fitted with a Ride Control System (RCS) by Naiad Dynamics to improve both comfort and performance. “The trend today is towards displacement yachts with lower speeds, longer range and more comfort,” says naval architect Frank Mulder. “But the owner still wanted something that was faster than your average yacht and Spectre can cruise at 20 knots with a top speed of 21 knots. The problem is that the faster you go, the worse the motions become in sea states that are anything less than ideal. Stabiliser fins are very good at damping roll, but in quartering seas, you also have the problem of pitching.”

Naiad Dynamics pioneered the science of ride control that coordinates multi-axis surfaces for controlling roll, pitch and other undesirable motions like yawing and slamming, by combining conventional fins with other appendages such as T-Foils, trim tabs, canards and interceptors. Optimal ride performance is achieved by accurately phasing the movement of each controlled surface with respect to the combined motions of the vessel. Mulder likens the system to “adjusting the suspension on your car when you want to go fast over rough terrain.”

Working closely with Mulder, Naiad ran initial simulations using three different configurations – with no RCS: with fins only; and with fins, canards and interceptors – at various speeds, wave heights and headings to find the best combination for Spectre. The results showed that having two active stabiliser fins amidships, two active interceptors mounted under the transom, and two active canards forward provided the most effective roll and pitch damping, as well as optimising the trim of the yacht. In fact, pitching was reduced by 25-47% in head seas and rolling by 79-88% at the worst case heading of 75 degrees.

Interceptors work much like the aerofoils on high-performance sports cars that pop out of the back at high speed to increase the down-force, except when applied to a hull they provide dynamic lift to reduce drag and control pitch. Canards are the small wing-like projections you see in front of the main wings on some jet aircraft; they work exactly the same way on a hull by angling up or down to alter the angle of attack and improve stability. Active T-Foils, which Naiad has successfully applied to high-speed catamarans and naval vessels, were considered but rejected early on as they are mounted under the keel and would have increased the draft, limiting the yacht’s cruising grounds.

With the initial data from Naiad’s simulations, Mulder Design continued to fine-tune the hull design and carried out 25 different CFD analyses before final tank testing. “The initial hull form was already very efficient, but each new CFD analysis resulted in incremental gains,” says Mulder. “That’s the beauty of CFD. We can arrive at a point after which we don’t see any improvements, and the tank tests are really just to confirm the CFD results.”

In addition to improving motion comfort, Spectre’s hull form and appendages provide a virtuous circle of efficiency benefits. Less drag means more speed, lower fuel consumption and longer range (well over 6,000nm at 12 knots) for the same installed power. Less fuel burned also means smaller tanks (just 127,000 litres), and thus less weight to carry around.

Spectre is powered by twin 2,580kW engines, the smallest in MTU’s 4000 series, but her top speed is at least 25% faster than a conventional displacement monohull. When Mulder was on board Spectre with the owner during final sea trials prior to delivery, he noted that at 15 knots the engines were running at less than one-third power.

“The wind later picked up and started gusting at 40 knots with white caps all around, but the yacht was still doing 20 knots with no pitch or roll,” Mulder recalls. “Like an airplane that needs forward speed to start flying, the interceptors and canards are less effective at slow speeds, but at anything over 12-14 knots, the motions just disappear. It’s really impressive. I’m not saying the owner can play pool on board, but she’s a very stable boat!”

Read more about Spectre