Optimal sailing performance
The main motivation behind specifying this unique retractable propulsion was to optimise sailing performance. CFD studies by Malcolm McKeon Yacht Design focused on the benefits of retractable propulsion drives versus fixed pods or conventional propeller and shaft arrangements. McKeon confirmed a considerable increase in performance by utilising the yacht’s twin RPS, especially when used in light to medium air. With the propellers retracted, the hydrodynamic drag is massively reduced, and the yacht accelerates and powers up to her full potential much faster. The optimised hydrodynamic hull compliments the powerful sail plan, resulting in a performance package not seen before on a yacht of this size.

Advantages while motoring
In addition to increased sailing performance, there are also significant motoring advantages. With electric power, Sarissa’s twin pulling propellers are optimised for efficiency and directly connected to permanent 500kW magnet motors. The unique features of these innovative RPS units are virtually undetectable noise and vibration levels which is further helped by the flexible insulation mounting to the hull, the single spiral bevel gear between e-motor and propeller and no gears or noisy mechanical drive inside the hull.

Redundancy enhances safety
Each unit of Sarissa can also serve as an underwater turbine, generating power for the systems and charging the batteries during sailing. There are also benefits when it comes to safety. Should the yacht encounter an unlikely fuel contamination issue and suffer mechanical generator failure, the propulsion units can serve as underwater turbines, generating power for the ship’s systems and charging the batteries.

Having plenty of redundancy is a key factor for sailing safely around the world, and this system further enhances the yacht’s safety.

Position of the units
Sarissa’s retractable drive units are much further forward compared to a more traditional shaft and propeller set-up. They are positioned directly below the machinery and midship water sports storage areas, rather than being fixed-mounted directly below the aft guest accommodation.

This positioning away from the guest area provides even further advantages to the onboard luxury experience for the guests when motoring. The drive units also cancel out the need for a stern thruster, as they can each be turned 90 degrees. This saves significant weight, which also improves performance.

In detail
The system consists of two retractable electrically driven azimuthing propulsion units. The propellers are optimized for vibrations and thrust. Each unit comprising of swing retractable, azimuthing with direct connected 500kW permanent magnet (PM) motors and a fixed pitch, 5-blade pulling propeller of 1300mm. diameter. The swing retractable propulsion units are installed underneath the mid lazarette.

The propulsion legs and hatches are integrated in the thruster casings. The thruster casings are bolted into the ships structure. Vibration damping is done in three ways. The drive legs are flexibly mounted in the thruster casings, the fixed ends of the retract/deploy cylinders are flexibly mounted to the thruster casings and the casing is concealed by two O-rings aiming to minimize transmittal of vibrations into the ships structure.

The non-buoyant thruster casings are covered with hydraulically operated hatches. The hatches are always closed, except when deploying/stowing. The maximum boat speed for deploying/retracting and/or opening the hull hatches is 7 knots. The hatches are not watertight.

When retracted, the underside of the hull shall be smooth and clean of any drive train appendages. When deployed, only the drive train appendages are exposed and the rest of the hull shall be smooth and clean.

The units are powered by permanent magnet electrical motors (PM motors). The PM motors are mounted in the propulsion unit, a 90 degree gearbox connects the vertical (when deployed) axis with the propeller axis. The PM motors are connected to inverters that will be fed by the DC bus.

Unique compared with existing sailing superyachts:

• Power level of 500kW each
• Minimized noise and vibrations level due to flexible mounting, single gear between e-motor and propeller and no gear inside the hull.
• Unit can serve as underwater turbine, generating power for the systems and charging the batteries.

Overwhelming rain of prestigious awards for Sarissa at Boat International’s Design & Innovation Awards:

• Eco award for her board systems
• Innovation of the year: the forward facing retractable electric propulsion
• Best exterior design sailing yachts
• Best interior design sailing yachts
• Best naval architecture sailing yachts

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